Acquisition and Tracking of NavIC L5 band signals

Transcription

1 Acquston and Trackng of NavIC 5 band sgnals Manjula T R Assstant Professor, Department of lectronc and communcaton engneerng, Jan Unversty, School of ngneerng and Technology, Global Campus, Bangalore, Karnataka, Inda. Orcd Id: Raju Garudachar Professor, Department of lectronc and communcaton ngneerng, Jan Unversty, School of ngneerng and Technology, Global Campus, Bangalore, Karnataka, Inda. Abstract NavIC (Navgaton wth Indan constellaton) s Indan regonal navgaton satellte ndgenously developed by Inda to cater to the needs of publc and mltary n the navgaton feld wth satelltes fully operatonal n orbts snce October 016. To extend NavIC utlty n number of applcatons rangng from navgaton to onosphere studes n Inda and worldwde, regular evaluaton and upgradaton of IRNSS system s sgnfcant. As a frst step n ths vew, real tme NavIC 5 band data s theoretcally analysed and processed n Matlab for sgnal acquston and trackng and the results of sgnal acquston and trackng are presented wth analyss. Further the lmtatons of sgnal trackng loop under drastc atmospherc condtons are hghlghted. Keywords: carrer trackng, code, navgaton, phase dscrmnator, Costas loop, loop flter INTRODUCTION Wth every country comng up wth ts own Navgaton system exhbtng self relablty n the navgaton feld, Inda s one among many, Amerca (GPS), Chna (Bedou), Russa (GONASS), Japan (Quas Zenth), SA( Galleo) whch has developed NavIC navgaton system to meet the regonal demands n the navgaton feld NavIC navgaton system consttute a constellaton 7 satelltes (1A, 1B, 1C, 1D, 1, 1F & 1G) wth 3 n geo synchronous orbts and 3 n geostatonary orbts contnuously (sec) transmttng navgaton sgnals contanng coded almanac and ephemers data [11]. NavIC recever on the user sde process the receved sgnal and navgaton data s extracted. Navgaton data s utlzed n the poston determnaton algorthm [6] to locate user poston. Navgaton sgnals are broadcast from NavIC constellaton n 5 ( MHz) and S (49.08MHz) band. The user equpped wth IRNSS recever operates on the receved sgnal to determne user poston. Although sgnals from 4 satelltes suffce to compute user poston, 3 more satelltes extend global coverage upto 1500km and further to compensate for sgnal outages caused durng unfavourable atmospherc condtons [8] NavIC SIGNA CHARACTRISTICS Navgaton nformaton consttutng phemers and Almanac data that descrbes orbtal moton and poston of satelltes n orbts s a 50 Hz navgaton data, encrypted by CA (Course acquston) code whch has chppng rate of 1.03MHz. Further t s BPSK modulated by 5 carrer sgnal. The transmtted 5 sgnal s gven by [5] where ( t ) D ( t ) Sn ( w )... 1 X ( t ) A CA t 5 CA course acquston code of th satellte D navgaton data of th satellte W 5 5 radan frequency ( *p*f 5) A Sgnal magntude ach NavIC satellte has unque dentfcaton number [1] referred to as CA (course acquston) code also called as pseudo random code (PRN) code, facltates multple access of satellte sgnals through code dvson multple access(cdma). Sgnal Acquston Sgnal acquston s a 3D process of detectng satellte sgnal and ts correspondng carrer frequency and code phase of the PRN code from the receved sgnal. The navgaton sgnal transmtted from satellte s recorded from the recever antenna. The sgnal s down converted nto Intermedate frequency (IF) n the front end of the recever [5]. IF data conssts of navgaton sgnals from all NavIC satelltes. Further IF data s processed n the recever to dentfy the vsble satelltes. Ths s accomplshed n sequence wthn a 8115

2 loop by frst wpng off the carrer by demodulatng wth replcated carrer sgnal and subsequently wpng off PRN code by correlatng the receved PRN/CA code wth replcated PRN (CA) codes of all satelltes [7] resultng n In phase(i) and quadrature phase correlator (Q) components The correlaton power (I + Q ) s measured for more than 1 code perod (N) s squared and accumulated. M such correlaton are averaged and compared aganst threshold. If resultant correlaton magntude s greater than threshold, satellte sgnal s detected; the carrer frequency values and code phase are extracted for the detected satellte. The correlaton power [4] s gven by R ( m) M k 1 N n1 IMPMNTATION x( n). ca[ n].cos( wn) N n1 x( n). CA[ n]. Sn( wn). Sgnal acquston s coded as search algorthm for carrer frequency and code phase at the pont of synchronzaton between the receved sgnal and replcated sgnal. For a samplng rate of 56MHz, one CA code perod has samples, the replcated carrer frequency at IF frequency s shfted n ncrements of 15Hz over Doppler frequency range of ±10KHz to have 81 frequency search bns and multpled wth receved carrer sgnal. Subsequently, receved CA code samples of 1ms (.e., samples) s correlated wth replcated CA code by crcularly shftng whch s equvalent to crcular convoluton. Ths correlaton between receved sgnal and the replcated sgnal results n correlaton peak power larger than the threshold for the detected satellte and the correspondng carrer frequency and code phase at the pont of correlaton peak s retreved; acquston results are tabulated n Results secton DFT/FFT tool s used to effectvely mplement the crcular convoluton, as crcular convoluton s equvalent to multplcaton n frequency doman. It s mathematcally expressed as R[ m] x[ n] CA[ n] F Sgnal Trackng 1 F( x[ n]. F( CA[ n])... 3 Once satellte sgnals are detected n sgnal acquston stage, contnuous trackng of the satellte sgnal s equally sgnfcant for further decodng and extracton of navgaton data and s referred to as sgnal trackng. The need for sgnal trackng arses from the fact the carrer sgnal experences frequency devaton due to Doppler shft. Relatve moton between satellte and recever nduces Doppler frequency offset n the carrer frequency and for statonary recever t s assumed to be ±10KHz. Smlarly CA code phase samples also undergo shft. Sgnal trackng s about detectng frequency and code shft of the detected satellte sgnal and synchronze wth the replcated sgnals wth the am of retrevng navgaton data. Ths s accomplshed n two stages 1. Carrer trackng loop. Code trackng loop Carrer trackng loop Carrer trackng loop tracks the frequency and phase of the receved sgnal by detectng the phase error between replcated sgnal and ncomng sgnal and accordngly replcated sgnal produced by numercally controlled oscllator (NCO) s adjusted to synchronze wth ncomng sgnal n both frequency and phase. For zero phase error detected, navgaton data s accurately extracted. Arctangent dscrmnator gven by atan(q/i) s used as phase dscrmnator (PD) to detect phase error angle between I and Q component. Though several phase dscrmnators [] are avalable, Atan PD has lnear output over half of the nput error range (±90 0 ) and hence preferable over other types. The PD output s fltered for nose by loop flter and drves the NCO sgnal frequency towards the ncomng carrer sgnal frequency. The loop flter parameters []: flter order, dampng rato and band wdth determne P s ablty to flter out the nose and track hgh sgnal dynamcs Code trackng loop Post the carrer sgnal synchronzaton, receved CA code samples s synchronzed by algnng wth replcated CA code samples by shftng rght or left. To determne the drecton of shft, the I and Q outputs are multpled wth prompt code (PRN code whch s phase algned), early code (prompt PRN code shfted by some samples to the rght) and late code (prompt PRN code shfted by some samples to the left) resultng n correspondng to I and Q channel respectvely. A code dscrmnator functon s gven by equaton 4, generates error ( proportonal to the code phase error between the replca and ncomng sgnal. Ths error s fltered and appled to code generator and output frequency s ncreased or decreased and accordngly the prompt code s phase shfted to be phase algned wth receved one. A code dscrmnator s gven by ( I Q ) I Q I Q I Q...4 The above two loops functon n sequence and satellte sgnal s tracked and navgaton data s extracted. The combned carrer and code trackng loop s mplemented as a basc 8116

3 P/Costas loop wth some modfcatons n archtecture as llustrated n the Fgure. 1. Satelltes (PRN) detected Table 1: Sgnal Acquston Results Acqured Frequency(Hz) Doppler frequency(hz) C/A Code Offset (samples) e e e e e e Fgure 1: Combned carrer and CA code trackng loop Matlab mplementaton of sgnal tackng NCO of the Carrer trackng loop and code generator of the code trackng loop s ntalsed to carrer frequency and code phase shft respectvely, acqured from sgnal acquston stage. The loop runs updatng the NCO and code generator at every Integraton tme (1ms) based on the dscrmnator output, remanng phase lock onto the sgnal, smultaneously retrevng the navgaton data [3]. Performance analyss of carrer trackng loop Performance of carrer trackng loop s measured n terms of ablty of the loop to mantan phase lock onto the receved sgnal and capablty of trackng hgh sgnal dynamcs. The carrer trackng loop tres to mantan phase lock of the sgnal as long as phase errors are wthn boundary. The domnant source of phase error n Costas P based recever s thermal nose phase jtter and dynamc stress error [9]. In order to mantan phase lock, the total phase error of the carrer trackng loop must not exceed one fourth the phase pull n range of arctangent dscrmnator of Costas loop. RSUTS AND DISCUSSION Real IF data for analyss was collected from IRNSS UR recever nstalled at IRNSS laboratory n Jan Unversty Campus. IF data at an ntermedate frequency of 16.1MHz s sampled to obtan dgtsed IF samples whch are processed wth acquston and Trackng algorthms mplemented n MATAB Fgure : Correlaton peak power of PRN3 satellte sgnal Wth respect to Sgnal trackng, the n-phase (I) and Quadrature phase (Q) correlaton power for arly () Prompt (P) and ate () channel s plotted n the Fgure.3 mples the maxmum power dstrbuton n the Prompt channel from where navgaton data s extracted. The performance of trackng loops s assessed wth respect to carrer phase error and code phase error. On an average, the phase resdual of P s wthn ±0.06 and code phase error s wthn ± 0.8 chps as llustrated n Fgure.4. & 5. respectvely, for an ntegraton perod of 1msec. The navgaton bnary data plot n the Fgure. 5 llustrates navgaton data was successfully extracted Table.1. descrbes the acquston results wth satellte ID, correspondng acqured frequency, Doppler frequency and C/A code offset. The magntude of correlaton power s plotted aganst Doppler frequency bn and code samples n Fgure. For the recorded msec of 5, IF data, satelltes PRN1,, 3, 4, 5, & 6 are detected. Fgure 3: Correlaton power plot of I and Q channel 8117

4 CONCUSIONS NavIC navgaton system has set started to be used n wde number of applcatons. As a performance study, real tme NavIC 5 band sgnals are processed n Matlab for sgnal acquston and trackng wth successful extracton of the navgaton data. Further the work can be extended to compute user poston by utlzng the navgaton data and analyse the performance of the carrer trackng loop under drastc sgnal condtons Fgure 4:. Phase error plot of carrer trackng loop ACKNOWDGMNT Authors are grateful to Indan space research organsaton (ISRO) for facltatng the Jan Unversty wth IRNSS recevers whch enabled to execute ths paper. And thanks to Jan Unversty for provdng all necessary help and academc encouragements Fgure 5:. Phase error plot of code trackng loop Fgure 6:. xtracted Navgaton bnary data RFRNCS [1] Indan Regonal Navgaton Satellte System, Sgnal In Space ICD for Standard Postonng Servce (Verson 1.0, ISRO-IRNSS-ICD-SPS 1.0), Indan Space Research Organzaton, June 014, avalable at: []. D. Kaplan, C. J. Hegarty (ds.), "Understandng GPS: Prncples and Applcatons, Artech House, nd dton, 006. [3] Borre, D. Akos, A Software-Defned GPS and GAIO Recever A Sngle Frequency Approach, Brkhauser Boston,006 [4] Fredrk Johansson, Rahman Mollae, Jonas Thor, and Jorgen Uustalo, GPS satellte sgnal acquston and trackng, Dvson of Sgnal Processng, ulea Unversty of Technology, Sweden, August 1, [5] Rshja Msra and Shubham Palod., Code and Carrer Trackng oops for GPS C/A Code, Internatonal Journal of Pure and Appled Scences and Technology [6] James Ba, Yen Tsu, Fundamentals of Global Postonng System Recevers: A Software Approach, John Wley & Sons, 000 [7] Faln Wu, and Kefe Zhang, GPS sgnal Acquston and trackng usng software GPS recever,proc.,, I, 005 [8] Manjula T R, G Raju, Comphrensve study of near Kalman flter Based Trackng technques under Ionosphere scntllatons, proc. RARST 016,, March 016 [9] Sopha Y. Zheng, Sgnal acquston and trackng for a software GPS recever, Thess for Master Degree, Department of lectrcal engneerng, Blacksburg, Vrgna,

5 [10] S. Thombre, M. Z. H. Bhuyan, S. Söderholm, M. Krkko-Jaakkola,. Ruotsalanen, H. Kuusnem, Investgatng the Indan Regonal Navgaton Satellte System usng a Software Mult- GNSS Recever n urope ", IT, 016. [11]