Use of raytracing in models to investigate ionospheric channel performance

Use of raytracing in models to investigate ionospheric channel performance G. Miro ´ Amarante * , S.M. Radicella Aeromony and Radiopropagation Laboratory, Abdus Salam ICTP, Strada Costiera 11, 34014 Trieste, Italy Abstract The main parameters that characterize an ionospheric channel, frequency, elevation angle, group path, ground range, and apogee have been obtained by means of 2D raytracing program called abcray03. The analysis has considered different hourly ionospheric conditions for four quiet days (25–28 March 2000) and five geographic locations distributed in the two hemispheres. Electron density profiles define the selected ionospheric conditions. Raytracing is run considering experimental electron density profiles from manually scaled measurements obtained with ionosondes. The resulting radio propagation parameters are compared with those obtained using the IRI and NeQuick models driven by measured F2 peak values. Keywords: Ray tracing; Ionosphere; Electron density profiles 1. Introduction The 2-dimensional raytracing technique used (called abcray03) is based on the expressions introduced by Croft and Hoogasian (1968) and Croft (1969), and has been considered in previous ionospheric studies (Moorhead and Radicella, 1998; Miro ´, 2000; Miro ´et al., 2002). The outputs obtained with this computing technique are the main radio propagation parameters for the selected frequency: time delay of the signal travelling from the transmitter to the receiver (related to group path), reflection height or apogee, ground range, and elevation angle. A particular ionospheric condition is inserted into the raytracing program using a model or a measured electron density profile. The electron density profile models used for this analysis are the International Reference Ionosphere (IRI) (Bilitza, 1990, 2001) and NeQuick (Radicella and Leitinger, 2001; Radicella et al., 2003). The aim of this paper is to illustrate the effects of the differences between these models and the experimental electron density profiles on radio system operations. Similar studies found in the bibliography show the importance of these effects. For example, the impact of the F region long-term trend on radio systems has been evaluated by Cannon Paul et al. (2004). This recent study uses an analytic raytracing technique – SMART (Norman and Cannon, 1997, 1999) – and electron density profiles fitted to CCIR coefficients and the results show that a 16 km drop in hmF2 can introduce ground range changes of 100 km with larger values under certain circumstances. The analysis presented in this paper has been divided into two parts: (1) study of the main radio propagation parameters for the four quiet days analyzed at five ionosonde stations; (2) statistical analysis of the differences in elevation angle, apogee and group path between the two models and experimental results. 2. Methodology The experimental electron density profiles were produced directly from the manually scaled ionograms by using the NHPC inversion technique (Huang and Reinisch, 1997). The model profiles obtained with the NeQuick and * Corresponding author. E-mail address: [email protected] (G. Miro ´Amarante). IRI models were fitted to foF2 and M3000F2 measured characteristics. The raytracing program considers the distance between transmitter and receiver (D) and the electron density profile as inputs. The electron density profile is located at the circuit midpoint. For this study, the transmitter–receiver links have been selected along a meridian. The analyzed distances are calculated by increasing the latitude in one degree steps up to 1500 km. The transmitter–receiver distance (D) was calculated using the expression D¼Reacos½sin kTsin kR þcos kTcos kRcosðuTuRÞ;ð1Þ where uand kare longitude and latitude of the transmitter (T) and the receiver (R) and Rethe radius of the Earth (6370 km). The program abcray03 was run for each distance and ionospheric condition (experimental and model electron density profiles at the circuit midpoint). The main radio propagation parameters (time delay, apogee, and ground range) were calculated in the frequency range of 1– 20 MHz (0.1 MHz step) and elevation angle sweep between 6 and 60 degrees (0.1 degree step). The analysis was run for different hourly ionospheric conditions (0, 6, 12, and 18 LT) for the four quiet days (25–28 March 2000) and for five geographic locations distributed in the two hemispheres (Table 1). The selected period corresponds to high solar activity with daily sunspot numbers greater than 155. 2.1. Study of the main radio propagation parameters for the four quiet days analyzed at five ionosonde stations The first study carried out using the raytracing results was the comparison between the different analyzed days. Ground range, time delay (or the equivalent group path), and apogee for each day (85–88 days of year 2000) were plotted. The examples shown in Fig. 1 correspond to Millstone Hill and Ascension Island at a transmission frequency of 12 MHz and 12:00 LT. The left panel shows the experimental electron density profiles used as ionospheric conditions at the midpoint. It can be seen in the case of Millstone Hill where the profiles have some variability, the group path and range show the biggest differences in the region between the E and F1 reflections. However, there are no significant differences at Ascension Island. For the rest of the ionosonde stations, the results show that Ramey has a similar behavior to the other selected Northern Hemisphere station, Millstone Hill, and the results obtained at Jicamarca and Grahamstown, in the southern Hemisphere, are closer to those found at Ascension Island. 2.2. Statistical analysis of the differences in elevation angle, apogee, and group path between model and experimental results A statistical analysis has been done comparing the two models (IRI and NeQuick) and the experimental results in elevation angle, apogee and group path. Table 2 shows the statistical results at Ramey for 00, 06, 12, and 18 LT corresponding to NeQuick (NQ) and IRI. The number of cases (number), average of absolute differences (Average), standard deviation (Stdev), median of absolute differences (Median), cases with positive differences (>0), and cases with negative differences (<0) were calculated for elevation angle, apogee, and group path. The differences shown for the group path are relative (divided by experimental values) because of the wide range considered. For the case of Ramey, the differences show that NQ overestimates the three parameters for 06, 12, and 18 LT and IRI underestimates them for 00 and 12 LT. IRI is worse than NQ for 00 LT with differences around 4 degrees in elevation angle, 63 km in apogee, and 4% in group path. NQ is worse than IRI for 06, 12, and 18 LT with differences around 4–5 degrees in elevation angle, 26–32 km in apogee, and 4–5% in group path. The shadow panels (Table 2–6)inAverage row correspond to the comparison between the IRI and NeQuick models (the largest value) and the shadow panels in positive (>0) and negative (<0) differences row show the overestimation or underestimation for each model. For the case of Millstone Hill (Table 3), the differences show that NQ overestimates the three parameters and IRI underestimates them for 00, 06, and 18 LT. NQ is worse than IRI with differences around 3–6 degrees in elevation angle, 25–38 km in apogee, and 3–5% in group path. At Jicamarca (Table 4), the differences show that NQ overestimates the three parameters and IRI underestimates them for 00 and 06. IRI is worse than NQ for 06 and 12 LT with differences around 5–10 degrees in elevation angle, 46– 80 km in apogee and 5–10% in group path. NQ is worse than IRI for 00 and 18 LT with differences around 3–6 degrees in elevation angle, 20–51 km in apogee, and 2–6% in group path. The differences obtained for Grahamstown (Table 5) show that NQ overestimates the three parameters for 06, 12, and 18 LT and IRI underestimates the three parameters for 00, 06, and 18 LT. IRI is worse than NQ for 00 LT with differences around 5 degrees in elevation angle, 47 km in apogee, and 5% in group path. NQ Table 1 Geographic coordinates, geomagnetic latitudes and modified dip latitudes of the selected ionosonde stations Station Lat. Long. Lat. Geomag. Modip Ascension Is 7.9 345.6 1.40 23.24 Grahamstown 33.3 26.5 33.80 50.97 Jicamarca 12.0 283.2 0.68 0.65 Millstone H. 42.6 288.5 53.99 55.90 Ramey 18.5 292.9 29.89 42.48 is worse than IRI for 12 LT with differences around 6 degrees in elevation angle, 41 km in apogee, and 5% in group path. For the case of Ascension Island (Table 6), the differences show that NQ overestimates the elevation angle and group path for 06 and 18 LT and IRI underestimates the three parameters. IRI is worse than NQ for 12 LT with differences around 4 degrees in elevation angle, 25 km in apogee and 4% in group path. NQ is worse than IRI for 06 and 18 LT with differences around 3–5 degrees in elevation angle, 28–30 km in apogee, and 3–6% in group path. Fig. 1. Range, group path, and apogee versus elevation angle for each selected day (right panel) and the corresponding experimental electron density profiles (left panel) for Millstone Hill and Ascension Island. Table 2 Statistical results at Ramey for elevation angle, apogee and group path differences corresponding to NeQuick (NQ) and IRI Table 3 Statistical results at Millstone Hill for elevation angle, apogee and group path differences corresponding to NeQuick (NQ) and IRI Table 4 Statistical results at Jicamarca for elevation angle, apogee and group path differences corresponding to NeQuick (NQ) and IRI Table 5 Statistical results at Grahamstown for elevation angle, apogee and group path differences corresponding to NeQuick (NQ) and IRI 3. Conclusions The differences in group path and range found in the comparison between the four quiet days when reflection occurs in the F1 region confirm previous results by the authors (Miro et al., 2005). The F1 region is critical to determine the main radio propagation parameters using raytracing through ionogram-derived electron density profiles. In summary, comparing the raytracing through measured and modeled profiles leads to the following conclusions (Table 7): (1) NQ overestimates the three parameters, (2) IRI underestimates the three parameters, and (3) NQ and IRI performances for the three parameters are similar with differences around 4 degrees in elevation angle, 33 km in apogee, and 4% in group path. Acknowledgements The authors are grateful to Dr. Moorhead of Neptune Radar Ltd., Glouscester, U.K., for his help in understanding the ray-tracing technique, to the Center for Atmospheric Research at University of Massachusetts Lowell, USA, for data and software support and to the IRI and NeQuick groups. References Bilitza, D. International Reference Ionosphere 1990. NSSDC 90-22, Greenbelt, Maryland, 1990. Bilitza, D. International Reference Ionosphere 2000. Radio Science 36 (2), 261–275, 2001. Cannon Paul, S., Rogers Neil, C., Hall Chris, M. Trends in critical frequencies and layer heights over Tromso and their consequential impact for radio system modeling. Advances in Space Research 34, 2085–2091, 2004. Croft, T.A. Methods and applications of computer raytracing. Stanford Electronics Labs tech. Report. SU-SEL- 69-007, 1969. Croft, T.A., Hoogasian, H. Exact ray calculations in a quasiparabolic ionosphere with no magnetic field. Radio Science 3, 69– 74, 1968. Huang, X., Reinisch, B.W. Vertical electron density profiles from digisonde ionograms. The average representative profile. Annali di Geofisica 39 (4), 751–756, 1997. Miro ´, G. Caracterizacio ´n del canal ionosfe ´rico para un enlace HF punto a punto mediante te ´cnicas de trazado de rayos, Ph.D. Thesis, Universidad Complutense de Madrid, 2000. (In Spanish). Miro ´, G., De la Morena, B.A., Radicella, S.M., Herraiz, M. Worst cases for a one-hop High Frequency Link. Annals of Geophysics 45 (1), 201–205, 2002. Table 7 Statistical results (number of cases (number), average of absolute differences (Average), cases with positive differences (>0) and cases with negative differences (<0)) for elevation angle, apogee and group path corresponding to NeQuick (NQ) and IRI The shadow panels show the overestimation or underestimation for each model. (All stations). Table 6 Statistical results at Ascension Island for elevation angle, apogee and group path differences corresponding to NeQuick (NQ) and IRI Miro Amarante, G., Zhang, M.L., Radicella, S.M. Ionogram inversion F1 layer treatment effect in raytracing. Annals of Geophysics 48 (3), 483– 489, 2005. Moorhead, M.D., Radicella, S.M. Using 2D raytracing to model the effects of vertical profile variability on oblique path propagation at HF frequencies. Proceedings of the Side Workshop of COST 251, 1998. Norman, R.J., Cannon, P.S. A two-dimensional analytic ray tracing technique accommodating horizontal gradients. Radio Science 32 (2), 387–396, 1997. Norman, R.J., Cannon, P.S. An evaluation of a new 2-D analytic ionospheric ray tracing technique – SMART. Radio Science 34 (2), 489–499, 1999. Radicella, S.M., Leitinger, R. evolution of the DGR approach to model electron density profiles. Advances in Space Research 27 (1), 35–40, 2001. Radicella, S.M., Leitinger, R., Nava, B., Coisson, P. A flexible 3D ionospheric model for satellite navigation applications. International Symposium on GPS/GNSS Proceedings, Tokyo, Japan, 305–310, 2003.