Survey of Positioning Techniques for improved Accuracy

FACULTY OF ELECTRICAL ENGINEERING, AUTOMATICS, COMPUTER SCIENCE AND ELECTRONICS DEPARTMENT OF TELECOMMUNICATIONS Master Thesis Report Name and Surename Daniel Abia Serrano Field of Studies Telecommunications Engineering Topic Survey of Positioning Techniques for Improved Accuracy Supervisor Prof. Andrzej G!owacz, Ph.D. Krakow, 2011 AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY ©2011 Daniel Abia Serrano All Rights Reserved ii To my parents and sister. iii Acknowledgments First of all, I would like to thank my tutor, Prof. Andrzej Głowacz, Ph.D., for proposing the topic of the Master Thesis and for the support and help during all this working period, as well as my supervisor, Prof. Zdzisław Papir, Ph.D. Hab., for accepting me as an Erasmus student at the AGH University (Krakow, Poland). Iwouldliketogivespecialacknowledgementtomyparents,PedroandAntonia,andmysister,Mavi, for the unconditional support since I began my studies. Without you nothing of this would have been possible. Besides, I am grateful to my grandmother Maravillas, whose permanent smile has always shined on us and warmed up the best family I could ever have. I would also want to thank the people I have met here in Krakow, Jon, Anni, Louise, Stefan, Alessandro, Julien, Manuel, Nico, Jean, Amandine and many more. Thank you for sharing your season far away from home with me, for the lunches, dinners and tea breaks when I have been stressed, as well as for the wonderful trips we have made together. In short, I will never forget the great moments that have made me stronger and kept me working. I also want to take the chance to mention the people who came to visit me here in Krakow, your presence has helped make this feel like a home away from home. In addition, thank you to all my friends from Mataró, specially to Jordi, Miquel, Pau, Ferran, Gemma, Mireia, Roser and Laura for always being there when I needed you and, why not, for standing my bad jokes. I hope that wherever or however we will be in the future, we will never stop doing things together and remembering the old times. Thank you also to my University colleagues from Barcelona, Dídac, Álvaro, Daniel A., Pau, Carles, Núria, Lledó, Albert, Xavi, Daniel G. and many more. Thank you for the support and the great moments spent together, both inside and outside the Campus, which helped me significantly to continue working hard during hard times. The hours of study together not only have brought me to the end of my degree, but to a friendship I will never forget. Likewise, I want to acknowledge the people I have met in Telecogresca, Saul, Anxo, Roger, Juackss, Miki, Alex, Sobri and Aleix amongst others, with whom I have lived the most epic nights. And last but not least, thank you Roser for being as you are and for enduring this time we have been apart and always with your best smile. Thank you for your advice and your comforting words when I most need them as well as for the unforgettable moments we have and we will spend together. By last, I would like to acknowledge the people who have made it possible for me to take part in the INSIGMA project num. POIG.01.01.02-00-062/09, and notice that the work has been co-financed by the European Regional Development Fund under the Innovative Economy Operational Programme. iv “Those things which I am saying now may be obscure, yet they will be made clearer in their proper place.” Mikołaj Kopernik (Poland, 1473-1543) v Abstract There is no doubt that positioning systems has been one of the greatest improvements in technology in the last quarter of the 20th century. Knowing the position of any object everywhere in the world by calculations of the signals received by the satellites has revolutionized different areas. What began as a military advances race between the U.S.A. and the U.R.S.S. during the Cold War, nowadays it seems to belong to the civil users instead to their developers. It is so much so, that in recent years these systems have been modified to be opened to the civil users, even proliferating new non-military systems. This Thesis is divided in three parts. The first part includes an introduction about positioning and there is an examination of the different current positioning systems, as well as the systems that are being developed. Due to their significance, GPS, GLONASS and Galileo are described widely than the rest. Even though, the main objective of this Thesis is the GPS. In the second part, there is an examination of the error sources that affect the accuracy in GPS positioning, to explain which of them can be corrected and how. Moreover, one can find a description of the techniques to improve accuracy used in GPS, called Augmentation Systems, with special mention to SBAS and AGPS. This last one, very popular nowadays thanks to the Smartphone’s. By last, there is an introduction about Tracking Systems , one of the most benefited by the positioning systems, in which is analyzed the different existing systems. Furthermore, a short description about a selection of significant products in the market is done. With this, the reader can get a global idea about the Tracking Systems and their possible applications. No cabe duda que los sistemas de posicionamiento han sido uno de los grandes avances en tecnología del último cuarto del siglo XX. El hecho de poder conocer la posición de un objeto mediante cálculos realizados por señales enviadas por satétiles y con cobertura global, ha supuesto una revolución en distintas áreas. Lo que comenzó como una carrera de avances militares entre los EE. UU. y la U.R.S.S. en plena Guerra Fría, ahora parece pertenecer más a los usuarios civiles que a los militares. Tanto es así que en los últimos años, esos sistemas se están abriendo más y más a los usuarios civiles, e incluso están proliferando otros sin fines militares. Esta Tesis está dividida en tres partes. En la primera parte se introduce el concepto de posicionamiento y se estudian los sistemas de posicionamiento que actualmente existen así como los que están en proceso de desarrollo. Debido a su importancia, se explican con mayor detalle los sistemas GPS, GLONASS y Galileo. Aún así el objetivo de esta tesis es el sistema GPS. En la segunda parte, se examinan las fuentes de error que afectan la precisión en el posicionamiento del sistema GPS, para ver cuáles de ellas pueden ser corregidas y cómo hacerlo. Con esto, se describen las técnicas existentes en GPS para mejorar la precisión, con carácter especial para los SBAS y AGPS. Éste último, en auge con la aparición de los smartphones. Por último, se introducen los Tracking Systems, uno de los grandes beneficiados por los sistemas de posicionamiento. Se analizan los diferentes sistemas existentes y se hace un pequeño estudio sobre una selección de los productos que el usuario puede encontrar en el mercado. Con esto, se pretende ofrecer al lector un idea global sobre los Tracking Systems y de sus posibles aplicaciones. vi Contents Acknowledgments iv Abstract vi IPositioningSystems 1 1Introduction 2 1.1 Basic principles in positioning . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 GPS, GLONASS, GALILEO and other Positioning Systems 6 2.1 Global Positioning System (GPS) . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.1 Principles of working . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.1.1 SpaceSegment......................... 8 2.1.1.2 Control Segment . . . . . . . . . . . . . . . . . . . . . . . . 21 2.1.1.3 User Segment . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.1.2 Position calculation by the receiver . . . . . . . . . . . . . . . . . . . 25 2.1.3 ProblemsinGPS............................. 30 2.2 Global Navigation Satellite System (GLONASS) . . . . . . . . . . . . . . . 30 2.2.1 GLONASS structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.1.1 Constellation of Satellites . . . . . . . . . . . . . . . . . . . 31 2.2.2 GLONASS & GPS Comparison . . . . . . . . . . . . . . . . . . . . . 40 2.3 GalileoSystem .................................. 41 2.3.1 Galileo Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.3.1.1 Space segment . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.3.1.2 Ground Segment . . . . . . . . . . . . . . . . . . . . . . . . 44 2.3.1.3 User Segment . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2.4 Othersystems................................... 45 2.4.1 COMPASS ................................ 46 2.4.2 Indian Regional Navigation Satellite System (IRNSS) . . . . . . . . . 48 vii 2.4.3 Quasi-Zenith Satellite System (QZSS) . . . . . . . . . . . . . . . . . 48 2.5 Problems in positioning systems . . . . . . . . . . . . . . . . . . . . . . . . . 49 II GPS accuracy 50 3 GPS Errors 51 3.1 GPS Errors and combinations to correct them . . . . . . . . . . . . . . . . . 51 4TechniquestoimproveGPSaccuracy 57 4.1 DifferentialGPS(DGPS) ............................ 58 4.2 Satellite-Based Augmentation System (SBAS) . . . . . . . . . . . . . . . . . 65 4.2.1 Wide Area Augmentation System (WAAS) . . . . . . . . . . . . . . 68 4.2.1.1 Ground Segment . . . . . . . . . . . . . . . . . . . . . . . . 70 4.2.1.2 Space Segment . . . . . . . . . . . . . . . . . . . . . . . . . 71 4.2.1.3 User Segment . . . . . . . . . . . . . . . . . . . . . . . . . . 72 4.2.2 European Geostationary Navigation Overlay Service (EGNOS) . . . 73 4.2.2.1 EGNOS Architecture . . . . . . . . . . . . . . . . . . . . . 74 4.2.2.2 Errors magnitude . . . . . . . . . . . . . . . . . . . . . . . 77 4.2.3 OtherSBAS................................ 79 4.2.3.1 System of Differential Correction and Monitoring (SDCM) 79 4.2.3.2 Multi-functional Satellite Augmentation System (MSAS) . 79 4.2.3.3 GPS Aided Geo Augmented Navigation (GAGAN) . . . . . 80 4.3 AssistedGPS(AGPS) .............................. 81 4.4 Conclusions.................................... 84 III Tracking Systems 86 5TrackingSystems 87 5.1 Passive and active tracking systems . . . . . . . . . . . . . . . . . . . . . . . 87 5.1.1 Tracking systems overview . . . . . . . . . . . . . . . . . . . . . . . . 87 5.1.2 Tracking systems applications . . . . . . . . . . . . . . . . . . . . . . 91 5.2 Products...................................... 93 5.2.1 Active and passive products . . . . . . . . . . . . . . . . . . . . . . . 93 5.2.1.1 Passive products . . . . . . . . . . . . . . . . . . . . . . . . 93 5.2.1.2 Active products . . . . . . . . . . . . . . . . . . . . . . . . 96 6 Conclusions 103 viii List of Figures 1.1 2DPositioning ................................... 3 1.2 Clock error effectinpositioning.......................... 4 1.3 Dilution of Precision effect ............................ 5 2.1 GPSSegments ................................... 8 2.2 GPSConstellation................................. 10 2.3 Orbitalparameters................................. 11 2.4 Evolution of the GPS satellites . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.5 GPSsignalstructure................................ 17 2.6 GPSsignalsevolution ............................... 21 2.7 Control Segment elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.8 Control Segment communication process . . . . . . . . . . . . . . . . . . . . . 23 2.9 GPS Receiver tracking system . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.10 Navigation Message frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.11 Signal Acquisition proces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.12 GPS Receiver Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . 29 2.13 GLONASS Antipodal Satellites . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.14 Uragan-Moverallview............................... 34 2.15 Uragan-Koverallview............................... 35 2.16 FDMA signals evolution in GLONASS . . . . . . . . . . . . . . . . . . . . . . 36 2.17 Constellation Current Status . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.18 Ground-Based Control Facilities . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.19 NovAtel OEMV Family receivers . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.20 GALILEOArchitecture .............................. 42 2.21 GalileoConstellation................................ 43 2.22 Galileo Frequency Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 2.23 Beidou-1 architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 2.24 COMPASS signals frequency bands . . . . . . . . . . . . . . . . . . . . . . . . 47 ix Figure 1.3: Dilution of Precision effect [1] In order to minimize synchronism errors, GPS satellites have highly accurate atomic clocks with stabilities of about 10−13. Commercial receivers, however, use quartz clocks much more unstable but less expensive. The way to combat the ambiguity for the end user is to predict the error of synchronism when the coordinates are calculated. 5 Chapter 2 GPS, GLONASS, GALILEO and other Positioning Systems 2.1 Global Positioning System (GPS) Positioning systems has been one of the greatest improvements in technology in the last quarter of the 20th century. Knowing the position of any object everywhere in the world by calculations of the signals received by the satellites has revolutionized different areas. What began as a military advances race between the U.S.A. and the U.R.S.S. during the Cold War, nowadays it seems to belong to the civil users instead to their developers. It is so much so, that in recent years these systems have been modified to be opened to the civil users, even proliferating new non-military systems. This part of the Thesis includes an introduction about positioning and there is an examination of the different current positioning systems, as well as the systems that are being developed. Due to their significance, GPS, GLONASS and Galileo are described widely than the rest, even though, the reader can get a global idea about each and every one of the systems. 2.1.1 Principles of working The NAVSTAR-GPS [22] (NAVigation System with Time And Ranging Global Positioning System) or commonly named GPS (Global Positioning System) is a Global Navigation Satellite System (GNSS) that determines in every part of the world the position of an object, a person, vehicle or vessel, with accuracy up to centimetre depending on the technology used, although is usual a few meters of accuracy [2]. The operational GPS satellites are nominally maintained within 24 orbital slots. These slots reside within circular orbits inclined 55ºwith respect to the equatorial plane. Four slots are contained in each of six orbital planes, with an orbital radius of 26,559.7 km. In order to determine the position, 6 the receiver automatically locate at least three satellites1in the network, from which receives a signal indicating identification and clock hour of every of them [23]. Once the receiver has gathered all that signals, the device synchronizes the GPS clock and calculates the time it takes to get the signals from the satellites to the antenna, and thereby measure the distance to the satellite using trilateration2, which is based to determine the distance of each satellite relative to the point of measurement. With the known distance, the determination of the relative position of the object to the three satellites is instantaneous. Therefore, knowing well the coordinates or position of each of the signal emitted by the satellites, the receiver can obtain the absolute position or actual coordinates of the measurement. The GPS is formed by three segments (see Figure 2.1) : •Space Segment (SS) •Control Segment (CS) •User segment (US) 1The location with three satellites determines 2 points in the interesection of the three spheres. In the case that the object is on the earth surface, one of these points could be discarded. If the object is in the air (i.e. an airplane) there will be ambiguity between both points, so another satellite will be needed at least to determine a unique position. 2Trilateration using GPS is to find the distance of each of the three signals to the point of measurement. Knowing the three distances, the pseudorange is easily determined by one’s position relative to the three satellites. It is also essential to know the coordinates or position of each satellite. In this way we obtain the absolute position or actual coordinates of the measurement. 7 Figure 2.1: GPS Segments [2] 2.1.1.1 Space Segment The main functions of this segment are, from the instructions received by the control segment, to provide an atomic clock reference signal, to generate the pseudorandom Radiofrequency (RF) signals and to save and forward the navigation message. In order to carry out that functions, the space segment is formed by: •The constellation •The Satellites •The GPS signal The constellation The space segment is nominally formed by a 24 satellite constellation (see Table 2.1), distributed in 6 orbital planes with an inclination of 55ºwith respect to the equatorial plane. The orbits of the satellites are almost circular, their eccentricity is less than 0.02, 8 with an orbital radius of 26,559.7km and a period of 12 sidereus hours (11h 58min. 2sec.). This configuration was designed so that at any point on the earth always had a minimum of 4 satellites visible above the horizon of the observer, with an elevation angle above 15 degrees. Table 2.1: Nominal GPS Constellation Slot Locations Slot Right Ascension of the Ascending Node (º) Argument of Latitude (º) A1 272.847 268.126 A2 272.847 161.786 A3 272.847 11.676 A4 272.847 41.806 B1 332.847 80.956 B2 332.847 173.336 B3 332.847 309.976 B4 332.847 204.376 C1 32.847 111.876 C2 32.847 11.796 C3 32.847 339.666 C4 32.847 241.556 D1 92.847 135.226 D2 92.847 265.446 D3 92.847 35.156 D4 92.847 167.356 E1 152.847 197.046 E2 152.847 302.596 E3 152.847 66.066 E4 152.847 333.686 F1 212.847 238.886 F2 212.847 345.226 F3 212.847 105.206 F4 212.847 135.346 In order to provide robustness in performance against satellite failures, the constellation design includes asymmetrical spacing in argument of latitude between satellites within each 9 plane. With this, when a new satellite is launched, it is placed near one of the slots that contain a satellite expected to require replacement in a short period of time. Since the first satellite launched on 1978 there has been 62 launches, although the current constellation is formed by 31 satellites (see Figure 2.2) [24]. Figure 2.2: GPS Constellation [3] The Kleperian elements needed by the satellites to be properly positioned in the constellation are (see Figure 2.3) [1]: [�]Rightascensionofascendingnodeis the geocentric angle between the ascending node direction and the Aries point. The node line is the intersection with the equatorial plane and the orbital plane. Its intersection with the unit sphere defines two points: the ascending node, through which the satellite crosses to the region of positive Z, and the descending one. [i] Inclination of orbital plane is the angle between the orbital plane and equator. [ω] Argument of perigee is the angle between node directions and perigee, measured in the orbital plane. The perigee is the point of closest approach of the satellite with 10 respect to the centre of mass of the Earth. The most distant position is the apogee. Both are in the semi-major axis direction. [a] Semi-major axis of orbital ellipse is the semi-major axis of the ellipse defining the orbit. [e]Numericaleccentricityoftheorbitis the eccentricity of the orbital ellipse. [T0] Perigee passing time is the time of the satellite passage through the closest approach with the Earth (perigee). Satellite orbital position can be obtained at a moment t using τ(t)=t−T0or any of the three following anomalies: [v(t)]Trueanomalyis the geocentric angle between perigee direction and satellite direction. [E(t)] Eccentric anomaly is the angle, measured from the centre of the orbit, between the perigee and the direction of the intersection point of the normal line to the major axis passing through the satellite with the circle of radius a. [M(t)] Mean anomaly is a mathematical abstraction. Figure 2.3: Orbital parameters [1] The satellites The space vehicles (SVs) have structures and mechanisms to maintain the correct orbit, communicate with the control segment and transmit the signals to the receivers. The most 11 critical point in the SVs is the synchronism in their clocks, as a result, all the satellites are equipped with atomic clocks (Rubidium (Rb) or Cesium (Cs)) which provide very high stability. Each GPS satellite is identified in several ways: regarding its position in the orbital plane, every satellite is placed in a slot (1, 2, 3. . . ) inside of the 6 orbits (A, B, C, D, E or F); by the NASA cataloging number; by the international identification number, by the PRN Code (Pseudorandom Noise code); and by the sequence launch number or Space Vehicles Number (SVN). During the GPS life, there have been developed some satellite groups [2]: •Block I: The Block I Space Vehicles, built by Rockwell International as developmental prototypes, were launched between 1978 and 1985 from VAFB, CA. These SVs supported most of the system testing. The Air Force launched the first Block I research and development satellite in February 1978, and as of February 1991, the GPS network consisted of six Block I R&D satellites. These satellites were used for the development and testing of Navstar receivers and user equipment and they were not equipped with the Selective Availability (S/A). The average life was around 4.5 years, although some of the last SVs launched worked during 10 years. These SVs were able to provide service to the users without the necessity of communicate with the Control Segment for 3-4 days. •Block II/IIA: Some of these SVs are currently operating. The first SVs launched were called Block II, but since 1990 there were improvements in them and the block was called Block IIA (Advanced) enabling the mutual communication between SVs. 9 Block II satellites were launched between 1986 and 1990; 19 Block IIA versions were launched between 1990 and 1997. Enhancements consisted in harden the electronics to radiation to prevent random memory upset events to improve SIS reliability and survivability [25]. These new SVs had the capacity to store 180 days of navigation message data to guarantee SIS3 availability. In order to provide SIS security, there were implemented the Selective Availability (S/A) and Anti-Spoof (AS), which will be described later (See 2.1.1.1). To maximize SIS integrity and protect user from tracking a faulty SV, the satellites are capable to automatically detect certain error conditions and switch to nonstandard PRN Code transmission or default navigation message data. 3SIS: Signal-in-Space 12 •Block IIR Operational Replacement Satellites: These SVs were made to replace the Block IIA satellites and the improvements were the ability to determine its orbit and generate navigation message data itself. Block IIR SVs are able to measure distances between SVs and transmit observations to other SVs or to the Control Segment. Thanks to these enhancements, SVs can operate up to half a year without control segment support with a non-degradation of the accuracy in ephemerides. 13 SVs have been launched with currently 12 of them active due to the launch failure of the first satellite on January of 1997. The last launch was on June of 2006. •Block IIR-M: The IIR-M capabilities include developmental military-use-only M-code on the L1 and L2 signals and a civil code on the L2 signal. There are eight satellites in the Block IIR-M series, which were built by Lockheed Martin. The first Block IIR-M satellite was launched on September 26, 2005. The final launch of a IIR-M was on August 17, 2009 with a total of 8 SVs currently working. •Block IIF, Follow-On Operation Satellites: The first launch of a Block IIF SV was on May the 27th of 2010 [26] and from August the 27th is correctly working [27]. These satellites are functionally equivalent to the IIR/IIR-M SVs and pave the way towards operational M-code after IOT&E in 2010. Block IIF satellites also add a new separate signal for civilian use, designed L5 (1,176.45 MHz.). The improvements also include an on-board reprogrammable processor, which will continue to deploy the modernization efforts of the IIR-M satellites, a 12-year design life, and a more robust military signal. Figure 2.4 [28] shows a representation of three different satellites, the first one corresponding to the Block I, in the middle one IIA satellite and on the right the latest Block IIF satellite launched. 13 Figure 2.4: Evolution of the GPS satellites [4] •Block III: The GPS Block III is an initiative of the US Air Force intended to meet the requirements for satellite-based navigation and timing capabilities over the next 30 years. Two industry teams led by Lockheed-Martin and Boeing are currently competing for the GPS Block III program. Finally, the US Air Force selected Lockheed-Martin [29] led team to build eight (up to 12) GPS IIIA satellites worth $1.4 billion with the first spacecraft to be placed into orbit in 2014. The US Air Force GPS Block III plans call for the procurement of eight GPS IIIB and 16 GPS IIIC satellites in later increments. The GPS IIIA satellites will deliver significant improvements over current GPS space vehicles, including a new international civil signal (L1C) and increased M-Code antijam power with full earth coverage for military users. GPS IIIB will enable a crosslinked command and control architecture, allowing these GPS III vehicles to be updated from a single ground station instead of waiting for each satellite to orbit in view of a ground antenna. GPS IIIC will include a high-powered spot beam to deliver greater M-Code power for increased resistance to hostile jamming [30, 31]. 14 the normal GPS receivers. Also, the aviation will also be benefited by these improvements, as this additional Safety-of-Life (SoL) civilian signal will make GPS an even more robust navigation service for many aviation applications. Figure 2.6 shows the evolution on the GPS signals from Bock I until Block III, with the frequencies where they are allocated and the relative power the signals have. Figure 2.6: GPS signals evolution [2] 2.1.1.2 Control Segment The Control Segment (CS) primarily consists of a Master Control Station (MCS) plus Monitor Stations (MS) and Ground Antennas (GA) at various locations around the world. The MCS is located at Air Force Base (AFB) in Colorado Springs, USA, while the monitor stations are located at Hawaii Kwajalein, Diego Garcia, Ascension and also at the Master Control Station, AFB. The mission of the MS is to receive all the transmitted signals by the satellites, save the received messages and transmit all the gathered information to the MCS. All monitor stations except Hawaii and Falcon AFB are also equipped with ground antennas (see Figure 2.7) in order to transmit the corrections to the satellites. The Control Segment includes a Prelaunch Compatibility Station (PCS) located at Cape Canaveral, USA, and a back-up MCS capability. The PCS primarily operates under control of the MCS to support prelaunch compatibility testing of GPS satellites via a cable interface. The PCS also includes an RF transmit/receive capability that can serve as a Control Segment ground antenna, if necessary [5]. 21 Figure 2.7: Control Segment elements [5] The MSC constitutes the general control centre of the GPS system. This centre receives the information gathered by the Monitor Stations, calculates the exact orbit of each satellite, evaluates the necessary information for the proper operation of the system, and then sends the corrections to be incorporated in the navigation message transmitted by each satellite. The MCS is also responsible for detecting any faults and provide corrective measures, as well as to ensure the normal maintenance of the satellites. The MCS operates 24 hours every day of the year. Corrections, as well as the updated almanac are transmitted to each satellite using the three ground antennas described above. The information provided also include messages concerning the operational status of the satellites. These ground antennas are also used to receive and transmit other control information. The information is transmitted in the S Band, at 1,783.74 MHz uplink and 2,227.5 MHz downlink [36]. Figure 2.8 graphically shows how the CS works. The CS has been planned to establish up to three satellite contacts per day, which guarantees a sufficient accuracy to the system. However, in general, with only two daily contacts the degree of accuracy required is achieved. Each satellite is observed by the CS for more than 95% of the time. The only critical case in which the integrity of the system can be degraded is if, during the time when the satellite is not observed, there is a failure in the operation. 22 Figure 2.8: Control Segment communication process [5] 2.1.1.3 User Segment The User Segment is formed by the GPS receivers. Its main function is to receive GPS satellite signals, to determine pseudoranges, and solve the navigation equations in order to get their coordinates and provide a very precise time. The basic elements of a generic GPS receiver is an antenna with pre-amplification, a radio frequency section, a microprocessor, an intermediate-precision oscillator, a feeding source, a some memory for data storage, and an interface with the user. The calculated position will be referred to the phase centre of the antenna. Figure 2.9 shows the generic GPS receiver tracking system. 23 Figure 2.9: GPS Receiver tracking system [5] Navigation Message The navigation message consists of 25 frames of data, each frame consisting of 1,500 bits. Each frame is divided into 5 subframes of 300 bits each (see Figure 1-5). At the 50 Hz transmission rate, it takes 6 seconds to receive a subframe, 30 seconds to receive one data frame, and 12.5 minutes to receive all 25 frames. Subframes 1, 2, and 3 have the same data format for all 25 frames. This allows the receiver to obtain critical satellite-specific data within 30 seconds. Each subframe begins always with the telemetry word (TLM), which is necessary for synchronization. Then appears the transference word (HOW), whose mission is to enable rapid commutation from the C/A code to the P code [5]. The content of each of the subframes is the following (see Figure 2.10): •Subframe 1: Contains information about the parameters to be applied to satellite clock status for its correction. These values are the coefficients that allow to convert time on board to GPS time. It also has data about satellite health condition and information about message ambiguity. •Subframes 2 and 3: These subframes contain satellite ephemeris. •Subsection 4: This subframe contains the ionospheric model parameters (to correct 24 the ionospheric refraction), UTC (Coordinated Universal Time) information, part of the almanac and indications of whether it is enabled on each satellite the AntiSpoofing, A / S (which transform the P code in the encrypted code Y). •Subsection 5: contains the almanac data and the status of the constellation. This allows a quick identification of which satellites are transmitting the received signals. 25 frames are needed to complete the almanac. Figure 2.10: Navigation Message frame [5] 2.1.2 Position calculation by the receiver The principle of working of the GPS receiver is based, like the old electronic navigation systems, on the mathematical principle of trilateration. Therefore, to calculate the position of a point will require the GPS receiver to accurately determine the distance that separates it from the satellites. Using the mathematical principle of trilateration one can know the position where is located, and even track and locate the source of a radio wave transmission. The GPS system uses the same principle but instead of using circles and straight lines, it creates virtual or imaginary spheres to achieve the same objective. From the moment the GPS receiver detects a radio frequency signal transmitted by a satellite from its orbit, it creates a virtual or imaginary sphere surrounding the satellite. 25 The satellite itself will act as the centre of the sphere whose surface is extended to the point or place where the receiver antenna is located, so the radius of the sphere is equal to the distance between the satellite and the GPS receiver. From that moment, the GPS receiver will measure the distances that separate at least two more satellites. It will have to calculate the time it takes for each signal to travel from the satellites to the point where it is located and to solve the corresponding mathematical equations. Radiofrequency signals are composed by electromagnetic waves moving through space concentrically from the transmitting antenna, in a similar way as do the waves generated on the surface of the water when a stone is thrown. Because of this property, radio signals can be picked up from any point around a transmitting antenna. Radio waves travel at the speed of light (300,000 km/s) measured in a vacuum, making it possible to calculate the distance between a transmitter and a receiver if known the time it takes the signal to travel from one point to another. To measure the time at which the satellite broadcasts the signal and the GPS receiver receives it, is needed that the clocks of the satellite and the receiver are perfectly synchronized. The satellite uses a cesium atomic clock, extremely accurate, but the GPS receiver has a quartz normal, not so accurate. To synchronize the GPS receiver clock, satellite broadcasts periodically a digital signal or pattern of control with the radiofrequency signal. This control signal reaches the GPS receiver always more delayed than normal radio frequency signal. The delay between both signals is equal to the time it takes the RF signal to travel from the satellite to the GPS receiver. The distance between each satellite and the GPS receiver is calculated by different mathematical operations, this distance is called pseudorange. To make this calculation the GPS receiver multiplies the delay time control signal for the value of the speed of light. If the signal has travelled in a straight line, without any interference by the way, the mathematical result is the exact distance that separates the satellite receiver. It is obvious that radio waves do not travel through a vacuum because of the gaseous mass that makes up the atmosphere, so their speed will not be exactly equal to that of light, but a little slower. There are also other factors may affect in the inaccuracy of the signal, such as local weather conditions, the angle between the satellite and the GPS receiver, relativistic effects, etc. To correct the effects of all these variables, the receiver uses complex mathematical models stored in memory. The results of the calculations are also complemented by the information about the corrections received by the satellites in order to get a more accurate position To obtain the exact position, the GPS receiver must locate at least 3 satellites that serve as benchmarks. Actually that is not a problem because usually there is always 8 satellites within the "sight" of any GPS receiver. To determine the exact location of 26 the orbit where the satellite should be at any given time, the receiver has an electronic almanac in its memory that contains the data. If the receiver does not have the almanac and position information stored, the receiver enters a "search the sky" operation that systematically searches the PRN codes until lock is obtained on one of the satellites in view. Once one satellite is successfully tracked, the receiver can demodulate the navigation message data stream and acquire the current almanac as well as the health status of all the other satellites in the constellation. To extract the satellite signal the receiver uses code correlation techniques. An internal replica of the incoming signal is generated and aligned with the received satellite signal. The receiver shifts the replica code to match the incoming code from the satellite. When the codes match, the satellite signal is compressed back into the original carrier frequency band. The process of the signal acquisition is shown in Figure 2.11. Figure 2.11: Signal Acquisition proces [5] The principle of operation of GPS receivers could be summarised as follows (see Fig27 ure 2.12) [5]: 1. When the receiver detects the first satellite generates a virtual or imaginary sphere whose centre is the satellite itself. The radius of the sphere, i.e. the distance from its centre to the surface, will be the same between the satellite and the GPS receiver, which assumes then that is located at any point on the surface of the sphere. Obviously, the receiver is not able to precise in which of the countless points of the sphere is located. 2. Calculating the distance to a second satellite, the receiver generates another virtual sphere. The area previously created overlays that other, as a result, an imaginary ring through spheres intersection appear. 3. The receiver calculates the distance to a third satellite and generates a third virtual sphere. This area is cut by the ring previously created at a point in space and on the surface of the Earth. The receiver discriminates as a location the point situated in the space (in case of GPS that are not made for aviation) using mathematical resources and takes as a correct position the point on Earth. 4. Once the receiver runs the three above steps on your screen can show values for the coordinates of your position, i.e. latitude and longitude. 5. In order to detect the height in which the GPS receiver is located, or in case of the GPS receiver is not over the Earth surface, another satellite is needed to discriminate one of the two points of the three spheres intersection. Many receivers will track more than four satellites, but less than all-in-view, as a compromise between complexity, accuracy, and robustness. I has to be noticed that, an acquisition with 3 satellites will be also very inaccurate due to the fact that the receiver clock will not be synchronised accurately with the satellites clock. 28 Figure 2.12: GPS Receiver Theory of Operation [5] 29 2.1.3 Problems in GPS As mentioned previously, GPS is a positioning system with several problems due to the amount of errors that affect it. The most noticeable problem for the user is the poor accuracy in some conditions. There are also some limitations of the system as a result of the kind of signals that are used by the system. These signals can only be detected in outdoor conditions, and depending on the environment (tress, buildings, atmospheric conditions, etc.) the power signal can be too poor to be detected by the receiver. Due to this, the more adverse conditions the receiver has, the worse accuracy the positioning will be. Another typical problem of the receivers is the inhability to synchronise the replica with the signal received. The clocks used in the receivers are not atomic, as a consequence, errors in synchronise affects the calculation of the pseudorange, which entails inaccuracy in positioning. A wider examination about the errors in GPS can be found in Chapter 3. Regarding the techniques to improve the GPS accuracy, in Chapter 4 there is a deep description of them. 2.2 Global Navigation Satellite System (GLONASS) GLONASS [37] (Russian: GLObalnaya NAvigatsionnaya Sputnikovaya Sistema; English: GLObal NAvigation Satellite System) is a Global Navigation Satellite System operated by the Russian Federation. This system was released only 4 years after the first GPS satellite was launched by the U.S. Firstly, the system was thought to be fully operational by the year 1991, but finally the whole constellation (24 satellites) was reached in December 1995 and begin to be fully operational the 18th January of 1996 [38]. The principles of working is very similar to the GPS. User equipment performs passive measurements of pseudoranges and pseudorange rate of four (three in case of not need the altitude) GLONASS satellites as well as receives and processes navigation messages contained within navigation signals of the satellites. The navigation message describes position of the satellites both in space and in time. Combined processing of the measurements and the navigation messages of the four (three) GLONASS satellites allows user to determine three (two) position coordinates, three (two) velocity vector constituents, and to refer user time scale to the National Reference of Coordinated Universal Time UTC(SU). The data ensuring of sessions scheduling for navigational determinations, selection of working "constellation" of SVs and detection of radiosignals transmitted by them, are transmitted as a part of the navigation message. The problem of the short life of the satellites, add to the low investment the Russian Federation gave to the project because of its difficult economic situation during the 90s, 30 Figure 2.17: Constellation Current Status Ground-Based Control Facilities As in the GPS Control Segment, this GLONASS segment is in charge to keep the system updated in order to guarantee a good service. The Ground Control Segment (GCS) is composed of the System Control centre (SCC), located in Moscow Territory, and several Telemetry, Tracking, and Control stations (TT&C) distributed throughout the Russian territory [6] (see Figure 2.18). In the GCC there is also the Central Synchronizer (CS), the Navigation Signal Phase Control System (PCS) and a Navigation Field Control Equipment (NFCE), which is also located in Komsomolsk-on-Amur [38]. The main tasks of the GCS are: •Monitoring of the orbital constellation’s normal operation •Continuous adjustment of satellite orbit parameters •Generation and uploading of time-tagged programmes, control commands, and especial information 37 Figure 2.18: Ground-Based Control Facilities [6] All the processes done by the GCS must be synchronized to be successful, the part of the segment in charge of that is the Central Synchronizer, a stationary ultra-stable hydrogen frequency standard, which is used as the basis for GLONASS time scale generation. The monitoring is based in the principle of comparison between the information received by the satellites and the precise coordinates that the different stations have registered. Once the stations have evaluated the differences between the estimated coordinates and the highaccuracy geodetics positions they have registered, a signal is transmitted to the satellites in order to correct the navigation message. These data are loaded at satellite every day [38] (on-board timing scale synchronism data are loaded at every rotation, which is two times per 24 hours). Regarding the navigation message, this is transmitted as a pattern of digital data that are coded by Hamming code and transformed into relative code. Structurally the data pattern is generated as continuously repeating super frames. A Superframe consists of the frames, and a frame consists of the strings. The boundaries of strings, frames and Superframe of navigation messages from different GLONASS satellites are synchronized within 2 milliseconds. The superframe has duration 2.5 minutes and consists of 5 frames. Each frame has duration 30 seconds and consists of 15 strings. Each string has duration 2 seconds. Within each frame a total content of non-immediate data (almanac for 24 GLONASS system satellites) are transmitted [37]. Concerning the content of the navigation message, it is not too much different as the GPS navigation message. Both messages have immediate and non-immediate data depending on the subframe. 38 The immediate data relate to the GLONASS satellite which broadcasts given RF navigation signal and include: •Enumeration of the satellite time marks •Difference between onboard time scale of the satellite and GLONASS time •Relative difference between carrier frequency of the satellite and its nominal value •Ephemeris parameters and the other parameters The non-immediate data contain almanac of the system including: •Data on status of all satellites within space segment (status almanac) •Coarse corrections to onboard time scale of each satellite relative to GLONASS time (phase almanac) •Orbital parameters of all satellites within space segment (orbit almanac) •Correction to GLONASS time relative to UTC(SU) (formerly Soviet Union and now Russia) and the other parameters User terminals The User Segment consists of equipment (such as a NovAtel OEMV family receiver (see Figure 2.19) that tracks and receives the satellite signals [40]. This equipment must be capable of simultaneously processing the signals from a minimum of four satellites to obtain accurate position, velocity and timing measurements. Like GPS, GLONASS is a dual military/civilian-use system. The system’s potential civil applications are many and mirror those of GPS. Figure 2.19: NovAtel OEMV Family receivers [7] 39 2.2.2 GLONASS & GPS Comparison Table 2.4 shows the main characteristics of each of the systems. Table 2.4: GLONASS and GPS technical specifications Parameter Detail GLONASS GPS Satellites Number of satellites (nominal) 21 + 3 spares 21 + 3 spares Number of orbital planes 3 6 Orbital plane inclination (degrees) 64.8 55 Orbital radius (km) 25,510 26,560 Signals Fundamental click frequency (MHz) 5.0 10.23 Signal separation technique FDMA CDMA Carrier frequencies (MHZ) - L1 1,598.06251,609.3125 1575.42 Carrier frequencies (MHZ) - L2 1,242.93751,251.6875 1227.60 Code clock rate (MHz) - C/A 0.511 1.023 Code clock rate (MHz) - P 5.11 10.23 Code length (chips) - C/A 511 1,023 Code length (chips) - P 5.11 ·1066.187104 ·1012 C/A-code Superframe duration (minutes) 2.5 12.5 Navigation Superframe capacity (bits) 7,500 37,500 Message Superframe reserve capacity (sec.) ∼620 ∼2,750 Word duration (sec.) 2.0 0.6 Word capacity (bits) 100 30 Number of words within a frame 15 50 Thechnique for specifying satellite ephemeris Geocentric Cartesian coordinates and their derives Kleperian orbital elements and perturbation factors Time reference UTC(SU) UTC(USNO) Position reference (geodetic datum) PZ-90 WGS84 40 2.3 Galileo System Galileo is the European GNSS currently being built by the European Union (EU) and European Space Agency (ESA) providing a highly accurate, guaranteed and global positioning service under civilian control. By now, the system is compatible with GPS and GLONASS. Furthermore, there are intentions to extend the compatibility to other navigation systems such as COMPASS, QZSS or IRNSS [41]. Galileo has been designed to offer two types of service, the navigation and the Search and Rescue (SAR) service. Regarding the navigation service, there are four different services on different levels of performance. Open access service: Designed to offer navigation service for the most of users. This service is free for the user and the positioning and timing is not as accurate as other navigation services offered by Galileo. This service can be compared with the Standard Positioning Service offered by GPS. Commercial service: The signal is encrypted due to the fact this service is not free. The highlights of this category is the high accuracy and that the service is guaranteed. Safety of Life service: Is composed by the open service with integrity added by providing timely warnings to the users when it cannot guarantee to meet certain margins of accuracy. Public regulated service: Like the commercial service, is encrypted to control the access to specific users requiring a high continuity of service. The SAR service is characterised by the near real-time access to the information. The positioning is precise and return link feasible. Table 2.6 shows the performance requirements for the Galileo Open and Safety-of-Life Services [2]. 41 Table 2.6: Performance Requirements for the Galileo Open and Safety-of-Life Services Galileo Services Open service Safety-of-Life service Coverage Global Global Accuracy (95%) H: 15 m H: 4 m V: 35 m V: 8 m (single frequency) (dual frequency) H: 4 m V: 8 m (dual frequency) Availability 99.8% 99.5% Alert limit N/A HAL: 40 m VAL: 20 m Time to Alert N/A 6s Integrity Risk N/A 2×10−7/150 s Continuity Risk N/A 8×10−6/15 s Certification and Service Guarantees No Yes 2.3.1 Galileo Architecture Similar to any other GNSS, Galileo is composed by three segments, the space, ground and user segment as shows Figure 2.20. Figure 2.20: GALILEO Architecture [8] 2.3.1.1 Space segment The satellites, in charge of broadcast the signals to the elements of the ground segment and to the end users, compose the Space Segment. The system is designed to work with 42 30 satellites (27 operation and 3 spares) distributed in three circular MEO planes at a nominal average orbit semimajor axis of 29,601.297 km, with an inclination of 56ºwith reference to the equatorial plane in a Walker 27/3/1 configuration (see Figure 2.21) [42]. Figure 2.21: Galileo Constellation [9] The main design drivers for the constellation were: •Provision of high quality services on a global base •Robustness of constellation design in the context of service availability •Quality of service and high service reliability in case of satellite failures. A further design driver was the visibility of Galileo satellites and the related constellation performance for commercial and mass market services [9]. Two satellite prototypes were launched in 2005 and 2008 called GIOVE A and GIOVE B [43, 44] respectively. The estimated life time of each was around 2-3 years, but by 2011 they still working correctly, which means a great achievement for the ESA. Both satellites are part of the Galileo test mission in order to provide experimental results based on real data to be used for risk mitigation for the In-Orbit Verification (IOV) satellites that will follow on from the testbeds. This second phase of Galileo (IOV) is scheduled for the beginning of 2011 by the launch of two new satellites [45]. As mentioned before, the satellites must broadcast the signals as well as receive the corrections transmitted by the ground segment. Each Galileo satellite will broadcast 10 different navigation signals making it possible for Galileo to offer the Open (OS), Safety-of-Life (SOL), Commercial (CS) and Public Regulated services (PRS). The open services are realized by using the 43 signals at L1, E5a and E5b, whether data or pilot (no data signals). Several combinations are also possible, such as a dual frequency service based on using L1 and E5a (for best ionospheric error cancellation) or single frequency services (at L1, E5a, E5b or E5a and E5b together) in which case the ionospheric error is removed using a model, and even triple frequency services using all the signal together (L1, E5a and E5b), which can be exploited for very precise, centimetric applications. The signals are centred in three different frequency bands, L1 (1,575.42 MHz), E6 (1,278.75 MHz) and E5 (1,191.795 MHz), providing a wide bandwidth for the transmission of the Galileo signals. These bands are included in the allocated spectrum for Radio Navigation Satellite Services (RNSS) and/or Aeronautical Radio Navigation Services (ARNS) (see Figure 2.22). The E6 signals are designed for PRS and CS services. The navigation data includes the ephemeris, time and clock correction parameters, service parameters and the almanac, very similar to the other GNSS described above [46]. The signals specifications can be checked in [41]. Figure 2.22: Galileo Frequency Plan 2.3.1.2 Ground Segment The ground segment of Galileo is in charge of: •Perform the measurement and monitoring of the satellites •Time synchronization •Uplink Navigation Messages to the satellites •Determine and disseminate the integrity status of the system •Continuously acquisition of relevant system information 44 •Enable permanent access to the satellites at any time The elements needed to achieve these objectives are: •About 30 GALILEO Sensor Stations (GSS) providing the instantaneous position of each satellite. Each GSS will be equipped with three parallel reception channels: –One channel for the determination of orbit data and clock synchronization –A second for the determination of integrity –A third redundant channel •Nine Mission Uplink Stations (ULS) equipped with a total of 31 (up to four per site) C-band antennas. These stations will uplink the navigation and integrity data from the control centre to the satellites. •Five Telemetry, Telecommand and Control (TT&C) stations equipped with S-band antennas to provide a secure exchange of data between the control centres and the satellites. •Galileo Control Centres (GCC), is the core of the GS, where all the information gathered by the GSS is processed to provide the Navigation Message and the corrections to the ULS. •Network to connect all the ground segment elements by radio or wired communication links [9]. 2.3.1.3 User Segment Due to the fact that the system is not operable for the end user, there are not commercial receivers available by the moment. Even though, some Galileo receivers have been implemented to perform tests with the signals broadcasted by the GIOVE satellites. When Galileo will be fully operational, it is expected that receivers will combine Galileo, GPS and GLONASS signals in order to get a more accurate position. 2.4 Other systems In this section are described three positioning systems that are currently being developed in different areas of the world. These three projects are less ambitious than the three main systems described in Sections 2.1, 2.2 and 2.3, as they are developed to provide regional coverage. Nevertheless, there is the possibility that, in a long-term future, some of them could provide global coverage. 45 2.4.1 COMPASS COMPASS [11] (also known as Beidou-2) is the name of the Chinese Global Navigation Satellite System currently under development. The name Beidou-2 becomes from the first project implemented by the Chinese government called Beidou1 whose objective is to be a Wide Area Positioning System. COMPASS is not an extension of this first system, but a new GNSS based in the same principles as GPS, GLONASS or GALILEO. Beidou-1, comprises three payloads on geosynchronous satellites. The system coverage of Beidou-1 is 70°−145°E (longitudes), 5°-55°N (latitudes). Namely, east to the east of Japan, west to Kabul, Afghanistan, South to the Nansha Islands and north to the Lake Baikal in Russia, covering the whole territory of China, the West Pacific Ocean, Japan, the Philippines, India, Mongolia, Southeast Asia and other neighbouring countries and regions. Despite all the regions covered, in terms of access and users, China is the most favoured. During the 2000 and 2003, the three satellites were launched (called Beidou1-A, Beidou1-B and Beidou1-C) (see Table 2.7) [10]. Table 2.7: Satellites of Beidou-1 Name Position Perigee Apogee Inclination Beidou1-A 140ºE35,722 km 35,803 km 0.10º Beidou1-B 80ºE35,753 km 35,821 km 0.00º Beidou1-C 110.5ºE35,760 km 35,836 km 0.30º Besides positioning, Beidou-1 systems can also be employed in the transmission of short messages, which clearly differs from the other systems mentioned above. The satellites are used to broadcast the signal transmitted by the ground or user segments. The Ground Segment monitors the constellation status, as well as receive the signals from the users broadcasted by the satellites to check whether the users are authorised to use the system or not. In case of acknowledge the authorisation, the answer is transmitted to the user terminal. The frequency of messages received varies depending on the class of authorised user. Due to the principle of working of the system, the number of users is limited. The systems architecture is shown in Figure 2.23. 46 Figure 3.1: Sunspot number observed and predicted from 1995 to now [12] Instrumental delays [Kj i]: Possible sources of these delays are antennas, cables, as well as different filters used in receivers and satellites. Multipath [Mj P1,i]: The interference by multipath is generated when a signal arrives, by different ways, at the antenna. Its principal cause is the antenna closeness to the reflecting structures, and it is important when the signal comes from the satellite with low elevation. Noise [εj P1,i]: In this term, the measurement noise of pseudorange is included and all non previously modelled effects. The contribution of these errors to the pseudorange is exemplified in Figure 3.2, Figure 3.2: Errors magnitude in pseudorange [1] The main part of the signals that the receivers need in order to get the position is the geometric distance [ρj i]. This is the parameter that contains the exact distance from the receiver to the satellite, so the next step is to correct all the errors described above. 53 Apparent distance between satellite and receiver can also be measured from the carrier signal phase, obtaining in this case: Lj i=ρj i+c�dti−dtj�+relj i+Tj i−α1Ij i+Bj i+wL+mj L,i +εj L,i Where: Wind-up [wL]: Is the term due to signal polarization. Ambiguity phase [Bj i]: Is an ambiguity phase term owing to the signal acquisition, an ambiguity of an integer number of wavelengths (Nλ)appears to which one has to add instrumental constants ki,kjfrom satellite and receiver, respectively (Bj i= ki+kj+λNj i). As mentioned in page 51, the errors can be classified as common mode errors or noncommon mode errors. The first group is referred to the errors that follows the same pattern (or similar) for the receivers located in the same region. With this, if a common mode error can be corrected by a receiver (or station), the rest of the receivers could easily correct it with the information given by the “corrector”. This is one of the principles of working of some GPS Augmentation Systems such as GBAS2or SBAS3. The errors are classified as [52]: •Common mode errors: –Dispersive ionospheric error [αIj i] –Non-dispersive atmospheric errors [Tj i] –Satellite clock bias [dtj] –Ephemeris error [E(t)] –Relativistic errors [relj i] •Non-common mode errors: –Offset of the receiver clock [dti] –Multipath error [Mj P1,i] –Instrumental delays error [Kj i] –Wind-up [wL] 2GBAS: Ground-Based Augmentation System (see page 58) 3SBAS: Satellite-Based Augmentation System (see page 65) 54 –Ambiguity phase [Bj i] –Random measurement noise [εj P1,i] Some of these errors can be corrected by the GPS receiver only by the combinations of the observables P1, P2, L1 and L2 [1]. The problem of these combinations (modelling) is that the receiver has to be able to detect both frequencies, which is only able for military or authorised users. Nevertheless, some civil-user receivers companies have implemented several tricky strategies to achieve an error correction acceptable taking into account the limitations of the receivers. The observables are: Codes (Pseudoranges) P1j i=ρj i+c�dti−dtj�+relj i+Tj i+α1Ij i+K1j i+Mj P1,i +εj P1,i P2j i=ρj i+c�dti−dtj�+relj i+Tj i+α1Ij i+K2j i+Mj P2,i +εj P2,i Phases (Carries phases) L1j i=ρj i+c�dti−dtj�+relj i+Tj i−α1Ij i+B1j i+wL1+mj L1,i +εj L1,i L2j i=ρj i+c�dti−dtj�+relj i+Tj i−α2Ij i+B2j i+wL2+mj L2,i +εj L2,i There are four important combinations that has to be explained depending on which of the error parameter is going to be corrected, or depending on the information that is going to be used to accurate de distance between the satellite and the receiver. •Ionospheric free combination: As the ionospheric effect depend on the square of the frequency (αi=40.3 /f2 i),this term can be easily cancelled by these combinations: PC =f2 1P1−f2 2P2 f2 1−f2 2 ;LC =f2 1L1−f2 2L2 f2 1−f2 2 The result of the combinations is a major accuracy in the determination of the distance due to the cancellation of the ionospheric error term. Solving the equations, the result obtained is: PC =ρ+c�dti−dtj�+rel +T+KC +MPC +εPC LC =ρ+c�dti−dtj�+rel +T+BC +wLC +mLC +εLC 55 •Narrow lane (PW)and wide-lane (LW)combinations: LW combination gives an observable with a wavelength λW = 86.2 cm, four times bigger than L1or L2, which makes it very useful for (cycle-slips) detections. To do so, Melbourne-Wübbena combination is used (W=LW−PW). PW =f1P1+f2P2 f1+f2 ;LW =f1L1−f2L2 f1−f2 Solving the equations one can see that the result is that the αWterm is αW=40.3 /f1f2, making easy the detection of cycle-slips. As a result, the distance accuracy increases. PW =ρ+c�dti−dtj�+rel +T+αWI+KW +MPW +εPW LW =ρ+c�dti−dtj�+rel +T−αWI+BW +mLW +εLW •Ionospheric combination: It cancels the geometric part of the measurement, leaving the ionospheric effect and the instrumental constants (besides multipath and observational noise). It is also used to detect cycle-slips in the phase. PI =P2−P1;LI =L1−L2 In these combinations the distance is cancelled, so the remaining terms enable an easily detection of the cycle-slips in the phase. PI =αII+KI +MPI +εPI LI =αII+BI +wLI +mLI +εLI Apart from these combinations implemented in the GPS receivers, there are several ways to improve the accuracy in positioning. Next chapter focus on the main techniques to improve the accuracy using external information. 56 Chapter 4 Techniques to improve GPS accuracy The GPS system is the most widely used positioning system worldwide, largely because its performance has been uninterrupted for decades, unlike the GLONASS system, giving the user confidence that the system will be operational when needed. The fact that the service offered to civilian users is free, added to the deactivation of the S/A in 2000, has made the marketing of GPS receivers has been a success. Nowadays, practically the majority of the cars are equipped with a GPS (either integrated or not), many mobile phones incorporate GPS in its functions, there are cars or people locators based on GPS, in aviation and marine navigation instruments are equipped with GPS receivers, etc. These are a few examples of how important is GPS in our technological society. However, despite the fact that GPS currently uses are much broader than a decade ago, the operating principle of many GPS receivers are still based on the same principles as 20 years ago. New generations of GPS satellites are introducing new signals to provide an improvement in the accuracy of civilian users (see Section 2.1.1.1), but many receivers still do not using them. The fact of need an improvement in accuracy or just to get a quick positioning at any given time, has promoted the development of techniques called GPS Augmentation systems. The main purpose of these methods are to enhance the performance of the current GNSS with additional information to: •Improve integrity via real-time monitoring •Improve accuracy via differential corrections •Improve availability and continuity These techniques are based on the use of external information from the GPS signal to help the position calculation. Information can be received in different ways, Radio Frequency (UHF, VHF, etc.); satellite signals (SBAS); data packages via UMTS, etc. As a result, depending on the technique used, the GPS receivers must have a new receiver module to 57 detect such signals, in order to use them in the positioning equations or error correction algorisms. 4.1 Differential GPS (DGPS) DGPS (Differential GPS) [13] is a particular case of GBAS (Ground Based Augmentation System) based on the use of the accurate location of a fixed point (receiver) to correct the errors of the GPS receivers. GBAS commonly consist of one or several ground stations, with a precise position known, which receive GPS data from the GNSS. Once the signal has been corrected, the information is transmitted by radio directly to the GPS users. GBAS System differs from SBAS mainly in that it is not designed to provide service over large geographic regions, due to the fact that the broadcasted signal from the stations is useful in less than 150 km from the base. That is why its main use is given in air traffic control to support precision approach phases, where base stations are located near the airport area. Although that, the use is becoming wider in other fields. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by S/A. Even with S/A eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS can yield measurements accurate from some centimetres to a couple of meters in stationary situations (in moving applications the accuracy is about 1-2 meters). Differential GPS involves the co-operation of two receivers, one that’s stationary and another that’s roving around making position measurements. The accuracy will be closely related with the distance between GPS receivers and the Base Station. As each GPS receivers use timing signals from at least four satellites to establish a position then each of those timing signals is going to have some error or delay depending on what sort of problems have occurred it on its journey down to Earth as mentioned in section 3.1. Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors (Dispersive ionospheric error, satellite clock drift, tropospheric error, etc.) [53]. However if two receivers are fairly close to each other, say within a few hundred kilometres, the signals that reach both of them will have travelled through virtually the same slice of atmosphere, and so will have virtually the same errors. This means that one of these receivers can measure the timing errors and then provide correction information to the other receivers that are roving around. This allows virtually all errors to be eliminated from the system. The error correction will be better or worse depending on the distance between the rover and the reference. A survey made in 1993 stated that estimated error growth of 0.67 m per 100 km from the broadcast site but more recent measurements of 58 accuracy across the Atlantic, in Portugal suggest a degradation of just 0.22 m per 100 km [54]. The reference station operates by receiving the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it uses its known position to calculate timing, rather than using timing signals to calculate position. Essentially determining what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor. The receiver then transmits this error information to the roving receiver so it can use it to correct its measurements. Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors. Then it encodes this information into a standard format and transmits it to the roving receivers. The roving receivers can then apply the corrections for particular satellites they are using (see Figure 4.1 [55]). Figure 4.1: DGPS Schematic Diagram [13] There are also different kinds of DGPS, for use when users do not need precise positioning immediately. This is termed Post Processing DGPS, and is used when the roving receiver just needs to record all of its measured positions and the exact time it made each measurement. Then later, this data can be merged with corrections recorded at a reference receiver for a final clean-up of the data, meaning you don’t need the radio link required in real-time systems. Another form of DGPS, called Inverted DGPS, which is used to save money when operating a large fleet of users. With an inverted DGPS system the users would be equipped with standard GPS receivers and a transmitter, and would transmit 59 their standard GPS positions back to the tracking station (the main office). Then at the tracking station the corrections would be applied to the received positions. Maybe one of the most important types of DGPS is the one based on Real-Time Kinematic (RTK DGPS). In standard DGPS technology, only corrections to pseudoranges based on the Navigation Message are being transmitted, which brings rover positional common mode errors down to values about 1m. The remaining DGPS error source is mainly multipath, which can be reduced by the use of special multipath mitigation methods. Highprecision navigation/surveying applications require RTK (Real-Time Kinematic) technology, which is based on the use of carrier phase. Carrier phase measurements are extremely precise (down to the fractions of millimetre), but they contain an unknown integer initialization constant, the so-called “phase ambiguity”. Therefore RTK positioning has to resolve integer ambiguities to achieve the high level of precision. In practice, RTK systems use a single base station receiver and a number of mobile units. The base station re-broadcasts the phase of the carrier that it measured, and the mobile units compare their own phase measurements with the ones received from the base station. There are several ways to transmit a correction signal from base station to mobile station. The most popular way to achieve real-time, low-cost signal transmission is to use a radio modem, typically in the UHF band (see Figure 4.2 [55]). Figure 4.2: RTK Schematic diagram [13] The base station of the RTK technology is normally not further than 20 km from the receiver in order to guarantee a centimetre accuracy to the user. These receivers are also the only system that can achieve complete repeatability, consequently, the cost of that technology is really expensive compared to other positioning systems. In addition, it has 60 to be noticed that, nowadays, standard DGPS needs an investment of around 30.000 €to install a Reference Base [53], which is also an expensive method. The problem of these expensive technologies is what drove a group of researchers from University of Blaise Pascal Clermont-Ferrand II (France) to develop a method called LCD-GPS (Low Cost Differential GPS and also Local Cooperative Differential GPS). This solution consists of a set of low cost standard civil GPS communicating receivers, part of them used in fixed manner as base stations and the other part is used as mobiles. All base stations have their position coordinates known with a good accuracy. They analyse continuously the instantaneous GPS errors and cooperate together to deduce a global error correction. This correction is sent wirelessly and has to be applied to mobile node positions (or fixed nodes which are not part of base stations) in order to improve their position accuracy [53]. The use of cheap prototypes to build the receivers and the base stations is what has allowed to reduce the cost of this technology. Nowadays, there are several companies that offer DGPS service all around the world. The National Geodetic Survey (NGS), an office of NOAA (National Oceanic and Atmospheric Administration) [56], coordinates a network of Continuously Operating Reference Stations (CORS). In Europe, there are some countries that have supported this technology, mostly the counties with large part of coast in order to improve the safety in the sea. Northern countries, as well as U.K., France and Spain, are the countries with more maritime stations (see Figure 4.3), the majority of them, following the recommendations of the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) [57]. Nowadays, apart from the stations situated on the coast, there are a great number of them also in land, the majority of them implemented by private companies. Figure 4.3 shows the location of the stations around the world and in Europe (with the coast and land stations). Australia, Russia and some Asian countries also count with a DGPS infrastructure [14, 58]. 61 Figure 4.3: DGPS Stations Coverage In Poland there are also DGPS coverage. Established since 1995 and following the IALA R-NAV Recommendations, DGPS-PL was implemented in the northern coast of Poland and has been modernised in 2007/2008 in order to provide Safety of Navigation in the Baltic Sea Area. This system is formed by two DGPS Stations called Dziwnów and Rozewie (see Table 4.1 and Figure 4.4 and), although 2 additional Integrity Monitor stations are planned. 62 •LPV, a WAAS operational service level with a HAL3equal to 40 meters and a VAL4equal to 50 meters [62]. LPV is similar to LNAV/VNAV except it is much more precise (40m lateral limit), enables descent to 200-250 feet above the runway, and can only be flown with a WAAS receiver. LPV approaches are operationally equivalent to the legacy instrument landing systems (ILS) but are more economical because no navigation infrastructure has to be installed at the runway. •LP (Localizer Performance) is a NPA procedure that uses the high precision of LPV for lateral guidance and barometric altimeter for vertical guidance. These approaches are needed at runways where due to obstacles or other infrastructure limitations, a vertically guided approach (LPV or LNAV/VNAV) can not be published. LP approaches can only be flown by aircraft equipped with WAAS receivers. The minimum descent altitude for the LP approach is expected to be approximately 300 feet above the runway. Figure 4.9 shows the area service and the availability of LP and LPV approaches. Note that in CONUS, the LP service is available the 100% of the time in the 100% of the service area (see Figure 4.9a), whereas the availability of the LPV is lower (see Figure 4.9b). There is another service derived from the LPV called LPV200, which specifies lower values of HAL and VAL. Consequently, the service volume size with a large availability is smaller that the other cases [16]. 3The Horizontal Alert Limit (HAL) is the radius of a circle in the horizontal plane (the local plane tangent to the WGS-84 ellipsoid), with its centre being at the true position, which describes the region that is required to contain the indicated horizontal position with a probability of 1·10−7per flight hour, for a particular navigation mode, assuming the probability of a GPS satellite integrity failure being included in the position solution is less than or equal to 10−4per hour. 4The Vertical Alert Limit is half the length of a segment on the vertical axis (perpendicular to the horizontal plane of WGS-84 ellipsoid), with its centre being at the true position, which describes the region that is required to contain the indicated vertical position with a probability of 1·10−7per flight hour, for a particular navigation mode, assuming the probability of a GPS satellite integrity failure being included in the position solution is less than or equal to 10−4per hour. 69 (a) WAAS LP Coverage Contours (b) WAAS LPV Coverage Contours Figura 4.9: WAAS Coverage [16] WAAS is composed by three segments: •Ground Segment •Space Segment •User Segment 4.2.1.1 Ground Segment The principal functions of the ground segment are to monitor the status of the GPS SVs, to process the data in order to correct the errors of the signals and to upload the data to the WAAS GEOs. The Ground Segment is composed by (see Figure 4.10): •Wide-Area Reference Stations •Wide-Area Master(s) Station(s) •Wide-Area Ground Uplink Station The Reference Stations are located at precisely surveyed locations in the U.S. territory and are in charge of monitor the status of the SVs. Each has three dual frequency GPS receivers that can be used to crosscheck the measurements. By taking measurements from two frequencies, the propagation delay caused by the signal passing through the ionosphere can be separated from the other error sources. The information is then transmitted to the Master Station, which performs the necessary corrections that has to be transmitted to the GEOs. WAAS sends corrections for the ionospheric delay as well as for the GPS satellites’ 70 clock and orbital errors. Each correction is sent to the user at least every five minutes. Because the reference stations know their location to within centimetres, they can determine what errors may be present on the ranging signals from the satellites. These errors are isolated to their individual components for efficient broadcast. One of the best enhancements of the WAAS System is that performs an estimation of the Ionospheric delay throughout the coverage area based on the delays calculated for each reference station, therefore, the receivers have a very good error correction regardless of location. Two primary types of messages are generated by the Master Station: integrity, and range corrections. Integrity messages are used to indicate which satellites are functioning within acceptable tolerances. Range corrections are parameters that permit the estimation of ionospheric delays, and satellite clock and ephemeris errors. All the data from the Master Station is transmitted by the ground network to the Ground Antennas, which broadcast the data to the WAAS GEOs [63]. Figure 4.10: WAAS architecture 4.2.1.2 Space Segment The Space segment is composed by the Geostationary Earth Orbit (GEO) satellites, which are located in fixed orbital positions over the equator. The GEO satellites broadcast the messages on L1 frequency for use by WAAS receivers. The WAAS GEO satellites may also serve as additional sources of navigation ranging signals, thereby increasing the number of usable “GPS-like” satellites. There are three satellites broadcasting in the WAAS Coverage area. Figure 4.11 shows the area covered by each satellite. 71 Figure 4.11: WAAS GEOs and coverage area [17] 4.2.1.3 User Segment User segment is referred to the equipment of the end user to detect the GPS and WAAS signals. The receivers must determine its location using the GPS signal received by the satellites and correct the pseudoranges with the messages from the GEOs WAAS satellites. Firstly, the receiver calculates the position by the fast type correction data (corrected satellite position and clock data), with this, the receiver can begin to use the slow corrections to accurate the location. The slow type correction data includes the ionospheric delay. There is a large variety of WAAS receivers. The next generation of the GPS with WAAS receivers is going to be implemented by NovAtel thanks to the three-year contract with the FAA worth up to US$9.7 million [65]. Figure 4.12 shows two examples of GPS/WAAS receivers. Furuno GP32 GPS/WAAS receiver and Brunton Atlas GPS/WAAS. (a) Furuno GP32 GPS/WAAS receiver (b) Brunton Atlas GPS/WAAS receiver Figura 4.12: GPS/WAAS receivers 72 4.2.2 European Geostationary Navigation Overlay Service (EGNOS) The European Geostationary Navigation Overlay Service (EGNOS) [18] is a SBAS under development by the European Space Agency (ESA), the European Commission and EUROCONTROL. It is intended to supplement the GPS, GLONASS and Galileo systems by reporting on the reliability and accuracy of the signals. by providing differential corrections to improve positioning accuracy and signal quality measurements to provide integrity to satellite navigation users in Europe [66]. Unlike WAAS, EGNOS can readily be used in a wide range of domains. As stated in the previous section, WAAS is clearly focused on improving navigation and positioning for the air service, which implies that many areas are not fully benefiting from the augmentation system. The main domains where EGNOS will be beneficial are: •Road navigation Accuracy, as well as integrity and availability are improved by EGNOS, consequently a better fleet tracking is possible. With this, transport companies and users of these services are benefit. •Aeronautics Air transport is becoming more and more important. EGNOS is designed to assist navigation both en-route as well as during landing, which will benefit the aerospace community. •Maritime Complementing EGNOS with the services already provided by marine radio beacons, maritime navigation will be safer. •Agriculture Due to the high positioning accuracy EGNOS provides and in combination with geodetic techniques an improvement in the area of property boundary mapping, land parcel identification and geo-traceability. Furthermore, this augmentation method enables the high-precision spraying of fertilisers and pesticides, reducing the amount of chemicals needed for achieving optimal yield and productivity. It can also support innovative applications such as automatic tractor guidance or remote livestock positioning and supervision. •Personal navigation applications EGNOS opens a wide range of new possibilities in applications such as guiding aids for the blind, emergency localisation, friend finding or geo-localised advertising. 73 •Clock synchronising EGNOS broadcasts a reliable time standard with unprecedented accuracy for use by computer and telecommunication networks. EGNOS will offer when fully operational three types of service [15]: •The Open Service (OS), freely available to the public in Europe. The main objective of the EGNOS OS is to improve the achievable positioning accuracy thanks to the correction of several error sources affecting the GPS signals. The minimum horizontal accuracy5is 3m horizontal, whereas the vertical accuracy6is 4m. •The Safety of Life Service (SoL), that will provide the most stringent level of signalin-space performance to all Safety of Life user communities in Europe. The main objective of the EGNOS SoL service is to support Civil Aviation applications up to LPV (Localizer Performance with Vertical guidance) operations. •The Commercial Data Distribution Service (CDDS) for customers who require enhanced performance for commercial and professional use. EGNOS CDDS provides authorised customers all EGNOS augmentation messages in real time (including satellite clocks and ephemeris corrections, propagation corrections and integrity information in the SBAS format) and raw data from the Ranging and Integrity Monitoring Stations (RIMSs) in real time (including satellite high precision pseudorange measurements). Regarding the EGNOS architecture, it is similar to WAAS, with ground, space and user segments. 4.2.2.1 EGNOS Architecture Ground Segment The Ground Segment is in charge of monitor the SVs, calculate error corrections and upload the GPS-like signals with the information to the EGNOS Satellites. The GS is composed by 34 Ranging and Integrity Monitoring Stations (RIMS), 4 Mission Control Centres (MCC), 6 Navigation Land Earth Stations (NLES) and the EGNOS Wide Area Network (EWAN). Two additional facilities are also deployed as part of the GS to support system operations and service provision, namely the Performance Assessment and Checkout Facility (PACF) and the Application Specific Qualification Facility (ASQF) (see Figure 4.13). 5Corresponding to a 95% confidence bound of the bi-dimensional position error in the horizontal local plane for the worst user location 6Correspondoing to a 95% confidence bound of the uni-dimensional unsigned position error in the local vertical axis for the worst user location. 74 •RIMS The main function of the RIMS is to collect measurements from GPS satellites and to transmit these raw data each second to the Central Processing Facilities (CPF) of each MCC. •MCC There are four MCC distributed around the EWAN. One of them is the main MCC and the mission of the other three are to support the main MCC in case of failure. The MCC is subdivided into the Central Control Facility (CCF) and the Central Processing Facility. –CCF These facilities are manned on a 24/7/365 basis in order to ensure permanent service monitoring and control. –CPF Provides EGNOS WAD corrections (clock corrections for each GPS satellite in view of the network of RIMS stations valid in all the broadcast area, ephemeris corrections to improve the accuracy of spacecraft orbital positions, model of ionospheric errors over the EGNOS service area in order to compensate for ionospheric disturbances on the navigation signals, estimation of the residual errors, etc.) and ensures the integrity of the EGNOS users. •NLES The NLES receive the message elaborated by the CPF at the MCC in order to transmit it to the GEO satellites for broadcasting to users and to ensure the synchronisation with the GPS signal. There are two NLES for each GEO Satellite. •EWAN EWAN is the network that links all the EGNOS components. 75 Figure 4.13: EGNOS Ground Segment distribution Space Segment The Space Segment is composed by three GEO Satellites whose mission is to broadcast the signals received by the NLES in order to provide the service in the whole service area of EGNOS (see Figure 4.14). The satellites broadcast the corrections and integrity information for GPS satellites in a right-hand circularly polarised (RHCP) signals in the L1 frequency band (1575,42 MHz). The broadcast signal is a combination of a 1023bit PRN navigation code of the GPS family and a 250 bits per second navigation data message carrying the corrections and integrity data elaborated by the EGNOS ground segment. The level of the received RF signal at the output of a 3 dBi linearly polarized antenna is within the range of –161 dBW to –153 dBW for all antenna orientations orthogonal to the direction of propagation [15]. 76 Figure 4.14: Geostationary satellite Broadcast Areas User Segment There is a wide range of EGNOS receivers available. There is a list of receivers made by Helios as part of a market study where the receivers are classified by the type of application the user demands. The receivers’ list can be seen at [67]. 4.2.2.2 Errors magnitude As mentioned above, an EGNOS receiver can correct the main sources of error that affects the pseudorange. Table 4.2 shows the contribution of the errors using EGNOS and in a GPS Stand-alone situation. The error sources analysed are the Satellite Residual Error for the Worst User Location (SREW), the User Ionospheric Vertical Delay (UIVD), troposphere, receiver noise and multipath contributions as well as the final User Equivalent Range Error (UERE) [15]. The improvement in accuracy is clearly noticeable (see Figure 4.15). 77 Table 4.2: Errors magnitude in positioning Error Sources (1σ�)Error Size (m) GPS SREW 2.3 Ionosphere (UIVD error) 0.5 Troposphere (vertical) 0.1 GPS Receiver noise 0.5 GPS Multipath (45ºelevation) 0.2 GPS UERE 5ºelevation (after EGNOS corrections) 4.2 GPS UERE 90ºelevation (after EGNOS corrections) 2.4 (a) Typical EGNOS SIS UERE Error Sources (1σ�)Error Size (m) GPS Clock and Ephemeris Errors 4.0 Ionosphere vertical error 2.0 to 5.0 Troposphere (vertical) 0.1 GPS Receiver noise 0.5 GPS Multipath (45ºelevation) 0.2 GPS UERE 5ºelevation (GPS Stand-alone) 7.4 to 15.6 GPS UERE 90ºelevation (GPS Stand-alone) 4.5 to 6.4 (b) Typical GPS Stand Alone SIS EURE Figure 4.15: Kinematic maritime test near Lisbon comparing GPS without and with EGNOS [18] 78 broadcasted by the mobile network operator. In case the user would need more accuracy in positioning, this technique is not useful because its main achievement is to reduce the TTFF and to provide an approximate positioning in areas with poor signal power. 85 Part III Tracking Systems 86 Chapter 5 Tracking Systems 5.1 Passive and active tracking systems This chapter is divided into two different sections. The first one is an introduction to the active and passive tracking systems and their applications. On the second section, the reader can find the description of some different types of products used in the applications described previously. 5.1.1 Tracking systems overview This part will be focused on tracking systems, mainly describing the principles of working for vehicle monitoring. The term tracking refers to the observation of people or objects in motion to supply their respective position in a sequence timely ordered. This data can be used subsequently (or simultaneously) for multiple applications, for instance, to be represented on a map. There are several types of tracking systems depending on the technology used (GPS, Radio beacons, 2G, 2.5G. . . ), the type of service used (real-time, near real-time. . . ), the final application, etc. The most commons, and the main purpose of this survey, are the systems that calculate the relative location of the target using GPS. Thus, that information can be stored in a flash memory (or equivalent) or simultaneously transmitted to a remote server by GSM Network. About the GPS tracking systems, there are two categories depending on which method is used to get the information from the GPS receiver to the final application. If the information is transmitted in real-time (or near real-time) by GPRS, SMS, CSD or other mean to the remote server, the device is called active. On the other hand, a passive device is the one that stores all the location data in an internal memory and afterward dumps it to the database. Although the architecture of each device or system is different, we can identify a number 87 of common parts in all. Figure 5.1 shows both active and passive architectures diagrams. (a) Active architecture diagram (b) Passive architecture diagram Figure 5.1: Active and passive architecture diagrams With regard to active systems, the main components of the device are: •Positioning Module (GPS) •Transmission Module (GSM/GPRS) •I/O Interfaces There could be many variations, such as a memory that can be used when the GSM/GPRS network is not available to transmit the data, or when the uses configures the device to work in a passive mode. Passive devices are all equipped with a memory (usually flash) to save all the parameters needed and do not have the GSM/GPRS Module. As mentioned before, many other features can be introduced to the devices, but this will be discussed in Section 5.2. The database and the server are another very significant parts of the tracking system. Depending on the application needed, some parts can be omitted, but as this part of the Thesis is focused in vehicle tracking applications, will be considered as part of the system. Now, the most significant elements of tracking system are described. Positioning Module (GPS) The basic function of this module is to calculate the relative location of the object at any time. Mostly, this part also gives information about the time, speed, altitude, etc. so the tracking can be more complete. Depending on the sophistication of the device, the GPS module can also have some augmentation system to improve the accuracy in positioning. The most common is A-GPS, due to the problems in positioning the vehicles inside the cities because of the multipath 88 error produced by the buildings. Another reason of using this system is that, in the active systems, a GSM antenna is added to the device to send the positioning information to the remote server, so during the time the antenna is not being used, the assisted data can be received [71]. There are devices designed with SBAS for a major accuracy as well. These kind of devices are mostly used when the tracking needs to be very precise (i.e. to calculate the coordinates of a road with a technical purpose). In order to avoid the loss of data in GPS black zones, some gadgets are implemented with an Inertial Navigation System (INS). This system is used to calculate the position of the object by a computer, motion sensors, and gyroscopes. With these elements and knowing the exact position in the time the GPS signal is poor, the system can easily calculate the position via dead reckoning. By the time the GPS signal stills undetectable, the tracking device can send or store the data. The accuracy of the INS is worse than the GPS, and the more time the INS is being used, the more error the measure contains. Transmission Module (GSM/GPRS) This part is only common in the active tracking devices. In fact, the transmitter module can be done by any other technology such as RF, satellite communication, Wi-Fi, etc. but in the area this part is focused the devices mainly use the GSM/GPRS bands. This module is in charge of the transmission of the parameters gathered by the GPS module in order to be sent to the remote server. Two types of technology have been mainly used in the design of the tracking gadgets, data sending by SMS (Short Message Service) or by GPRS (General Packet Radio Service). The first method used was by SMS due to the large coverage of GSM all over the world, but several problems appeared. First of all, in one SMS there is only space for 160 characters of 7 bits (140 characters of 8 bits), as a consequence, the update of long data to the server is very expensive. There is no guarantee of the deliver time and also the time delay of the transmission is a bottleneck for real-time applications. The bit-rate of SMS is 9,6 kbps. The General Packet Radio Service is an enhancement of GSM networks to support packet switched data services (See Figure 5.2). This technology provides data rate of 56114 kbps and the transmitter knows at any time whether the server is receiving the data or not. GPRS operates on the existing GSM network infrastructure that it utilizes available time slots during each frame transmission. Thus, it does not overload the existing GSM network traffic and can efficiently provide data services. As a consequence, the price for sending big quantities of data is cheaper than in SMS. The main problem of the devices that uses this technology is that the coverage is smaller than the GSM 2G (SMS) coverage. As a result, the majority of the devices can switch from one mean to the other depending 89 on the quality of the signal, ensuring that the data is sent to the server at all times [72, 73]. When the device is located in an area without GSM coverage, the GPS Module sends the data to an internal or external memory which backups the data to be further transmitted, when the device detects GSM signal [74]. Figure 5.2: GPRS Architecture I/O Interfaces All the devices have some I/O interfaces to permit the manipulation of the stored data and to configure the device. Many of the gadgets do not have any screen or buttons to configure it, so a connection to a computer is needed. In the case of the passive tracking devices, the I/O interface will be the only way to get the saved data. Several types of interfaces can be implemented in a gadget such as serial port RS-232, Ethernet, USB, etc. In addition, I/O interfaces can be used to monitor the status of some parts of the vehicle (wheels pressure, fuel, engine problems, electronic problems, gas detection, etc.) or get information about the environment (temperature, humidity, air pressure, etc.) that can be useful for some applications, i.e. in real-time monitoring of dangerous goods transport [75]. Database and remote server This part of the system is clearly separated from the tracking device and is responsible for processing the information transmitted by the GSM/GPRS module. Firstly, the data 90 reaches the database, where they are stored for later use. The most common application is to print the vehicles’ path or position on a map (GIS). The application will be responsible of obtaining the data from the database to process it and finally represent it on the map [76]. In cases in which communication between devices is via GPRS, the device will have its own IP, so that communication will be much faster than if done via SMS. With GPRS Network communication, real-time applications are possible. Regarding the database can be implemented using any database management system such as DB2, PostgreSQL, Oracle or MySQL. As well as the server can be configured with any web server software in case of the data has to be displayed in Internet. In cases the data is not needed to be in a server, this element can be omitted. Figure 5.3 shows an example of active tracking system architecture with the elements described above. Figure 5.3: Example of active tracking system architecture 5.1.2 Tracking systems applications There are several different applications related with tracking systems, not only for vehicle tracking but also for people tracking (i.e. for offenders or people who are not allow to leave a specific area or country). In this section, only vehicle tracking applications will be 91 described. Four different types of tracking can be distinguish: •Fleet management •Urban transit management •Monitoring driving behaviour •Stolen vehicle recovering Although the main idea of all of this applications is to get the vehicles’ path at any time, many different variances can be considered in each type. Fleet management For many companies, being able to track all their vehicles in real time has a significant value. In transport companies, tracking systems allow them to have precise control of the place where each package is. Consequently, they can offer the clients an added value to the product, as the client would be able to consult the exact point where the package is and the estimated time delivery. In the case of private ambulances companies, efficient service can be ensured when sending a vehicle to a particular area. Knowing the exact location of each ambulance makes easy to notify the nearest. With regard to carriers of dangerous goods, this technology provides a large increase in security. As explained above, there are gadgets that monitor external and internal parameters to the vehicle, so that by the time the sensor detects any abnormality, a signal is sent to the server. With this, the server can act consequently on warning the driver or even controlling somewhere the vehicle remotely [75]. Urban transit management In this category are included monitoring schedule adherence of buses applications, making easy to check the level of success of the service and more comfortable for the user as he is always inform about the incidences. Some cities have developed a system in which the time left for the next bus/tram/train is showed in a screen or even transmitted, via Bluetooth or SMS, to the passengers. Another widely used application in many cities is to remotely trigger changes of buses’ signs, such as the name of the next stop in the screen or the change of direction when the bus reaches the end of the line. As the track is being processed in real-time, it is easy to remotely control the dynamic parts of the bus in order to not disturb the driver. 92 Monitoring driver behaviour These types of application are not as usual as the other two described above, although are more and more used nowadays than time ago. These applications monitor the location, time, speed, etc. of the vehicle in order to inform a third person. The relationship between the driver and the third person is usually employer/employee or parents/teen. The main purpose of this is to know at every time if any irregularities are being committed, such as exceeding the speed limit or to avoid the use of firms’ vehicles for personal purposes. Stolen vehicle recovering This application is based on the incorporation of a tracking device in the vehicle, properly disguised, to know the position of the vehicle in case of theft. Depending on the sophistication of the product, some parts of the vehicle can be controlled remotely, i.e. activate the car alarm, stop the engine or disconnect all the electronic parts preventing the use of the vehicle. With the advent of this technology, many insurance companies have lowered prices to customers who incorporate a tracking system, as the odds of loss of vehicle theft decrease significantly. 5.2 Products 5.2.1 Active and passive products As mentioned in page 87, there are two different types of tracking products according to the method used in the transmission of data. In this section some products will be presented to provide the reader a global idea about the actual products in the market and their characteristics. The range of products is very wide depending on the features of the devices, for that, only a selection of the most remarkable gadgets is going to be examined. Due to the amount of possibilities the active products have in respect with the passive, this part will be more focused in active products. 5.2.1.1 Passive products Passive products are the devices that do not send the location data to the server while is being gathered, but afterwards. The best advantage of these products is the fact the owner does not have to pay monthly fees because all the data is transmitted directly from the device to the computer. In cases the device is designed to support SBAS or A-GPS (which is not common in these kind of devices), the owner will have to pay the according fees of the augmentation system in use. The most common augmentation system in a 93 passive device is SBAS due to the fact is not needed another antenna in the range of, for example, GSM to receive the augmentation information (like in A-GPS). In case the device is designed with A-GPS, the data receiver module can be used to transmit the navigation data to a server via GSM, so the device becomes active. There are several ways to store the data depending the manufacturer. However, the most widely used is the NMEA protocol. Devices implemented with the NMEA protocol make sure the compatibility with many other devices and software [77]. In the end, the software must easily interpret all the data gathered by the gadget in order to make the information comprehensive for the end user. An example about how the NMEA data is stored is given in [78]. In passive products the accuracy of the data is not as important as in the active devices. Normally active products with high accuracy are used to send messages to the vehicle when it is arriving to some position, but this is not the case of the passive products. Due to that fact, these devices are not going to be deeply examined, as it is not strictly related with the main objective of the Thesis. Nevertheless two different devices will be described; the first one is standard; the second is equipped with SBAS support. The standard device chosen is the GPS Tracking Key Pro developed by LandAirSea, the more sophisticated is the GT-750F(L)-Lite developed by CanMore Electronics Company [78]. Two features have been the most significant to select this last device among the other competitors, firstly the fact that SBAS is supported, and secondly that the NMEA protocol is used. GPS Tracking Key Pro This device is one of the best for a non-accurate tracking, such as stolen vehicle recovery or monitor driver behaviour. This model has been designed to be placed anywhere in the car, either inside or outside the car. The gadget is provided with a powerful magnet and is water resistant, so there is no problem to install it under the vehicle. The GPS Tracking Key Pro (see Figure 5.4) records every second the speed, location of stops, duration of stops, time en route, arrival addresses and direction of vehicle within 2.5 meters of accuracy. The principle of working, as mentioned before, is to calculate the position and the rest of parameters using the L1 C/A Code and store all the data every second in a non-volatile flash memory in NMEA format. The capacity of the flash memory is about 100 hours of driving. The device uses 2 AA batteries, which provides a two-week battery life driving 4 hours per day, although it can be plugged to the vehicle’s main power supply. 94 Real-time RaceFX More than a device like in the other cases described above, this is an example an application only possible with the best accuracy in positioning. The main idea of this application is to monitor all the cars in Nascar races, which provide data about speed and position. The design of RaceFX had to overcome several adversities. Accurate vehicle positions needed to be obtained, calculated, and transmitted under conditions in which GPS satellite signals are frequently blocked or reflected. In addition, installing the GPS vehicle tracking system without affecting the aerodynamics of a car travelling 200 mph and yet maintaining good satellite visibility requires an innovative design supposed a non-trivial challenge. RaceFX is composed by four subsystems: GPS, telemetry, time synchronization, and video overlay. Each racecar has a GPS receiver and a 900 MHz transceiver. RaceFX employs a sophisticated telemetry system that transfers position and other vehicle information from all race vehicles to a central processor at the rate of five times per second. DGPS, pseudorange and carrier phase tracking techniques generate vehicle coordinates accurate to 50 centimetres (1σ). The telemetry conveys differential messages from GPS base stations to the racecar rover units at 0.5 Hertz and racecar rover information to the video subsystem at 5 Hertz. Six broadcast cameras are instrumented to measure their pan, tilt, zoom, and focus 30 times per second or once per video frame. RaceFX interpolates racecar position information to correspond with the camera orientation in each video frame. High-speed computers combine this data to appropriately juxtapose the car and data in the video frame. The video overlay accepts information from all rovers and reformats it for individual video frames of the particular camera used in the broadcast. Racecars travel at speeds up to 90 meters/second. Relative timing between the video, and GPS must be accurate to one millisecond to keep time-induced errors below 10 centimetres. The GPS-based system maintains the timing to about 10 microseconds, or 100 times better than the minimum requirement. Racetracks must be accurate digitized for the correct working of the system. Due to the speed of the cars and the buildings near the racetrack, satellites are constantly entering and exiting from the field vision of the GPS receiver. Consequently, there is a significant degradation on positioning. To overcome that problem, NovAtel and Sportvisionengineers developed several techniques to minimize these shadowing effects by incorporating a computerized model of the track into filters that provide more precise positioning. With this and the incorporation of a DGPS station in a visible place from all the track, racecars are able to calculate the exact position at every moment. Figure 5.9 shows a frame on TV of the RaceFX result. 101 Figure 5.9: RaceFX frame example [21] As the reader can see, the final result is a real-time racecars monitoring with several parameters that can be checked at any time. This gives the television company a differentiation attribute amongst its competitors, impossible to achieve without the accuracy in positioning and the post processing techniques. 102 Chapter 6 Conclusions This report is a theoretical description of the different positioning systems and the GPS Augmentation Systems. With the information that can be found in this Thesis the reader can get a global idea about positioning and navigation, how the systems work, advantages and disadvantages of them, history, new developments in each one, etc. The fact that all of the systems are characterised in the same document will help the reader the comparison between them, as all the significant features, principles of working and evolution are included. As the reader can see, the most significant systems described are GPS, GLONASS and Galileo due to the fact they are the most developed and popular nowadays (and in a short-term future). About the GPS Augmentation Systems,themainpurposeofthispartwastogivethe reader the bases of the techniques to improve the accuracy in the GPS System. Most of them can be also applied to other system, since all the positioning system described are based in the same principles. In that part the reader can see how the accuracy is improved with each technique and the difficulty in implementing each one. With this, it was pretended to get the reader the necessary information to evaluate which is the best Augmentation System according to their necessities. In conclusion, about DGPS, the main problem is the initial and maintenance cost of the technology. The base stations are very expensive and the coverage area is not too big as well. Nevertheless, the accuracy achieved with this method is the best that a receiver can have, and can be even higher if it is combined with another technique. Regarding the SBAS techniques, are very useful to have high accuracy in areas not covered by a DGPS base station. The WAAS system is widely used by the aviation community, although civil users can also use it. The problem of this system is that it only has coverage in the United States. For the European users, the best SBAS option is to use the EGNOS system. About the accuracy provided by these systems, thanks to the ionospheric model the user can correct almost completely the ionospheric error, one of the 103 most important errors in GPS. The combination of a GPS receiver and a transmission module to receive the assistance is more and more common in mobile devices, providing the user with the possibility of receiving the assistance broadcasted by the mobile network operator, which makes AGPS one of the most widely used augmentation system. The last part of the Thesis is an introduction to the Tracking Systems.Inthatpart,the main purpose is to show the reader the possibilities that the combination GPS + Augmentation Systems have. There is nothing clearer to achieve this than some practical examples, so in the last part the theoretical aspects are justified with examples and products available nowadays in the market. About the devices described, passive systems are more useful for those applications that do not need real-time monitoring, but further analyse of the track of the object. For real-time or near real-time applications, active devices are perfect. Depending on the price of the device and the technology used (use of augmentation systems, IMU’s, transmission’s module protocols, etc.) the product’s possible applications will be wider. If the user is not looking for high accuracy to remotely track an object, the Airlink PinPoint XT or a smartphone with tracking system software would be enough. In case the user would need more accuracy in positioning to remotely control a vehicle, a product with similar characteristics to the AsteRxi would be a good option. A future work on this area could be interesting if some products could be tested and compared in real conditions, not only in the theoretical conditions given by the manufacturers. Even though, the information given should is enough to give the reader a clear idea about the types of devices in the market and its applications. 104 ! 105 Bibliography [1] M. Hernández-Pajares, J.M. Juan Zornoza, and J. Sanz Subirana. GPS data processing: code and phase Algorithms, Techniques and Recipes . CPET (Centre de Publicacions del Campus Nord, UPC), 2005. [2] Christopher J. Hegarty and Eric Chatre. Evolution of the global navigation satellite system (GNSS). IEEE. [3] Extracted from Celestia 3D Space Simulator. [4] Extracted from boeing official website. URL: http://boeing.mediaroom.com/ [5] Navstar GPS User Equipment Introduction. Technical Report, 1996. [6] G. M. Polischuk, V. I. Kozlov, V. V. Ilitchov et Al. The Global Navigation Satellite System GLONASS: Development and usage in the 21st century . Technical report, 34th Annual Precise Time and Time Interval (PTTI) Meeting. [7] Extracted from NovAtel official website, URL: www.novatel.com [8] Extracted from the European Comission Enterprise and Indsutry official website. URL: http://ec.europa.eu/enterprise/policies/satnav/galileo/media/galileo_architecture.jpg [9] Werner Enderle, Thomas Weber, and Hans Kuhlen. The architecture of the European Global Navigation Satellite System - Galileo . Technical Report AIAC 2003–111, 10th Australian International Aerospace Congress, 2003. [10] Zhu Yonghui and Bai Zhengdong. The COMPASS Communications System and its applications . Technical Report 978-1-4244-5824-0, Tsinghua University, Beijing, China, 2010. [11] Thomas GrelIer, Joel Dantepal, Antoine Delatour, Alain Ghion, and Lionel Ries. Initial observation and analysis of COMPASS MEO Satellite Signals . Technical report, Inside GNSS, May/June 2007. ! 106 [12] Extracted from the nasa official website. URL: http://solarscience.msfc.nasa.gov/SunspotCycle.shtml [13] Tomasz Lipecki. The modern technologies of DGPS and RTK corrections transfer . Geomatics and Environmental Engineering, 1(3), 2007. [14] Marek Dziewicki. Position accuracy evaluation of the modernized Polish DGPS . Polish Maritime Research, 16(4):57–61, 2009. [15] Paul Verhoef. EGNOS, Service Definition Document . Technical report, European Commission Directorate-General for Energy and Transport, 2009. [16] NSTB. URL: http://www.nstb.tc.faa.gov/index.htm [17] FAA - Federal Aerospace Agency, WAAS Coverage. URL: http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/ gnss/waas/news/media/GEO_Coverage.jpg [18] Elisabeth Klaffenböck and Bernhard Hofmann-Wellenhof. European Satellite Navigation - A Challenge . Technical report, 50th International Symposium ELMAR, Zadar, Croatia, September 2008. [19] System of Differential Correction and Monitoring (SDCM), Federal Space Agency (Roscosmos), URL: http://www.oosa.unvienna.org/pdf/icg/2008/expert/2-5b.pdf [20] Jizhong Li and Muqing Wu. A positioning algorithm of AGPS . Technical report, 2009 International Conference on Signal Processing Systems, 2009. [21] Extracted from a GPSWorld article. URL: http://www.gpsworld.com/transportation/road/realtime-gps-fx-on-screen-positioning-racecars-729 [22] GPS official website. URL: http://www.gps.gov/ [23] Global Security, 2010-2011. URL: http://www.globalsecurity.org/space/systems/gps.htm [24] United States Naval Observatory. GPS constellation status. URL: ftp://tycho.usno.navy.mil/pub/gps/gpstd.txt [25] Director, operational test and evaluation. Technical report, U.S. Department of Defense and U.S. Department of Army, 2003. [26] GPSWorld, May 2010. URL: http://www.gpsworld.com/gnss-system/gps-modernization/news/block-iif-launch-delayeduntil-thursday-9982.106 ! 107 [27] GPSWorld, May 2010. URL: http://www.gpsworld.com/gnss-system/gps-modernization/news/block-iif-successfullylaunched-cape-canaveral-10003 [28] InsideGNSS, June 2010. URL: http://www.insidegnss.com/node/2124 [29] Lockheed Martin Corporation. Denver, May 2008. URL: http://www.lockheedmartin.com/news/press_releases/2008/515_ss_gps.html [30] Lockheed Martin Corporation, 2010. URL: http://www.lockheedmartin.com/products/GPS/ [31] Deagel, 2010. URL: http://www.deagel.com/C3ISTAR-Satellites/GPS-Block-III_a000238005.aspx [32] R. M. Alkan, H. Kamman, and M. Sahin. GPS, Galileo and GLONASS Satellite Navigation Systems and GPS modernization . Technical report, Istanbul Technical University, Faculty of Civil Engineering Department of Geodesy and Photogrammetry. [33] Irving Leveson. Benefits of the new GPS civil signal, the L2C . Technical report, Inside GNSS, 2006. [34] John W. Betz et al. Enhancing the future of the civil GPS. Overview of the L1C signal . Technical report, Inside GNSS, 2007. [35] Global Positioning System Wing (GPSW). Systems Engineering and Integration Integrase Specification. Technical report, IS-GPS-800, 2010. [36] Paul Correia. Guide Pratique du GPS . Éditions Eyrolles, París, 2000. [37] Multiple authors. Global Navigation Satellite System GLONASS. Interface Control Document. Navigational Radiosignal in Bands L1, L2 . Technical report, Russian Institution of Space Device Engineering, 2008. [38] V. N. Kazantsev, M.K Reshetnev, A. G. Kozlov, and V.E Cheremisin. Overview and design of the GLONASS System . Technical report. [39] Russian Federal Special Agency. URL: www.glonass-ianc.rsa.ru [40] NovAtel Inc. URL: http://www.novatel.com/products/gnss-receivers/oem-receiver-boards/ [41] European GNSS (Galileo) Open Service, Signal in Space Interface Control document. Technical report, European Union and European Space Agency, September 2010. ! 108 [42] Galileo overview. Technical report, Eero Ailio, European Commission, COPUOS S&T Committee, February 2008. [43] ESA - European Space Agency, Giove information. URL: http://www.giove.esa.int/page_index.php?page_id=35&menu=102 [44] ESA - European Space Agency, Giove information. URL: http://www.giove.esa.int/page_index.php?menu=103&page_id=36 [45] ESA - European Space Agency. URL: http://www.esa.int/esaNA/SEMGGGF098G_index_0.html [46] Frantisek Vejrazka. Galileo and the other satellite navigation systems . Technical Report IEEE 1-4244-0822-9/07, Dept. of Radio Electronics, Czech Technical University, 2007. [47] Gao Grace Xingxin, Alan Chen, Sherman Lo, David De Lorenzo, Todd Walter, and Per Enge. Compass-m1 broadcast codes and their application to acquisition and tracking. Technical report, Stanford University, San Diego, 2008. [48] United Nations Office for outer Space Affairs official website. URL: http://www.oosa.unvienna.org/ [49] N. Neelakantan. Overview of Indian Satellite Navigation Programme . Technical report, Satellite Communciation and Navigation Programme, Indian Space research Organization, India. [50] JAXA - Japanese Aerospace Exploration Agency, September 11, 2010. URL: http://www.jaxa.jp/press/2010/09/20100911_h2af18_e.html [51] Quasi-Zenith Satellite System, Navigation Service Interface Specification for QZSS (ISQZSS). Technical Report V1.1, Japan Aerospace Exploration Agency, July 2009. [52] Jay Farrell. Experimental Differential GPS Reference Station evaluation . Technical report, Department of Electrical Engineering, University of California, Riverside, 1999. [53] Kara Messaoud and Hou Kun-Mean. Low Cost Differential GPS receivers (LCD-GPS): The differential correction function . Technical report, LIMOS Laboratory UMR 6158 CNRS, University of Blaise Pascal Clermont-Ferrand II, France, 2008. [54] Luís Sardinha Monteiro, Terry Moore, and Chris Hill. What is the accuracy of DGPS? , 58, pp 207-225. Journal of navigation, European Navigation Conference GNSS 2004, 2005. [55] NOAA National Geodetic Survey. URL: http://www.ngs.noaa.gov/CORS/CorsPP/WA-SlideShow/index.htm ! 109 [56] National Oceanic and Atmospheric Administration (NOAA). URL: http://www.noaa.gov/ [57] IALA. URL: http://www.iala-aism.org/ [58] AMSA - Australian Maritime Safety Authority. URL: http://www.amsa.gov.au/Shipping_Safety/Navigation_Safety/Differential_Global_Postitioning_ System/Service_Status/ [59] Product survey. Technical report, Hydro International, October 2007. URL: http://www.hydro-international.com/productsurvey/id10-RTK_DGPS_Receivers.html [60] Combined performances for SBAS receivers using WAAS and EGNOS. Technical report, EUUS Cooperation on Satellite Navigation (Working Group C), July 2010. [61] Shau-Shiun Jan, D. Gebre-Egziabher, T. Walter, and P. Enge. Improving GPS-Based landing system performance using an empirical barometric altimeter confidence bound . Aerospace and Electronic Systems, IEEE Transactions on, 44(1):127 –146, 2008. ISSN 0018-9251. doi: 10.1109/TAES.2008.4516994. [62] NSTB/WAAS T&E Team. Wide-Area Augmentation System performance analysis report num. 32. Technical report, FAA/William J. Hughes Technical Center, April 2010. URL: http://www.nstb.tc.faa.gov/ [63] ToddWalter and J. David Powell. Flight inspection of GNSS SBAS procedures . Technical report, Aero/Astro Department, Stanford University. [64] FAA - Federal Aviation Administration. WAAS status and LPV. URL: http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/ gnss/hill/media/WAAS-LPV-Q_A-020607.pdf [65] InsideGNSS, July/august 2009. URL: http://www.insidegnss.com/node/1576 [66] Richard Farnworth. Eurocontrol’s support to EGNOS certification . [67] Super-ENGOS receiver supplier consultation, european GNSS supervisory authority. URL: http://www.egnos-portal.eu/files/dmfile/OPO-09-004_Receiver_list_091021.pdf [68] Overview of MSAS, Office of Aeronautical Satellite Systems, Japan Civil Aviation Bureau. URL: http://www.oosa.unvienna.org/pdf/icg/2009/icg-4/05-2.pdf [69] K. N. Suryanarayana Rao. GAGAN - The Indian Satellite Based Augmentation System . Indian Journal of Radio and Space Physics, 36:293–302, August 2007. [70] Lidija Bugif, Renato Filjar, and Sasa Desic. Assisted satellite positioning . Technical report, 47th International Symposium ELMAR, Zadar, Croatia, June 2005. ! 110 [71] D. Belton, T. Haddrell, and C. de la Fuente. Integrated Assisted GPS solutions for security applications . Technical report, 2005. [72] Peng Wang, Zhiwen Zhao, Chongbin Xu, Zushun Wu, and Yi Luo. Design and implementation of the Low-Power Tracking System based on GPS-GPRS module . Technical report, 2010. [73] Wen Leng and Chuntao Shi. The GPRS-Based location system for the long-distance freight . Technical report, 2006. [74] N. Chadil, A. Russameesawang, and P. Keeratiwintakorn. Real-time Tracking Management system using GPS, GPRS and Google Earth . Technical report, May 2008. [75] F. Valente, G. Zacheo, P. Losito, and P. Camarda. A telecommunications Framework for realtime monitoring of dangerous goods transport . Technical report, 2009. [76] A.U. Alahakone and V. Ragavan. Geospatial information system for tracking and navigation of mobile objects . Technical report, 2009. [77] NMEA official website. URL: http://www.nmea.org/content/nmea_standards/nmea_083_v_400.asp [78] GT-750F(L)-Lite Data sheet, Canmore Electronics Co., Ltd. Hsinchu County, 302, Taiwan. URL: www.canmore.com.tw [79] Septentrio Satellite Navigation. AsteRxi: IMU Enhanced GPS/GLONASS Dual-frequency receiver dataste.