Weak-Signal, Fast-Acquisition GPS Receiver Technology for Space Applications: Navigator GPS Receiver

1. Weak-signal, fast-acquisition GPS receiver technology for space applications: Navigator GPS receiverLuke WinternitzCode 596 TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAAA

2. Operating “above the constellation” • GPS receivers are standard guidance components for LEO missions • Prior technology targets LEO only • Signals too weak and sparse beyond LEO

3. Talk outline • GPS in GEO/HEO: what is needed to operate effectively above the GPS constellation (3 slides) • GPS signal processing • Basic concepts for GPS (3 slides) • Standard approach to acquisition and its limitations (11 slides) • Detection theory approach (9 slides) • Navigator GPS Overview (8 slides) • Applications/Simulations (10 slides) • (44 slides with content, 55 total)

4. GPS signal strength at GEO

5. GPS signal strength in lunar transit Navigator Sensitivity + Passive Antenna Navigator Sensitivity + 10 dB Antenna Navigator Duo Sensitivity + 10 dB Antenna 35 dB-Hz C/N0 with 10dB antenna 25 dB-Hz 15 dB-Hz

6. Summary of introduction • Traditional GPS receiver design gives sensitivity of 35dB-Hz • To operate effectively at GEO and other moderately high altitude above constellation apps requires 10 times improved sensitivity 25dB-Hz • To operate effectively in cislunar transit desires 100 times improved sensitivity 15dB-Hz

7. Talk outline • GPS in GEO/HEO: what is needed and how to get there. • GPS signal processing • Basic concepts for GPS • DSSS comm. systems, and an SNR approach to detection • Detection theory approach • Navigator GPS Overview • Applications/Simulations

8. R2 R1 R3 GPS positioning concept • In two dimensions, range to three known reference points gives position. Two ranges often suffice. • In three dimensions, three ranges suffice • If each range is subject to a common bias then four suffice. • This is how GPS works.

9. GPS subframe start transmitted Subframe start seen at Rx Measurement GPS pseudorange measurement Four unknowns need four equations A similar set of equations arise for velocity involving Doppler measurement and clock rate bias.

10. GPS system • Constellation of 24-32 satellites in 12hr circular orbits • Below the constellation, a user should see 8-16 signals at all times • The GPS system provides a data message containing precise ephemerides (positions and velocities) for the GPS satellites • With four GPS signals, receiver position, velocity and time can be determined • Fewer than four signals can still provide very useful information, particularly if the receiver dynamics are very predictable, e.g., in orbit

11. Talk outline • GPS in GEO/HEO: what is needed and how to get there. • GPS signal processing • Basic concepts for GPS • GPS signal structure, and traditional approach to acquisition. • Detection theory approach • Navigator GPS Overview • Applications/Simulations

12. -1 +1 1ms periodic 1023 bit (chip) C/A Code : 20 per data message bit -1 +1 GPS C/A code signal 20ms +1 GPS Data Message 50bps 1.023Mbps Carrier 1.57542 GHz +1 -1

13. Received signal • Transmitted signal • Passes through channel which adds delay, Doppler and wideband noise • Receiver down-converts and samples the result

14. Receiver task • Acquisition • Determine presence/absence of particular GPS signal, and obtain estimates of Doppler frequency shift and time delay (!, ¿) • Very difficult, involves large amount of processing as we will see • Tracking • Provides continuous estimates of Doppler, delay and carrier phase (!, ¿, µ) needed for data demodulation and for making the fundamental pseudorange measurement • This task is relatively easy, involving only a simple feedback control loop • Current technology receivers are limited by their acquisition sensitivity: 25dB-Hz signals can be tracked but not acquired with standard techniques • Two points • We are interested in unaided cold-start acquisition • Ignore data bits for now

15. Antenna RF Front End Microprocessor Tracking Loop Software Navigation Software Accum Removes Carrier (1.57542 GHz) Replica GPS signal generator Correlator GPS correlator

16. Traditional acquisition:serial search true signal replica/test signal

17. Example acquisition

18. +1 -1 Correlation power and grid spacing • Rules of Thumb: • |¢¿| <0.25 chips • |¢!|<0.25/T Hz • For T=1ms correlations spacing = 250Hz • For T=10ms correlations spacing = 25Hz • Weak signal acquisitions require large T and thus have decreased bin size and more bins. 1023 chip periodic

19. Going from 1ms to 10ms integration time SNR improves 10-fold Doppler spacing decreases 10-fold Dwell time increases 10-fold Acquisition time increases 100-fold 20min acquisition time becomes 2000min! Increasing T to 10ms

20. Calculation for acquisition times • Time delay spacing • 1023 chips checked in ½ chip increments = 2046 delay divisions • Doppler • LEO: 140kHz Doppler range checked at 250Hz spacing = 560 • GEO: 20kHz Doppler range checked in 25Hz bins = 800 • Acquisition time for a single GPS signal • LEO: 1ms x 2046 x 560 = 19min • GEO: 10ms x 2046 x 800 = 272min • HEO: even worse! • (multiple correlators will usually cover the GPS satellite dimension in a cold-start scenario)

21. Signal dynamics, another limit on T In reality, we are looking for a moving target The signal moves in the search space due to dynamics between the receiver and GPS transmitter Longer integration times imply less tolerance to signal dynamics

22. Limits of traditional design • Traditional receivers use T=1ms correlations and serial search algorithms, this results in: • Cold-start acquisition times can be very long • Acquisition sensitivity limit around 35dB-Hz • Increase of integration time is implausible without external aiding • Need better acquisition algorithm/hardware • use multiple independent correlators • or seems reasonable to try

23. Talk outline • GPS in GEO/HEO: what is needed and how to get there. • GPS signal processing • Basic concepts for GPS • Traditional approach to GPS acquisition and limitations • Detection theory approach • Navigator GPS Overview • Applications/Simulations

24. Detection theory approach • Is the correlation approach the best that can be done? • We want to be as efficient as possible, would like some theoretical guidance. • Systematic and generic approach to the problem is through statistical detection or hypothesis testing theory • Binary hypothesis testing problem: which of two possible distributions (states of nature) generated the data

25. Likelihood ratio test • Optimal tests are always Likelihood-ratio tests • Threshold set according to different criteria to essentially trade off false alarms and missed detections.

26. Composite tests • Testing between two parametric classes of probability distributions defined • Optimal test does not exist! • Extremely popular approach: generalized LRT (GLRT): estimate the unknown parameters by maximum likelihood, do binary test. We need estimates for (w,t) anyway.

27. Example of GLRT concept

28. Under a Gaussian noise model • The GLRT test boils down to a threshold test on • Precisely the parallel correlation statistic we already were considering! • Not only is the parallel search necessary for practical implementation of GPS acquisition, it is the GLRT • With a statistical model, we are able to set thresholds and compute expected performance

29. Predicted performance for GLRT (parallel search) Threshold set to enforce 5% false alarm probability when searching 10kHz frequency swath 10 dB increase in T gives 10dB increase in sensitivity

30. Data message bits limit integration time T to 20ms Must avoid correlating across a GPS data bit transition to prevent unpredictable cancellation of correlation power 20ms +1 GOOD BAD -1 This sets an upper limit on T to one GPS data bit period:

31. BAD GOOD “Half-bits” technique Compute two consecutive 10ms correlations, one is guaranteed to be free of transitions Repeat and add up squared magnitudes (to remove data bits)

32. Performance of non-coherent integration Threshold set to enforce 5% false alarm probability when searching 10kHz frequency swath 10 dB increase in T gives only 5dB increase in sensitivity

33. Talk outline • GPS in GEO/HEO: what is needed to operate above the GPS constellation • GPS signal processing • Basic concepts for GPS • Traditional approach to GPS acquisition and limitations • Detection theory approach • Navigator GPS overview • Applications/simulations

34. Navigator specifications/goals • Build a HEO qualified GPS receiver • Stringent radiation requirements (> 100 kRad) • Implement algorithms in FPGAs • Allows for modification, upgrade, and customization • Acquisition requires no knowledge of receiver’s position/velocity/clock - truly autonomous • Acquire and track weak signals down to 25 dB‑Hz • Acquire GPS signals quickly • Within one second for strong signals (>40 dB‑Hz) • Within one minute for weak signals (<40 dB‑Hz)

35. Methods • Acquisition sensitivity achieved by an extended integration period • Acquisition speed attained with FFT based circular correlation and specialized algorithms and hardware • Employs “frequency domain” correlation of the C/A code • Employs handful of tricks to improve efficiency: can simultaneously search a grid of more than 700,000 test bins • Many details of implementation follow key paper by Psiaki [2001] • Improved tracking sensitivity using standard PLL/DLL methods with correlations extended to 20ms

36. Navigator acquisition – parallel search • Entire delay dimension and large bands of Doppler dimension processed in 1ms using stored 1ms block of data • Computing a 10ms correlation is done by adding up 10 consecutive 1ms correlations coherently • Longer integrations use half-bits method of non-coherent integration

37. Acquisition modes • Strong Signal Mode • Based on 1 ms correlation • Effective to 40 dB-Hz, appropriate for “below the constellation” • Can complete 140kHz Doppler search for all signals in less than 0.5 seconds • Weak Signal Mode • Based on half-bits method described previously • Can search 10kHz Doppler at a time at 25Hz granularity (>700,000 grid points) • 300ms integration time achieves 25dB-Hz • Algorithms embedded in FPGA • Commanded with GPS satellite number, Doppler range, and integration time • Returns largest correlation and delay-Doppler estimate

38. Acquisition engine

39. Tracking • Standard channel/correlators • FPGA implementation provides easy upgrade • L2C in development • Efficient design allows for up to 36 correlators useful for many applications using multiple antenna • Spinning spacecraft • Attitude determination • Bistatic Radar

40. Tracking Processor Antenna Analog Front End Motorola Coldfire RH-CF5208 RF Daughter card (1) Actel RTAX2000 Acquisition 4 MB SRAM (2) Actel RTAX2000 Hardware

41. Navigator signal processing card • Rad-hard ColdFire • 65.536 MHz • 5 RTAX-2000 FPGAs • 3 used for GPS • 2 for applications, e.g., crosslink transceiver (IRAS) • Total dose > 100 kRad • RS422, RS644, Spacewire

42. Navigator flight box • NavSP + RF Daughtercard + Power Converter Card • Dimensions: ~ 4”x10.5”x10.5” • Power: 20W • Weight: 5.5 Kg • RF: Currently 4 coherent GPS inputs • Radiation Tolerances: > 100 kRad • Navigator GPS Receiver is TRL 6 • Finished EMI/EMC, Thermal/Vac, and Vibration testing • Delivered to HST SM4 project for launch ~ 10/8/08

43. Tracking Loops Navigation GEONS Measurements Measurements Tracking status PVT solution Pipewall Spacecraft state Satellite ephemerides Tracking channels Ephemeris & Almanac Telemetry Data message Detected signal Acquisition Module Acquisition Control To spacecraft Software • New in-house development • Low-level hardware interface and control • Basic PVT solution • GEONS • Flight heritage orbit determination filter • Special Applications • Attitude determination • Bistatic radar

44. Talk outline • GPS in GEO/HEO: what is needed and how to get there. • GPS signal processing • Basic concepts for GPS • Traditional approach to GPS acquisition and limitations • Detection theory approach • Navigator GPS Overview • Simulations and Applications

45. Number of GPS signals simulated vs. tracked simulated vs received power levels days into sim hours Baseline GEO results: visibility Simulations conducted using high fidelity Spirent GPS constellation simulators

46. Baseline GEO results: performance • Assumes “ideal conditions” • No ionosphere, no SV clock or ephemeris errors • OCXO oscillator

47. Lunar re-entry application • Direct re-entry trajectory provided by LM • Simulation assumes immediate visibility after blackout • Re-acquisition performed during max acceleration

48. Test and results • Ran 250 trials of a 2 minute looping simulation • Measured time from simulation reset to first GPS position, velocity and time solution

49. Re-entry demo

50. Missions currently supporting • HEO GPS navigation for Magnetospheric MultiScale (MMS) mission • Navigator is the GPS portion of the Interspacecraft Ranging and Alarm system (IRAS). • TRL 6 prototype IRAS scheduled for completion 3/09. • Bistatic Radar Ranging on HST SM4 • Navigator is part of Relative Navigation Sensor system. Flight unit delivery in 1/08. Launch; 10/08. • Will estimate range between Shuttle and HST from weak signal reflected GPS • GOES • Project interested in commercializing high altitude GPS capability for potential vendors of next GOES spacecraft. • Prototype receiver being developed for in-house testing at Boeing, Lockheed Martin.