An Internet DGPS service for precise outdoor navigation

An In e ne DGPS Se ice o P ecise Ou doo Na iga ion Manuel G. Soa es Benedi a Malhei o F ancisco J. Res i o Dep . o Elec ical Enginee ing, Dep . o Elec ical Enginee ing ISEP, ISEP, and Compu e s, FEUP, Rua D . Robe o F ias, Rua D . An onio Bema dino de Dep . o Elec ical Enginee ing, Rua D . An onio Bema dino de Almeida, 431, Almeida, 43 I, P-4200-465 Po o PORTUGAL PORTUGAL P-4200-072 Po o P-4200-072 Po o PORTUGAL Abs ac - The goal o he wo k p esen ed in his pape is o p o ide mobile pla o ms wi hin ou compus wi h a GPS based da a se ice capable o suppo ing p ecise ou doo na iga ion. This can be achie ed by p o iding campus-wide access o eal ime Di e en ial GPS (DGPS) da a. As a esul , we designed and implemen ed a h ee- ie dis ibu ed sys em ha p o ides In e ne da a links be ween emo e DGPS sou ces and he campus and a campus-wide DGPS da a dissemina ion se ice. The In e ne da a link Se ice is a wo- ie clien lse e whe e he se e -side is connec ed o he DCPS s a ion and he clien -side is loca ed a he campus. The campus-wide DGPS da a p o ide dissemina es he DGPS da a ecei ed a he campus ia he campus ln ane and ia a wi eless da a link. The wi eless b oadcas is inlended o po able ecei e s equipped wi h a DGPS wi eless in e ace and he lnl ane link is p o ided o ecei e s wi h a DCPS se ial in e ace. The applica ion is expec ed o p o ide adequa e suppo o accu a e ou doo campus na iga ion asks. I INTRODUCTION The need o imp o e he accu acy o he posi ion eadings ob ained wi h s anda d Global Posi ioning Sys em (GPS) ecei e s in ou doo na iga ion asks ca ied a ou campus led o he esea ch and de elopmen desc ibed in his pape . One solu ion o imp o e he accu acy o GPS measu emen s is he join use o Global Posi ioning Sys em da a and Di e en ial Global Posi ioning Sys em da a. Recei e s a e hen able o co ela e da a om bo h sou ces ( he GPS sa elli e da a and he DGPS co ec ion da a), g an ing use s highe accu acy eadings, Howe e , while he a ailabili y o wo ldwide con inuous GPS da a ( h ee-dimensional posi ion, eloci y and ime) o use s is gua an eed by he GPS se ice, he same is no ue wi h aDu oD ia e DGPS da a sou ces. The sol e his p oblem we designed and implemen ed a h ee- ie dis ibu ed sys em ha es ablishes ln eme da a links be ween DGPS sou ces and he campus and p o ides a campus-wide DGPS da a dissemina ion se ice. This pape desc ibes in de ail his p ojec . Sec ion 1 in oduces he eade o ou mo i a ion and goal. Sec ion 2 p o ides a b ie desc ip ion o he ea u es o he GPS and DGPS sys ems. Sec ion 3 p esen s he de eloped applica ion and sec ion 4 desc ibes he cu en s a us o he p ojec . Sec ion 5 epo s on ela ed wo k and sec ion 6 p esen s he conclusions. II REAL TIME POSITIONING Nowadays, eal ime posi ioning is an ac i i y suppo ed by sa elli e-based sys ems, namely, he No h-Ame ican NAVSTAR Global Posi ioning Sys em (GPS) and he Russian Global Na iga ion Sa elli e Sys em (GLONASS). Bo h sys ems p o ide con inuous, wo ldwide, h ee-dimensional posi ion, eloci y and ime in o ma ion o end-use s. In he nea u u e, he Eu opean GALILEO sa elli e adio na iga ion sys em is expec ed o become he i s non-mili a y eal ime posi ioning sys em. GPS u ilises he concep o ime-o -a i al (TOA) anging o de e mine he use posi ion. This concep en ails measu ing he ime a signal ansmi ed by an emi e a a known loca ion akes o each a use ecei e . This ime in e al, e e ed o as he signal p opaga ion ime, is hen mul iplied by he speed o he signal ( he speed o ligh ) o ob ain he emi e - o- ecei e dis ance. By measu ing he p opaga ion ime o signals b oadcas ed om mul iple emi e s a known loca ions ( he GPS sa elli es), he ecei e can de e mine i s posi ion. A Di e en ial Global Posi ioning Sys em a ailabili y and adequacy o DGPS da a sou ces depends, no only, on he exis ence o nea by DGpS base - me pe manen ques o highe posi ion eadings accu acy he alidi y and accu acy o he co ec ions depend on he dis ance be ween base and o e , bu also, on app op ia e da a dissemina ion O e he a ea unde conside a ion. led o he O a GPS subsys em he Di e en ial Global Posi ioning Sys em (DGPS). By de ini ion, he DGPS me hod uses well-known geog aphic loca ions as e e ences o de ec he ange e o s o he GPS sa elli es. The me hod elies on a se o s a ions, called As a esul , in o de o inc ease he posi ioning accu acy DGPS base s a ions, equipped wi h elabo a ed GPS ob ained wi h GPS ecei e s wi hin ou campus, ecei e s (12 channel single o dual ecei e s) and si ua ed app op ia e eal ime DGPS da a mus be p o ided. To a p ecisely geo- e e enced loca ions, o compu e and 0-7803-7937-3/03/$17.00 02003 IEEE 512 b oadcas he co ec ions o be applied o he GPS measu emen s (Fig. I). Wi h base s a ions and o e s eco ding obse a ions a he same ime, GPS p ocessing so wa e can educe o elimina e “common e o s” (such as he a iable delays in oduced in he GPS signal when i c osses he ionosphe e and he oposphe e) and by he epheme is and clock e o s o he GPS sa elli es Fig. I -Componen s o a DGPS Sys em. The e a e wo eal ime di e en ial posi ioning me hods: code phase and ca ie phase. DGPS u ilizing code phase measu emen s - pseudo ange co ec ions - can p o ide a ela i e accu acy o a ew me e s (I m o 3 m). DGPS u ilizing ca ie phase measu emen s - eal ime kinema ic (RTK) co ec ions - can p o ide a ela i e accu acy o a ew cen ime es. The ecei ing equipmen and he access ules applied o he di e en ypes o DGPS da a ne wo ks a y. While he DGPS da a ansmi ed ia adio beacon is public, ee and complies wi h s anda d comme cial DGPSGPS ecei e s, DGPS da a b oadcas ed ia sa elli e is p op ie a y, equi es he paymen o an annual ee and he use o speci ic ecei e equipmen . The access o In e ne links depends on he policy adop ed by he se ice p o ide and is in ended o s anda d comme cial DGPSEPS ecei e s. 111 DESIGN AND IMPLEMENTATION The de eloped dis ibu ed sys em implemen s wo main asks: he es ablishmen o he In e ne da a links be ween he DGPS sou ces and he ISEP campus and he campus-wide dissemina ion o he DGPS da a. The o e all applica ion is a h ee- ie clien ise e dis ibu ed applica ion. The i s ie - he clien ie - ep esen s he end clien applica ions (wi eless o ln ane clien s loca ed wi hin he campus). The in e media e laye - he campus se e - e ches and dissemina es he DGPS da a o he end clien applica ions. The hi d laye - he se e s o DGPS da a - ac s as he da a p o ide laye o he in e media e laye . The i s and hi d laye s a e insula ed. This app oach p e en s he po en ial conges ion o he da a sou ce se e s and allows he adop ion o di e en anspo p o ocols be ween he i s and second laye s and be ween he second and i s laye s. In Fig. 2 we p esen an o e iew o he sys em’s a chi ec u e. 4- mU,mI*. h Y E.b*m”” Fig. 2 -O e iew o he Sys em’s A chi ec u e. The applica ion p o ides h ee ypes o da a links: ln ane and wi eless da a links - be ween he i s and second laye s - and In e ne da a links - be ween he second and hi d laye s. A DGPS In e ne Da a Link Applica ion The In e ne link is accomplished h ough a wo- ie clien ise e applica ion. The se e applica ion, loca ed a he hi d ie , is ins alled in a hos a he DGPS base s a ion and is con inuously ed by he DGPS da a gene a ed by he DGPS base s a ion. The in e ace be ween he DGPS base s a ion equipmen and he DGPS da a link se e applica ion is con olled by he DCPS base s a ion equipmen and consis s o a RS232 in e ace. The da a link clien applica ion, which is loca ed a he in e media e ie , has a double ole: i ac s as he In e ne da a link clien and as he inpu module o he campus-wide da a p o ide . 4- OPI s., ,,im DGPlDu&iink s *a Fig. 3 - DGPS ln eme Da a Link Two- ie Applica ion. The implemen ed DGPS In e ne Da a Link applica ion (Fig. 3) p o ides wo ypes o da a links: a ull-duplex, poin - o-poin , connec ion-o ien ed and a simplex, poin - o-mul ipoin , connec ionless ype o communica ion. While he i s link elies on he anspo laye ’s T ansmission Con ol P o ocol (TCP) o p o ide a eliable communica ion se ice, he second ype o link uses he anspo laye ’s Use Da ag am P o ocol (UDP) o p o ide a mul icas message-o ien ed se ice. Addi ionally, bo h links allow mul i h eading, i.e., i is possible o use he 513 se e -side applica ion o p o ide, simul aneously, da a o mul iple clien s. In ou case, since we in end o use mul iple DGPS da a sou ces, he clien applica ion can, simul aneously, connec o di e en DGPS da a se e s. Each se e -side applica ion da a link is uniquely iden i ied by i s po and IP add ess. When he se ice link is suppo ed by TCP, he IP add ess is he hos IP add ess bu when he se ice link suppo ed by UDP, he 1P add ess is he selec ed class D IP add ess (class D IP add esses a e in he ange 224.0.0.0 o 239.255.255.255, inclusi e). The po numbe assigned by he ln eme Assigned Numbe s Au ho i y (IANA) o he dissemina ion o DGPS co ec ion da a is po 2101. c A. I Da a T ansmission The de eloped ln eme Da a Link Applica ion allows da a ansmission o e TCP (unicas ) and UDP (mul icas ). The da a ansmission can be implemen ed using da a ames ( ame mode) o , simply, by es ( aw mode). In ame mode, once a comple e, e o ee RTCM message has been ecei ed by he se e applica ion, a da a ame con aining he indi idual RTCM message and o he addi ional da a is c ea ed and ansmi ed. In he aw mode, he by e se ha cons i u es an RTCM message is ansmi ed by he se e applica ion (a he same a e i is gene a ed by he DGPS base s a ion) wi hou any addi ional p ocessing o e i ica ion. This la e mode was implemen ed in o de o p o ide compa ibili y wi h o he exis ing end-use In e ne DGPS da a clien s. The DGPS Da a Campus Se e expec s o ecei e DGPS da a in ame mode. wn n~~wiq,xnis y"lp, I, A.I.1 F ame Mode When in ame mode, he se e applica ion c ea es a da a ame o each indi idual RTCM message ecei ed. In o de o ob ain a RTCM message om he in o ma ion gene a ed by a DGPS base s a ion, he se e applica ion has o p ocess he ecei ed da a. The DGPS base s a ion ou pu s a con inuous by e s eam ia he se ial in e ace ha mus be collec ed, decoded and assembled in o alid RCTM messages. The by e s eam ecei ed h ough he se ial in e ace needs app op ia e p ocessing o se e al easons: (i) each 8-bi by e only con ains 6 RTCM bi s; (ii) he 6 RTCM bi s a i e in e e se o de (leas signi ican bi i s ); (iii) e e y 30 bi s o RTCM da a holds 6 pa i y bi s ha mus be checked in o de o alida e and decode he RTCM da a. The RTCM messages a e made o RTCM wo ds. Each RTCM wo d is 30-bi long and con ains 24 da a bi s and 6 pa i y bi s. Since one 8-bi by e only con ains 6 RTCM bi s, each RTCM wo d occupies i e 8-bi by es. The wo mos signi ican bi s o each %bi by e ( he 2 non-RTCM bi s should be se o space and ma k (OI), espec i ely. The RTCM messages include, no only, 6 pa i y bi s in each da a wo d bu , also, he da a in he ollowing da a wo d is encoded acco ding o he las 2 pa i y bi s o he p e ious da a wo d. The algo i hm used o compu ei e i y ,; IC3 ~ I .. :.! 3 *. Fig. 4 ~ Da a F ame In ame mode, he clien -applica ion ecei es he ames con aining he RTCM-SC 104 messages and is immedia ely capable o e i ying he quali y o he da a link ansmission as well as he age o he messages being ecei ed. I he da a ames we e ansmi ed wi hou e o s and he messages hey hold a e s ill applicable, he da a ames a e o wa ded o he DGPS da a campus se e applica ion. B DGPS Da a Campus Applica ion The DFPS Da a Campus applica ion p o ides campus-wide DGPS da a bo h ia a adio beacon and ia he campus ln ane (Fig. 5). The idea is o be able o use DGPSiGPS ecei e s wi h and wi hou a DGPS da a wi eless in e ace [ 121. The DGPS/GPS ecei e s wi hou a DGPS da a wi eless in e ace ha e a RS232 DGPS inpu in e ace. In his la e case, a DGPS Da a Campus Clien 514 can be ins alled, o example, in a lap op compu e o ecei e he co ec ions da a and o o wa d i ia he RS232 in e ace o he DGPSiGPS ecei e . Fig. 5 - DGPS Da a Campus Applica ion The se e da a inpu module is he In e ne Da a Link Clien . This inpu module is capable o es ablishing links wi h mul iple emo e DGPS se e s using connec ion-o ien ed o he connec ionless (message-o ien ed) links. Once a link is es ablished, he clien -side applica ion ecei es he da a ei he in ame o in aw ansmission mode. When mul iple da a sou ces a e used, addi ional p ocessing is needed. B. I DGPS Da a Campus Se e The DGPS Da a Campus Se e is composed o h ee unc ional uni s: he da a inpu module (In e ne Da a Link clien ), he ln ane se e and he wi eless se e (Fig. 6). Fig. 6 - DGPS Da a Cnmpu Sewe . The DGPS Da a Can pus Se e ecei es he DGPS da a di ec ly om he DGPS In e ne Da a Link and o wa ds i immedia ely o bo h modules in o de o minimize he ansmission delay. The ansmission delay is c i ical when using RTK co ec ions since he co ec ions upda e a e should no exceed he GPS upda e in e al a e. B. I. I In ane Se e Module The ln ane se e module ecei es he da a ames con aining he RTCM messages, e i ies he quali y o he ansmission and he age o he RTCM messages. I he da a ames a e e o ee and he messages a e s ill applicable, he da a ames a e o wa ded o all clien s connec ed. The ln ane se e mul icas s DGPS da a ames o all ln ane clien s ha join he speci ied mul icas g oup a he applica ion po c ea ed o his se ice. Cu en ly, he da ag ams ime- o-li e (TTL) pa ame e is se o one (jus o he Elec ical Enginee ing Depa men sub-ne wo k). The implemen ed DGPS ln ane Se e es ablishes simplex, poin - o-mul ipoin , message-passing ype o communica ion, The dis ibu ed applica ion elies on he anspo laye Use Da ag am P o ocol (UDP) o p o ide a mul icas se ice wi hin he campus In ane . Al hough UDP has no low con ol mechanism, i is a message-o ien ed p o ocol, i.e., p o ides dynamic alloca ion o ne wo k bandwid h. In ou case, since we need o es ablish simul aneously se e al connec ions wi hin he local campus ne wo k, we adop ed a mul icas ype o communica ion. B. I.2 Wi eless Se e Module In ou case, he clien applica ion ecei es he DGPS co ec ions ia he In e ne link om he DGPS base s a ions and eeds hem o he wi eless se e . The ole o he wi eless se e is h ee old: i s , i o ma s and bu e s he DGPS messages, hen, i modula es he da a on o he ansmi e ca ie and, inally, b oadcas s he esul ing da a. Fig. 7 - Wi eless Se e . The da a modula o ecei es he RTCM co ec ion messages, encodes hem as digi al in o ma ion using a Minimum Shi Keying (MSK) encoding algo i hm, and o wa ds hem o he compu e sound ca d. Some special ca e mus be aken in his p ocessing since he RTCM p o ocol uses 30-hi leng h wo ds and di ides hem in o 6-bi leng h by es ha a e sen o he modula o in s anda d IRA (ASCII) %bi by es occupying he six leas signi ican bi s. The modula o s ips he s a , s op and he wo mos signi ican bi s om his by e and only ansmi s he six RTCM SC-104 da a bi s o e he ai , A his poin he da a consis s o jus he RTCM SC-104 da a p o ocol and he ela ionship be ween he s a o a RTCM 30-bi wo d and an asynch onous IRA by e has been los . E e y ime RTCM da a is sen o he sound ca d, he anscei e con ol uni sends an au oma ic Push-To-Talk (PTT) command o he adio anscei e so ha he RTCM da a ge s b oadcas ed. B. 1.3 Wi eless Do a Link DGPS adio beacons modula e he ca ie wi h he co ec ion da a and o he in o ma ion such as he heal h o he e e ence s a ion and he iden i ica ion o he ansmi e . The modula ion is Minimum Shi Keying (MSK), a special o m o F equency Shi Keying (FSK). In he case o pseudo ange co ec ions, he modula ion a e is usually 100 b/s o 200 b/s. The adio egula ions go e ning he use o his band a e speci ied in he 515 In e na ional Telecommunica ions Union (ITU) Recommenda ion M.823 [IO], inco po a e he RTCM SC-I04 p o ocol and allow he ansmission o supplemen a y in o ma ion (such as he DGPS signals), The ange o an indi idual ansmi e is a unc ion o powe ou pu , an enna heigh and e iciency, as well as he cha ac e is ics o he su ounding land. The b oadcas da a consis s o a selec ed subse o he message ypes con ained in he RTCM Special Commi ee No. 104, "Recommended S anda ds o Di e en ial GNSS Se ice", Ve sion 2.2, da ed Janua y 15, 1998 [I I] and e e ed in his pape as "RTCM SC-104". The co ec ions encoded as MSK and a e hen modula ed on o he ca ie o he adio beacon. MSK encoding esul s in app oxima ely a 125 Hz shi in he ca ie equency o he adio beacon (a 100 his). Du ing no mal ope a ion he minimum ield s eng h o he DGPS b oadcas signal will be 75 pVim in he speci ied co e age a ea, a a ansmission a e o 100 bls. 8.2 DGPS Da a Campus Clien The DGPS Da a Campus Clien is used o p o ide DGPS co ec ion da a o DGPSiGPS ecei e s wi hou a wi eless in e ace. In his case, he clien -side applica ion is used o es ablish he da a link be ween he DGPS Da a Campus Se e and he se ial in e ace o he ecei e . The clien connec s o DGPS Da a ln ane Se e by c ea ing a mul icas socke and by joining he se ice mul icas session (mul icas IP add ess o he hos and he applica ion po o he se ice). Once he link is es ablished, he DGPS da a ames a e ecei ed. Whene e a da a ame is e o ee and he RTCM message i holds is no oo old, he message is immedia ely o wa ded o he RS232 in e ace. 1V STATUS AND RESULTS A e some esea ch, we ound h ee DGPS base s a ions in he icini y (IO km adius a ea) o ou campus. A he ime, none o he p op ie a y en i ies was b oadcas ing DGPS da a in a o ma complian wi h s anda d DGPSGPS ecei e s. One en i y, he ein called en i y A, has a DGPS base s a ion loca ed a a dis ance o app oxima ely 8 km o he no hwes (NW) o ou campus. The DGPS base s a ion ecei e has 12 single equency (LI) acking channels and gene a es RTCM V2.1 messages numbe s I and 3 (pseudo ange co ec ions). Howe e , he adio b oadcas equency band (VHF) and he echnology used a e no complian wi h s anda d GPSiDGPS ecei e s. En i y B uns a DGPS base s a ion loca ed a app oxima ely 6 km o he sou h (S) o ou campus and was collec ing DGPS da a only o pos -p ocessing (I2 hou s RlNEX da a iles). Base s a ion B has 12 dual equency (LI and L2) acking channels and is capable o ou pu ing RTCM V2.1 and V2.2 messages numbe s 1, 2, 3, 9, 18, 19, 20, 21 and 22 (RTK and pseudo ange co ec ions). Cu en ly, as a esul o his p ojec , base s a ion B is al eady gene a ing eal ime DGPS da a (RTCM V2.1 and V2.2 messages numbe s 1, 2, 3, 20, 21 and 22). Now, we a e wai ing o ins alla ion o he ne wo k cable be ween he base s a ion and he nea es ln e ne node. The hi d en i y, he "lns i u o pa a o Desen ol imen o Tecnologico" (IDT), is loca ed a ou campus and uns a DGPS base s a ion equipped wi h a T imble Pa h inde P o XR GPS ecei e wi h 12 single equency (LI) acki lg channels which can p o ide RTCM V2.l messages numbe s 1 and 3 (pseudo ange co ec ions). When his p ojec s a ed, IDT p o ided only nea eal ime da a (hou ly RlNEX da a iles o pos -p ocessing), As a esul o his p ojec , he IDT base s a ion is now a eal ime DGPS base s a ion. Cu en ly, we a e unning a se ies o es s using he IDT base s a ion as he sole da a sou ce. Since i only p o ides di e en ial pseudo ange co ec ions, we can only p o ide posi ioning esul s wi h his ype o co ec ions, As soon as we ha e an In e ne connec ion wi h en i y B, we will be able o show esul s using RTK co ec ions. The es s we e ca ied ou using wo Ga min GPS/DGPS ecei e s: a Ga min 176 (G176) and a Gannin 35 (G35). Bo h ecei e s accep DGPS co ec ion ia he se ial in e ace. To compa e he accu acy ob ained wi h and wi hou he applica ion o he pseudo ange DGPS co ec ions, he ecei e s emained s a ic h ough ou he es and we e posi ioned side by side. While he G35 only ecei ed GPS da a, he G176, which was connec ed ia he se ial in e ace o a DGPS Da a Campus Clien , ecei ed bo h GPS and DGPS da a. In he absence o di e en ial co ec ions (Fig. 8), he la i ude, longi ude and ele a ion s anda d de ia ion alues we e 1.538 m, 1.335 m and 2.561 m. Wi h he applica ion o di e en ial co ec ions (Fig. 9) du ing he same pe iod o ime, hese alues d opped o 0.899 m, 1.023 m and 2.042 m, espec i ely. Fig. 8 -GX wi hou di e enlial comeclions While in he case o he G35 ecei e , he mean a e age posi ioning alues we e 41" 10.7493 N, 8'36,4708 W and 516 133.19 m (ele a ion), in case o he (3176, hey we e 41” 10.7467 N, 8’36,4713 Wand 132.60 m (ele a ion). Fig. 9 -GI 76 wi h di e en ial co ec ions The posi ioning esul s ob ained a e shown using VisualGPS, a copy igh ool o displaying GPS posi ioning da a om VGPS [ 161. Addi ionally, we ha e also used exis ing on-line DGPS se e s loca ed in o he pa s o he globe. We we e able o es ablish success ul da a links be ween ou DGPS campus-se e applica ion and hese emo e da a sou ces. Al hough he da a ecei ed was no app op ia e o ou campus ( he ecei ed co ec ions we e o sa elli es ou o sigh ), i was possible o e i y he compa ibili y be ween sys ems. We a e unning in ensi e es s, collec ing sys em pe o mance indica o s and hope o be able o p o ide a de ailed sys em e alua ion in a nea u u e. So a , we ha e been able o e i y ha he implemen ed unc ionali ies a e wo king and ha he da a collec ed suppo s his claim. Meanwhile, he au oma ic con ol o he anscei e and he implemen a ion o he PCh anscei e in e ace ha e been es ed wi h success. V RELATED WORK The idea o dissemina ing RTCM co ec ions o e he In e ne in eal ime o p ecise di e en ial posi ioning and na iga ion pu poses was also in es iga ed by W. Rupp ech [15]. In 1999, Rupp ech de eloped a DGPS da a se e called DGPSIP ha dissemina ed DGPS da a ecei ed h ough a adio in e ace a an a e age o 284 bis. The adio was (and s ill is) no mally uned o POINT BLUNT, CA, Coas Gua d ansmi e : he se e sends ou a packe oughly e e y second wi h 35 by es o da a and 40 by es o IP heade . The DGPSIP is a mul i h eaded se e , suppo s up o 64 concu en connec ions and also ansmi s mul icas UDP co ec ions (cu en ly on 224.0.1.235 po 2101). In 2002, he EUREF (Eu opean Re e ence F ame) which is a Sub-Commission o IAG’s (In e na ional Associa ion o Geodesy) Commission X on Global and Regional Geode ic Ne wo ks, decided o se up and main ain a di e en ial GNSS in as uc u e (DGNSS) on he In e ne using s a ions o i s Eu opean GPSGLONASS Pe manen Ne wo k (EPN). The objec i e was o dissemina e RTCM co ec ions o e he In e ne in eal ime o p ecise di e en ial posi ioning and na iga ion pu poses [5]. The ac onym o hese ac i i ies is EUREF-IP (IP o In e ne P o ocol). DGNSS ial se e s cu en ly p o ide RTCM co ec ions as gene a ed in a numbe o Eu opean coun ies. The EUREF p o ides since 2002 ee clien so wa e o access he app op ia e da a s eams. This implemen a ion ansmi s he DGPS co ec ions h ough he In e ne using he s anda d RTCM SC-104 p o ocol o e TCP. Mo e ecen ly, se e al esea ch eams ha e p oposed and designed sys ems ha use o GPRS as ye ano he suppo echnology o he dissemina ion o DGPS da a. VI CONCLUSION In o de o ha e access, wi hin ou campus, o DGPS co ec ion da a, we de eloped a h ee- ie clien lse e dis ibu ed applica ion. The i s ie - he clien ie - ep esen s he end clien applica ions (wi eless o n ane clien s loca ed wi hin he campus). The in e media e laye - he campus se e - e ches and dissemina es he DGPS da a o he end clien applica ions. The hi d laye - he se e s o DGPS da a - ac s as he da a p o ide laye o he in e media e laye . The i s and hi d laye s a e insula ed. This app oach p e en s he po en ial conges ion o he da a sou ce se e s and allows he adop ion o di e en anspo p o ocols be ween he i s and second laye s and be ween he second and i s laye s. As a esul , o he campus da a dissemina ion we use mul icas and o he In e ne da a links we p o ide bo h connec ion-o ien ed (TCP) and message-o ien ed (mul icas ) ypes o communica ion. Fu he mo e, he da a can be sen in ame mode o in aw mode. Whe eas he by e s eam mode was kep o easons o compa ibili y wi h exis ing DGPS clien so wa e applica ions ( o example, he clien so wa e p o ide by [5] and [15]), he ame mode was designed o allow he simul aneous connec ion wi h mul iple DGPS sou ces and o p o ide an almos ins an aneous way o checking i he ansmission was e o ee and o ge ing he age o he RTCM message. The p elimina y esul s we ha e ga he ed so a ha e shown ha he se ice p o ided by he o e all dis ibu ed sys em p o ides low-cos suppo o accu a e ou doo campus na iga ion. We expec o u he inc ease he posi ioning accu acy wi hin ou campus as soon as we ha e access o a di e en ial RTK da a sou ce (en i y B). Ou cu en wo k is ocused on es ing and e alua ing he sys em he ein desc ibed. 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