Weak-signal fast-acquisition GPS receiver technology for space applications

Weak-signal, fast-acquisition GPS receiver technology for space applications: Navigator GPS receiver Luke Winternitz Code 596 GODDARD SPACE FLIGHT CENTER

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 GODDARD SPACE FLIGHT CENTER 2

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) GODDARD SPACE FLIGHT CENTER 3

GPS signal strength at GEO GODDARD SPACE FLIGHT CENTER 4

GPS signal strength in lunar transit C/N 0 with 10 d. B antenna Navigator Sensitivity + Passive Antenna Navigator Sensitivity + 10 d. B Antenna Navigator Duo Sensitivity + 10 d. B Antenna 35 d. B-Hz 25 d. B-Hz 15 d. B-Hz GODDARD SPACE FLIGHT CENTER 5

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

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 GODDARD SPACE FLIGHT CENTER 7

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. • R 1 GODDARD SPACE FLIGHT CENTER • R 2 • R 3 8

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

GPS system • Constellation of 24 -32 satellites in 12 hr 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 GODDARD SPACE FLIGHT CENTER 10

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 GODDARD SPACE FLIGHT CENTER 11

GPS C/A code signal +1 GPS Data Message 50 bps 20 ms -1 +1 1 ms periodic 1023 bit (chip) C/A Code : 20 per data message bit 1. 023 Mbps -1 Carrier 1. 57542 GHz +1 -1 GODDARD SPACE FLIGHT CENTER +1 12

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

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: 25 d. B-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 GODDARD SPACE FLIGHT CENTER 14

GPS correlator Antenna RF Front End Accum Microprocessor Tracking Loop Software Removes Carrier (1. 57542 GHz) Navigation Software Replica GPS signal generator Correlator GODDARD SPACE FLIGHT CENTER 15

Traditional acquisition: serial search true signal replica/test signal GODDARD SPACE FLIGHT CENTER 16

Example acquisition GODDARD SPACE FLIGHT CENTER 17

Correlation power and grid spacing • Rules of Thumb: – |¢ ¿| <0. 25 chips – |¢ !|<0. 25/T Hz +1 -1 • For T=1 ms correlations spacing = 250 Hz 1023 chip periodic • For T=10 ms correlations spacing = 25 Hz • Weak signal acquisitions require large T and thus have decreased bin size and more bins. GODDARD SPACE FLIGHT CENTER 18

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

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

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 GODDARD SPACE FLIGHT CENTER 21

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

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 GODDARD SPACE FLIGHT CENTER 23

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 GODDARD SPACE FLIGHT CENTER 24

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. GODDARD SPACE FLIGHT CENTER 25

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. GODDARD SPACE FLIGHT CENTER 26

Example of GLRT concept GODDARD SPACE FLIGHT CENTER 27

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 GODDARD SPACE FLIGHT CENTER 28

Predicted performance for GLRT (parallel search) Threshold set to enforce 5% false alarm probability when searching 10 k. Hz frequency swath 10 d. B increase in T gives 10 d. B increase in sensitivity GODDARD SPACE FLIGHT CENTER 29

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

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

Performance of non-coherent integration Threshold set to enforce 5% false alarm probability when searching 10 k. Hz frequency swath 10 d. B increase in T gives only 5 d. B increase in sensitivity GODDARD SPACE FLIGHT CENTER 32

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 GODDARD SPACE FLIGHT CENTER 33

Navigator specifications/goals • Build a HEO qualified GPS receiver – Stringent radiation requirements (> 100 k. Rad) • 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 d. B‑Hz • Acquire GPS signals quickly – Within one second for strong signals (>40 d. B‑Hz) – Within one minute for weak signals (<40 d. B‑Hz) GODDARD SPACE FLIGHT CENTER 34

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 20 ms GODDARD SPACE FLIGHT CENTER 35

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

Acquisition modes • Strong Signal Mode – Based on 1 ms correlation – Effective to 40 d. B-Hz, appropriate for “below the constellation” – Can complete 140 k. Hz Doppler search for all signals in less than 0. 5 seconds • Weak Signal Mode – Based on half-bits method described previously – Can search 10 k. Hz Doppler at a time at 25 Hz granularity (>700, 000 grid points) – 300 ms integration time achieves 25 d. B-Hz • Algorithms embedded in FPGA – Commanded with GPS satellite number, Doppler range, and integration time – Returns largest correlation and delay-Doppler estimate GODDARD SPACE FLIGHT CENTER 37

Acquisition engine GODDARD SPACE FLIGHT CENTER 38

Tracking • Standard channel/correlators – FPGA implementation provides easy upgrade – L 2 C in development – Efficient design allows for up to 36 correlators useful for many applications using multiple antenna • Spinning spacecraft • Attitude determination • Bistatic Radar GODDARD SPACE FLIGHT CENTER 39

Hardware Tracking Antenna Processor (1) Actel RTAX 2000 Motorola Coldfire RH-CF 5208 Analog Front End RF Daughter card Acquisition (2) Actel RTAX 2000 GODDARD SPACE FLIGHT CENTER 4 MB SRAM 40

Navigator signal processing card • Rad-hard Cold. Fire – 65. 536 MHz • 5 RTAX-2000 FPGAs – 3 used for GPS – 2 for applications, e. g. , crosslink transceiver (IRAS) • Total dose > 100 k. Rad • RS 422, RS 644, Spacewire GODDARD SPACE FLIGHT CENTER 41

Navigator flight box • Nav. SP + RF Daughtercard + Power Converter Card – Dimensions: ~ 4”x 10. 5” – Power: 20 W – Weight: 5. 5 Kg • RF: Currently 4 coherent GPS inputs • Radiation Tolerances: > 100 k. Rad • Navigator GPS Receiver is TRL 6 – Finished EMI/EMC, Thermal/Vac, and Vibration testing – Delivered to HST SM 4 project for launch ~ 10/8/08 GODDARD SPACE FLIGHT CENTER

Software Data message Ephemeris & Almanac Satellite ephemerides Tracking channels Navigation Detected signal Acquisition Module Acquisition Control GODDARD SPACE FLIGHT CENTER Measurements PVT solution GEONS Spacecraft state Measurements Tracking status Pipewall • 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 Tracking Loops Telemetry To spacecraft 43

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 GODDARD SPACE FLIGHT CENTER 44

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

Baseline GEO results: performance • Assumes “ideal conditions” • No ionosphere, no SV clock or ephemeris errors • OCXO oscillator Position Error (m) Velocity Error (cm/s) Mean STD Max Baseline Mean STD Max 1. 344 0. 6639 2. 835 0. 010 0. 0057 0. 024 GODDARD SPACE FLIGHT CENTER 46

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

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 GODDARD SPACE FLIGHT CENTER 48

Re-entry demo GODDARD SPACE FLIGHT CENTER 49

Missions currently supporting • HEO GPS navigation for Magnetospheric Multi. Scale (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 SM 4 – 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. GODDARD SPACE FLIGHT CENTER 50

Missions currently supporting • Global Precipitation Measuring Mission (GPM) – Primary navigation and time sensor – LEO mission: Interested in fast acquisition • Air Force Research Lab’s Plug and Play spacecraft – Testbed for fast satellite assembly – Potential LEO flight • Orion/CEV – Fast acquisition for re-entry navigation after blackout – Working with avionics subcontractor Honeywell • Shuttle Technology Demonstration (DTO) (planned) last shuttle flight 2010 – Test fast-acquisition for Orion during nominal LEO orbit and during re-entry GODDARD SPACE FLIGHT CENTER 51

MMS and the Intersatellite Ranging and Alarm System (IRAS) • MMS consists of four (spinning) spacecraft flying in a tetrahedral formation • The IRAS provides orbit determination (GEONS) and precision timing achieved through – High sensitivity Navigator GPS: required sensitivity 28 d. B-Hz – Crosslink ranging using GPS like method: required accuracy of 30 m 1 -sigma • Pass science alarm message among all spacecraft within 10 seconds • Pass 10 k. B navigation message among all spacecraft GODDARD SPACE FLIGHT CENTER 52

IRAS TRL 6 test environment • Four Navigator/IRAS boxes synchronized through simulated GPS • Transceivers talk through 6 PERFs devices passing data and making ranges off one another using GPS-like ranging technique. • PERFs: Path Emulator for RF Systems – Simulates RF path between IRAS units • RF path loss • Delay • Dynamics GODDARD SPACE FLIGHT CENTER 53

Possible Future Capabilities • Reception of new GPS signals – Modernized GPS broadcasts of L 2 C and L 5 – L 2 C acquisition and tracking near testing in Navigator receiver. Data-less signal allows for greatly improved tracking sensitivity • Ultra-Weak signal acquisition and tracking – Acquisition thresholds below 15 d. B-Hz using combination of improved signal processing and adaptive antenna technology – Applicable to HEO, lunar, and cislunar orbits for Constellation Program (Cx. P) • GPS-derived ranging crosslink communications – Developed for MMS - IRAS – S-band communications link with code phase ranging – Signal processing and RF down conversion integrated into present Navigator receiver design – Applicable to future spacecraft formation flying missions and Cx. P automated rendezvous and docking sensing needs. GODDARD SPACE FLIGHT CENTER

Conclusion • Traditional receivers have sensitivity of no better than C/N 0=35 d. B -Hz. They are limited by acquisition sensitivity • Dedicated acquisition hardware allows the Navigator to use signals C/N 0<25 d. B-Hz, where tracking sensitivity becomes limiting factor • Fast-acquisition is a very nice side benefit • Space qualified Navigator receiver has been developed to meet these specs • Hi-fi simulations verify theoretical performance is achieved • The demand for such a receiver is real – HSM 4, GPM, MMS, AFRL-Pn. P, GOES-R, Shuttle DTO, Orion CEV GODDARD SPACE FLIGHT CENTER 55

Thank you GODDARD SPACE FLIGHT CENTER

Performance Characteristics • Cold Start Capable: – Time to first fix in LEO < 1 minute with no a-priori knowledge • Fast Acquisition: – All satellites acquired within 2 seconds in LEO • Position Accuracy: – < 10 m ( < 1 m with GEONS) • Velocity Accuracy: – < 5 cm/s ( < 1 cm/s with GEONS) • PPS Accuracy: < 50 ns • Weak signal capable – Acquisition and tracking down to 25 d. B-Hz GODDARD SPACE FLIGHT CENTER