PRECISE ORBIT DETERMINATION OF THE CHAMP SATELLITE WITH STAND-ALONE GPS. Sunil B. Bisnath and Richard B. Langley

Transcription

1 PRECISE ORBIT DETERMINTION OF THE CHMP STELLITE WITH STND-LONE GPS Sunil B. Bisnah and Richard B. Langley The European Navigaion Conference 2002 GNSS May 2002 Copenhagen, Denmark

2 OVERVIEW Inroducion of single-receiver processing echnique. Descripion of UNB echnique: Principle. Modelling. Spaceborne daa processing: Measuremen descripion. Resuls. nalysis. Conclusions and furher research.

3 SINGLE RECEIVER PROCESSING Purpose of research: To develop processing sraegy o provide precise posiioning of any plaform using a single, geodeic-grade GPS receiver. Raionale for capabiliy: Ress in desire o remove requiremen of reference receiver(s) for high-precision posiioning. Processing echnique possible because: GPS can provide direc, coninuous, accurae, 3-dimensional posiioning. High-qualiy GPS daa producs (orbis and clocks) available. Removal of Selecive vailabiliy (S). Have developed GPS-only echnique make no assumpions regarding plaform dynamics hence plaform-independen.

4 PROCESSING STRTEGY () Purely geomerical sae-space, GPS-based soluion. Simulaneously: Uilize code daa o compue mobile receiver posiion, and carrier daa o compue mobile receiver posiion change, in a kinemaic, sequenial, leas-squares filer/smooher. Inpus (all readily available aside from mobile receiver daa): Dual-frequency code and carrier measuremens from mobile receiver (preferably high-rae). Precise GPS consellaion ephemerides. Precise GPS consellaion saellie clock offses from GPST. Inerpolae GPS-deermined posiions for mobile receiver sae hroughou rajecory.

5 PROCESSING STRTEGY (2) No need for: Reference receiver or reference receiver nework ha informaion basically provided by IGS daa producs. ssumed dynamic models ime-differenced, carrier-phase observaions precisely measure moion. Eniled: Carrier-phase-conneced, pseudorange poin posiioning.

6 CRRIER-PHSE-CONNECTED, PSEUDORNGE POINT POSITIONING GPS measuremen low precision (0s of cm), unambiguous pseudoranges high precision (sub-mm), ambiguous carrier phases decimere accuracy measuremen consiuens which mus be esimaed or eliminaed parially addressed fully addressed saellie posiion error saellie clock receiver clock roposphere ionosphere mulipah receiver noise saellie posiion error saellie clock receiver clock roposphere ionosphere mulipah receiver noise carrier-phase ambiguiy

7 FLOWCHRT OF DT PROCESSING IGS GPS precise ephemerides and clock offses daa preprocessing plaform GPS receiver daa filer / smooh daa plaform sae and sae covariance inerpolae beween posiions plaform rajecory

8 SCHEMTIC OF FILTER PRINCIPLE code and carrier measuremens -, iniial sae and covariance esimae from code - compue funcional and sochasic models -, esimae filered sae -, sae covariance esimae sae esimae -, sequenial processing

9 FILTER/SMOOTHER EQUTIONS () Linearized observaion equaions: + =, 0 ε e δx δx δφ δφ P P 0 0 C δφ C P Leas-squares soluion: ˆ ˆ x x ( ) T T T T 0 0 δ δ δ δ + + = C C C C C C x x Φ p Φ Φ x Φ + δ δ δφ Φ P P δφ Φ w C w C w C T T

10 FILTER/SMOOTHER EQUTIONS (2) Pseudorange measuremen conribuion: P P 0 = δx + e ; C P Carrier-phase measuremen conribuion: δφ δφ 0 = δx + δx + ε, ; C δφ Opimal smoohed soluion: ˆ ˆ x = C x + C xˆ s f f b b

11 DDITIONL MODELLING Observables are ionosphere-free, undifferenced pseudorange and ionosphere-free, ime-differenced carrier phase. Modelling consideraions more acue han for relaive posiioning: Relaivisic GPS SV clock correcion (due o orbial eccenriciy). GPS SV anenna phase-cenre o cenre-of-mass offse. GPS SV phase wind-up (especially for large sampling inervals of differenced phase). Model compaibiliy wih IGS producs.

12 BENEFITS OF SPCEBORNE GPS The following capabiliies can be realized by placing a GPS receiver modified for spacefligh (a spaceborne GPS receiver) aboard a Low Earh Orbier (LEO) spacecraf: Orbi deerminaion (posiion and velociy). Real-ime [few-mere-level]. Pos-processed [decimere-level]. iude deerminaion [sub-degree-level]. Timing [sub-microsecond-level]. Ranging informaion beween spacecraf and GPS saellies for GPS signals ransiing he amosphere (limb sounding).

13 BENEFITS OF GPS-EQUIPPED GEOSCIENCE SPCECRFT Graviy field recovery and geoid deerminaion. Radar and laser alimery. Inerferomeric Synheic perure Radar mapping. Oher remoe sensing insrumenaion posiioning. Time-agging and synchronizaion of scienific sensor measuremens. Ionospheric limb sounding. Tropospheric limb sounding. Sea-surface profiling.

14 SPCEBORNE DT TESTING Daa descripion: Orbis of he dual-frequency daa from he CHMP saellie. JPL BlackJack dual-frequency receiver, racking up o eigh SVs. Zenih-mouned POD anenna on choke-ring. January 5 h second daa inerval. No elevaion angle mask, bu signal-o-noise mask used. IGS precise GPS saellie orbis and clock offses. Processing: Invesigae he geomeric srengh of measuremens. Tes pracicaliy and performance of echnique agains high-qualiy CHMP convenional dynamic orbi soluions.

15 CHMP STELLITE ND SMPLE GROUNDTRCK Near-circular, polar orbi liude 450 km Orbial Period 90 minues srium

16 MESUREMENT QULITY NOMLIES: ISSUES Measuremen gaps in CHMP RINEX files. Poor qualiy measuremens a low elevaion angles rapid degradaion. Periods of unexpeced behaviour in all racked carrier-phase observables. Enire carrier-phase oulier arcs. Individual epochs of single carrier-phase measuremen ouliers no relaed o cycle-slips. Periods of unexpeced behaviour in all racked pseudorange observables. Enire pseudorange oulier arcs. Daa gaps in IGS precise GPS clock offse esimae files. Given all of hese measuremen ouliers, a grea deal of pre- and posprocessing mus be carried ou.

17 MESUREMENT QULITY NOMLIES: EXMPLES Geomery-free phase r.o.c. (m/s) Periods of unexpeced behaviour in all racked carrier-phase observables Widelane phase minus narrowlane code r.o.c. (m/s) Enire carrier-phase oulier arcs. Enire pseudorange oulier arcs.

18 DT VILBILITY mean sd original # SVs edied # SVs PDOP 2.6.0

19 PROCESSING SOLUTIONS: CODE-ONLY, FILTERED ND SMOOTHED norm diff. (m) UNB - JPL code-only forward filer backward filer smooher max r.m.s smooher Code-only for. filer back. filer

20 POST-FIT RESIDULS res. (cm) iono-free pseudorange iono-free phase diff. r.m.s

21 ORBIT SOLUTION COMPRISON: DYNMIC JPL ND DYNMIC GFZ (24 HRS) diff. (cm) radial along cross norm max. bias r.m.s Processing ime: hours.

22 ORBIT SOLUTION COMPRISON: GEOMETRIC UNB ND DYNMIC JPL (24 HRS) diff. (cm) radial along cross norm max bias r.m.s Processing ime: minues.

23 ORBIT SOLUTION COMPRISON: GEOMETRIC UNB ND DYNMIC JPL (4 HRS) diff. (cm) radial along cross norm max. bias r.m.s

24 CONCLUSIONS Kinemaic, sequenial leas-squares filer described. Dynamics-free naure of filer allows for applicaion o any plaform. Near-decimere posiion componen r.m.s difference beween UNB geomeric soluions and dynamic soluions in he along-rack and crossrack componens. Comparison in radial componen a he few-decimere r.m.s. level. Very low processing coss few minues of processing ime wih only CHMP GPS measuremens and RINEX GPS ephemeris and clock daa versus hours wih convenional dynamic processing.

25 FURTHER RESERCH Process addiional CHMP daa. Improve sochasic models in filer. dd some form of residual analysis o remove remaining measuremen ouliers. Simulae real-ime processing wih IGS prediced GPS ephemeris and clock producs.