State of the Art and Future Trends in Radionavigation

1. State of the Art and Future Trends in Radionavigation Todd Humphreys, UT Austin Aerospace Dept. (with slide contributions from Mark Psiaki, Cornell MAE Dept.) Briefing to USPTO| April 14, 2011

2. Outline of Topics • Overview of Radionavigation/GPS • Advances in Weak-Signal GNSS Tracking and Indoor Navigation + Network-aided Navigation • Vector Tracking for Improved Navigation Accuracy and Robustness • Multipath Mitigation

3. Radionavigation Radionavigation Systems Radionavigation Systems GPS GPS GNSS GNSS

4. The Three GPS Segments (Courtesy of U.S. Air Force)

5. 24 Satellites in 12-Hour Orbits Distributed in 6 Orbital Planes of 55 deg Inclination: (Courtesy of B.W. Parkinson)

6. f L 1 Spread Spectrum Radio Ranging RFTrans. Link PRNDespreading PRNSpreading In Receiver In Transmitter 1-Chip Interval +1 Received CPRN(t-r/c) t TransmissionDelay =r/c -1 +1 Transmitted CPRN(t) t -1 PRN Chip Values (earliest to latest): +1, -1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1

7. GPS Position & Time Determination Pseudorangemeasurementequations • Data: r1, r2, r3, r4,p1, p2, p3, p4 Unknowns: ruser, dt User ReceiverLocation • Need >= 4 signals • Pseudorange measurement: p = c(trcvd+dtrcvd-ttrans)

8. GPS Errors & Accuracy • Ephemeris errors in r i: 2 m • Transmitter clock errors: 2 m • Residual Ionospheric delay: 4 m* • Tropospheric delay: 0.5 m • Multipath (reflected signals): 1 m# • Receiver noise: 0.5 m • Multiplicative effect of geometry (GDOP) • Typical accuracy: 10 m/axis, 30 nsec in time, 0.01 m/sec velocity * for single-frequency receiver w/model corrections, error > 15 m possible in unusual ionospheric conditions, low elevation # error > 15 m possible in strong multipath environments

9. 12 12 Local Area Differential GPS BroadcastPosition & Time FAA LAAS version accuracy: 0.5 m within 45 km of ref. receiver at airport Xs ActualPosition & Time s Measured Scalar Correction User applies correctionto range measurements Known location Reference Receiver CORRECTION = expected pseudorange - measured pseudorange (Courtesy of B.W. Parkinson)

10. Wide Area Differential GPS using SBAS (Courtesy of B.W. Parkinson) • FAA WAAS version accuracy: 1-2 m over North America • Europe system: EGNOS; Planned Japanese system: QZSS • Systems provide integrity signals

11. Carrier-Phase Differential GPS Two Possible Alternate Locations of Antenna B GPSSatellite i RMS precision ofrelative rangemeasurement:5 mm Antenna B l Measurement Equations: Antenna A 3.5l

12. The End of Selective Availability

13. GPS–Equipped Cell Phones • Motivations: • FCC Phase-II E911 requirement (50 m 67% of calls, 150 m 95% of calls) • Location based services (please tell me where is the nearest restaurant) • Money (projected $26B market in 2010, $104B in 2020) • Challenges: • Weak signals/multipath in urban canyons & indoors • Solutions: • Assisted GPS (as in figure) • MEMS • Coarse position fix from communications link (Fig. 3 of Ballantyne et al., "Achieving Low Energy-per-fix with A-GPS Cellular Phones," Proc. ION GNSS 2005)

14. GPS–Equipped Automobiles • Motivations: • Never-lost/driving directions • Location based services • Money (projected $86B market in 2020) • Challenges: • Weak signals/multipath in urban canyons • Solutions: • Aiding from inertial sensors • Dead reckoning aids from odometer/steering data • Maps & velocity/direction information included in fix solution • Near-zenith augmentation system satellites, e.g., QZSS (Fig. 1 of Normark & Ståhlberg "Hybrid GPS/Galileo Real Time Software Receiver," Proc. ION GNSS 2005)

15. New GPS Signals L2 L1 P(Y) C/A LegacySignals L2 Civil Block IIR-M (1st Launch 2005) Military M L5 Block IIF(1st Launch 2010) 3rd Civil (Courtesy of B.W. Parkinson)

16. Galileo Signals (GIOVE-A Launched 12/2005) (Fig. 1 of Wallner et al., "Interference Computations Between GPS and Galileo," Proc. ION GNSS 2005)

17. Opportunities Afforded by New Signals • Dual-frequency ionospheric corrections in civil receivers • GPS, Galileo, or a combination • Flight-certifiable with 2 aviation-protected frequency bands • Better performance for indoor & high-altitude space applications • Pilot signals (i.e., no data) allow better signal processing • More power on some signals • Access to more spacecraft signals via interoperability • GPS L1/L5 & Galileo L1/E5a bands are same • 54 or more satellites available from combined constellation • Improved GDOP, availability in urban canyons, RAIM

18. Non-Standard Applications of GNSS (Courtesy of B.W. Parkinson) (Courtesy of B.W. Parkinson) Note 4 antennas forattitude Blind Landing, 0.30 m accuracy Robotic Farming, 0.08 m accuracy (Fig. 4.f from Mitchell et al. Proc. ION GNSS 2004) • GPS Solves a Murder Mystery • Headline: “Jury will hear how GPS tracked murder suspect” – 9 April 2005, Citizens Voice, Wilkes Barre, PA • Facts: CSI-wireless Asset-Link GPS tracker in suspect’s rented Lincoln navigator placed him at crime scene minutes before firefighters discovered victim Ionospheric Remote Sensing

19. GNSS Economics • Over 10M civil sets in use, > 200,000/month sold at costs >= $100 • World sales for cell-phones & automobiles alone projected at $190B in 2020

20. Outline of Topics • Overview of Radionavigation/GPS • Advances in Weak-Signal GNSS Tracking and Indoor Navigation + Network-aided Navigation • Vector Tracking for Improved Navigation Accuracy and Robustness • Multipath Mitigation

21. The Problem

22. Approaches to Indoor and Weak-Signal Nav. (1/3) • Non-GNSS Solutions • Wi-Fi access points • received signal strength + large database = ~10 m accuracy • Pseudolites: GPS-like signals from terrestrial transmitters • Center frequency is usually offset from GPS, but same signal structure; as good as ~5 cm accuracy • Navigate off of cell phone towers • Coarse but robust fallback: Cell tower ID • Better: Advanced forward link trilateration in CDMA systems • Use sensors to dead reckon during short GNSS unavailability • Inertial measurement units (accelerometers, gyros) • Cameras • Magnetometers • Altimeters • IMES: Indoor measurement system • Extremely weak (0.1 to 0.4 nano-watt) GPS-like signals used only for data transmission • “If you’re near enough to detect my signal, you must be within 10 meters of my location, which is ...” • No need for synchronization with GNSS signals since they are not used for ranging • Scalable? (Would have to be densely deployed.)

23. Approaches to Indoor and Weak-Signal Nav. (2/3) • Increasing Receiver Sensitivity • Narrow the search space: obtain rough user position and time and GNSS spacecraft ephemeris from the network (aided GPS (AGPS)) • Massively parallel processing: search thousands of hypothesis cells simultaneously (currently implemented in high-performance chips by CSR, Broadcom, others) • Extend the coherent integration time • Track pilot channels in new GNSS signals (no navigation data bits) • Integrate across navigation data bit boundaries by “wiping off” the data bits with data provided over network (e.g., via AGPS) • Stabilize or compensate for clock and receiver dynamics to extend the receiver’s coherence time • Use high-quality, stable clock or frequency stability transfer to reduce unpredictable clock variations • Use IMU to compensate for receiver dynamics

24. Approaches to Indoor and Weak-Signal Nav. (3/3) • Cafeteria Navigation: Cobble together a solution based on a subset of the following sensors and signals: • GNSS • Non-GNSS Sensors • IMU • Magnetometer • Altimeter • Camera • Signals of opportunity • Wi-Fi • Cellular telephone signals • HDTV • Iridium • Non-GNSS radionavigation signals • Pseudolites • IMES • Nav-enhanced Iridium (future) • Nav-enhanced Wi-Fi (future) “The most suitable technology for indoors is a combination of GNSS with accelerometers, gyros, and Wi-Fi.” -- KanwarChadha of CSR, Oct. 2010

25. Massively Parallel Correlation Figure: Frank Van Diggelen Silicon- and FFT-based MPC techniques allow all code offsets to be searched simultaneously, reducing TTFF and indirectly improving sensitivity

26. 26 State-of-the-Art AGPS: CSR’s EGPS Loose coupling between GPS & CDMA is practical and cheap, but prevents nanosecond-level time aiding and further improvement in sensitivity

27. Future: Tightly-Coupled Opportunistic Navigation Enabling configuration: (1) Same clock: Downmix and sample GPS and SOP with same oscillator (2) Same silicon: Sample GPS and SOP in same A/D converter

28. Details on Improving Sensitivity by Extending Coherence Time (1/2) Example: For C/N0 = 7 dB-Hz, T must be > 7 dB-sec (about 5 seconds)

29. Details on Improving Sensitivity by Extending Coherence Time (2/2) Stable signals from CDMA cell towers can be used to discipline local clock TCXO: Temperature-compensated crystal oscillator OCXO: oven-controlled crystal oscillator

30. Outline of Topics • Overview of Radionavigation/GPS • Advances in Weak-Signal GNSS Tracking and Indoor Navigation + Network-aided Navigation • Vector Tracking for Improved Navigation Accuracy and Robustness • Multipath Mitigation

31. Traditional Receiver Architecture (Fig. 1 of Lashley, 2009)

32. Vector Tracking Loop Architecture (Fig. 2 of Lashley, 2009)

33. Vector Tracking • Improves cross correlation immunity, helping to solve the near/far problem • In independent channel tracking, a tracking loop can get fooled into tracking a cross-correlation peak instead of the autocorrelation peak • In vector tracking, the centralized tracking loop is not fooled by cross-correlation peaks because these do not follow the predicted trajectory • Improves robustness • Navigation solution less sensitive to loss of individual channels. A solution is still possible with fewer than 4 satellites visible (degrades gracefully). • Faltering channels are “helped along” by the combined information contributed by the other channels • Amenable to “cafeteria navigation” • “Hungry” estimator can take in signals of opportunity and data from a diverse sensor suite

34. Outline of Topics • Overview of Radionavigation/GPS • Advances in Weak-Signal GNSS Tracking and Indoor Navigation + Network-aided Navigation • Vector Tracking for Improved Navigation Accuracy and Robustness • Multipath Mitigation

35. Multipath: A Dominant Error Source (Van Diggelen, InsideGNSS, 2011)

36. Long Coherent Integration Time Provides Some Protection Against Multipath (Fig. 8 of Pany, 2009)

37. Optimal Approaches To Multipath Mitigation: Maximum Likelihood Multipath model: Likelihood function: ML approach: Choose A_i, tau_i, and ph_i to maximize the likelihood function (See Sahmoudi, 2008)

38. More Information http://radionavlab.ae.utexas.edu

39. Backup Slides

40. Emerging Threat: Civil GPS Spoofing

41. Civil Anti-Spoofing Techniques • Data bit latency defense (weak but easy to implement) • Multi-antenna defense (patented in 1996; strong against single spoofer; fails against multiple spoofers; requires additional hardware) • Vestigial signal defense (work in progress; appears strong) • Navigation message authentication (strong, practical, requires cooperation of control segment) • Cross-correlation using P(Y) code (pioneered by Lo, refined by Psiaki, very strong but not so practical)

42. Software-Defined GNSS Receivers: The GRID Receiver (2006) V1

43. GRID Receiver Evolution (2006-2010) V2 V3 V4

44. GRID Receiver (2011) V5