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State of the Art and Future Trends in Radionavigation

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. Outline of Topics . Overview of Radionavigation/GPS

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State of the Art and Future Trends in Radionavigation

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  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

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