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Augmented GNSS: Fundamentals and Keys to Integrity and Continuity

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Augmented GNSS: Fundamentals and Keys to Integrity and Continuity ION GNSS 2011 TUTORIAL Augmented GNSS: Fundamentals and Keys to Integrity and Continuity Sam Pullen Department of Aeronautics and Astronautics Stanford University, Stanford, CA. 94305-4035 USA Tuesday, Sept. 20, 2011 1:30 – 5:00 PM Oregon Convention Center, Portland, Oregon (last updated 9/16/11) Acknowledgements: B. Belabbas, J. Blanch, R. Braff, M. Brenner, J. Burns, B. Clark, K. Class, S. Datta-Barua, P. Enge, M. Harris, R. Kelly, R. Key, J. Lee, M. Luo, A. Mitelman, T. Murphy, Y.S. Park, B. Parkinson, B. Pervan, R.E. Phelts, J. Rife, C. Rodriguez, J. Scheitlin, C. Shively, K. Suzuki, R. Swider, F. van Graas, T. Walter, J. Warburton, G. Xie, G. Zhang, and more… ©2011 by Sam Pullen Outline • Augmented GNSS Terminology • Introduction to GNSS and GNSS Augmentation – Differential GNSS (DGNSS) • GBAS and SBAS System Architectures • Aviation Applications and Requirements • Principles of Integrity and Continuity • Specific Examples: – Nominal Error Bounding – Signal Deformation Monitoring – Ephemeris Monitoring – Ionospheric Anomaly Mitigation • Summary 20 September 2011 Augmented GNSS: Integrity and Continuity 2 Augmented GNSS Terminology • GPS: Global Positioning System • GNSS: Global Navigation Satellite Systems • DGPS: Differential GPS (or GNSS) • L(A)DGPS: Local-Area Differential GPS • WADGPS: Wide-Area Differential GPS • CDGPS: Carrier-Phase Differential GPS (usually a subset of Local-Area DGPS) • LAAS: Local Area Augmentation System (FAA) • GBAS: Ground-Based Augmentation System (international; includes LAAS) • WAAS: Wide Area Augmentation System (FAA) • SBAS: Space-Based Augmentation System (international; includes WAAS) 20 September 2011 Augmented GNSS: Integrity and Continuity 3 Augmented GNSS Terminology (2) DGPS CDGPS GBAS LADGPS SBAS WADGPS (aka “RTK”) • SB JPALS (US • SLS-4000 • LD JPALS • WAAS (FAA, • WAGE (GPS DOD) LAAS (US DOD) USA) Wing, DOD) (Honeywell) • IBLS (with • ParkAir • EGNOS (ESA, • OmniSTAR Pseudolites) • DGRS 610/ SCAT-I Europe) (Trimble) 615 (Thales) • Surveying • NDGPS (US • MSAS (JCAB, • StarFire • KIX GBAS Coast Guard) Japan) (NavCom) • Precision (NEC, Japan) Farming • Commercial • GAGAN • Other • LCCS-A-2000 services (India) commercial • Other cm-dm (NPPF Spectr, services level apps Russia) • Many other • SNAS (China) meter-level • SELEX-SI apps GBAS 20 September 2011 Augmented GNSS: Integrity and Continuity 4 Augmented GNSS Classifications Global Category GBAS SBAS (ICAO SARPS) WAAS National Program EGNOS (e.g., FAA; RTCA LAAS Standards for U.S.) MSAS etc. Honeywell SLS- 4000 Raytheon Contractor Systems Thales DGRS-615 Thales Alenia KIX GBAS NEC/Raytheon etc. etc. 20 September 2011 Augmented GNSS: Integrity and Continuity 5 Aviation GNSS Terminology • ICAO: International Civil Aviation Organization • SARPS: Standards and Recommended Practices (ICAO Requirements) • MASPS: Minimum Acceptable System Performance Standards (sys. arch.) Used • MOPS: Minimum Operational Performance by Standards (user avionics) RTCA • ICD: Interface Control Document • NPA: Non-Precision Approach (2-D horizontal) • LNAV/VNAV:Lateral/Vertical Navigation Approach • LPV: Lateral Position Vertical Approach • CAT-I Category I Precision Approach (200 ft DH) • CAT-II Category II Precision Approach (100 ft DH) • CAT-III Category III Precision Approach (0-50 ft DH) 20 September 2011 Augmented GNSS: Integrity and Continuity 6 Outline • Augmented GNSS Terminology • Introduction to GNSS and GNSS Augmentation – Differential GNSS (DGNSS) • GBAS and SBAS System Architectures • Aviation Applications and Requirements • Principles of Integrity and Continuity • Specific Examples: – Nominal Error Bounding – Signal Deformation Monitoring – Ephemeris Monitoring – Ionospheric Anomaly Mitigation • Summary 20 September 2011 Augmented GNSS: Integrity and Continuity 7 The Evolution of GPS • 24+ Satellites since FOC in 1995 (space vehicles, or SVs) • 6 orbit planes, 60 degrees apart • 55 degrees inclination • 12-hour (11 hr, 58 min) orbits • 26,560 km from earth’s center • 20,182 km mean altitude • moving ~ 2.7 km/sec NDGPS WAAS LAAS Early interest S/A off “IOC” IOC SDA in DGPS 1970 1975 1980 1985 1990 1995 2000 2005 2010 Program “kickoff” 11 Blk I SVs 9 Blk II 14 Blk IIA 12 Blk IIR 8 Blk IIR- SVs SVs SVs M SVs 1st Blk IIF 20 September 2011 Augmented GNSS: Integrity and Continuity 8 The GPS Space Segment (as of Sept. 2010) Source: Lt. Col M. Manor, “GPS Status (Const. Brief),” CGSIC, Sept. 2010 Total of 35 SVs (31 SVs Healthy; 14 near EOL) 20 September 2011 Augmented GNSS: Integrity and Continuity 9 The GPS Ground Segment Today Source: Col. B. Gruber, “GPS Mod. & Prog. Upd.,” Munich SatNav Summit, March 2011 GPS is, in itself, a differential system. Gaylord Green 20 September 2011 Augmented GNSS: Integrity and Continuity 10 GPS Measurements: “Pseudoranging” SV #1 SV #2 1 2 cbu cbu cbu 3 SV #3 20 September 2011 Augmented GNSS: Integrity and Continuity 11 Elements of a Pseudorange |R| / c B b M I T SV ( not to scale ) • = measured pseudorange (sec) RX • c = speed of light in vacuum 3 × 108 m/s • |R| = true (geometric) range from RX to SV (m) • B = SV clock error (previously included S/A) (sec) • b = RX clock error (sec) • = RX noise error (sec) • M = RX multipath error (sec) • I = Ionospheric delay at RX location (sec) • T = Tropospheric delay at RX location (sec) • = other receiver errors (sec) 20 September 2011 Augmented GNSS: Integrity and Continuity 12 True Range and Ephemeris Error SVerr R R| R | 1 R R s u rs s u SV • R = true vector from RX to SV ( R ) rs Rs_err • 1rs = true unit vector along R (1’ = estimate) |R| R • Rs = true vector from Earth center to SV s • Ru = true vector from Earth center to RX 1rs Ru • Rs’ (estimate of Rs) derived from broadcast Earth navigation data (ephemeris messages) center RX • Ru’ (estimate of Ru) is derived from estimated user position improved by iteration during position determination (meter- level accuracy not needed) • What is the impact of errors in Rs? (Come back to this later…) 20 September 2011 Augmented GNSS: Integrity and Continuity 13 “Corrected” Pseudorange and Position Solution c c Best c ( Test + Iest ) • = “corrected” pseudorange measurement (sec) c • Best = SV clock error correction from navigation data (m) • Iest = ionospheric error correction based on Klobuchar model with parameters included in navigation data (m) • Test = tropospheric error correction based on external meteorology model (temp., pressure, humidity inputs) (m) T Iterate and Linearize: x = x0 + xb = b0 + b X [ x b ] - T 1 1rs' _ 1 - T 1 G X + 1'rs_ 2 c G where - T 1 1'rs _ N T 1 T Xest (G W G ) G W c W diag [ w1, w2, …, wN ] (default: w = w = … = w = 1) 1 2 N 20 September 2011 Augmented GNSS: Integrity and Continuity 14 Range-Domain Error Breakdown • Examine pseudorange error relative to “perfect” range, meaning range to true satellite position: r er c ( B + b + T + I + C ) + A ( S – U) + A S • err pseudorange error relative to perfect range • Y = residual error in (generic) vector/matrix Y after applying correction or broadcast information (sec) • C M + + (sum of uncorrected receiver errors) (m) - ' 1 T 0 0 0 R ' R s_ 1 - s1 u1 T 0 '1 s_ 2 0 0 Rs2 ' Ru2 AN 3N S U 0 0 0 3N 1 3N 1 - T 0 0 01 s _ N ' RsN ' RuN T 1 T Xest (G W G ) G W rer 20 September 2011 ented GNSS: IntAugmuitynegrity and Conti 15 “Dilution of Precision” (DOP) • A very useful (if imprecise) result comes from taking an idealized covariance of the position state error estimate Xest from the previous slide • For default weighting matrix (W = INxN ) and case where for each satellite is zero-mean and i.i.d.: err T 1 T 1 2 Cov (Xest) ( G G ) Cov (err ) = ( G G ) 2 – Where = variance of i.i.d., zero-mean pseudorange error 2 XDOP 2 Only a T 1 YDOP H GG function of NN 2 VDOP SV geometry 2 TDOP HDOP2 XDOP2 + YDOP2 PDOP2 XDOP2 + YDOP2 + VDOP2 2 2 2 2 2 GDOP XDOP + YDOP + VDOP + TDOP 20 September 2011 Augmented GNSS: Integrity and Continuity 16 The Usefulness of DOP • (Unweighted) DOP separates the two primary sources of GNSS errors: 1. Errors in ranging measurements 2. Impact of satellite geometry • Differential GNSS primarily addresses the first error source by eliminating common-mode range errors. – One exception in SBAS: additional ranging measurements from GEO satellites • GNSS modernization addresses both error sources, but the second one is typically of more benefit to differential GNSS users. 20 September 2011 Augmented GNSS: Integrity and Continuity 17 Local Area DGNSS: The Basic Concept • Exploit the spatial and temporal correlation of several GNSS error sources to (mostly) remove them from user range measurements. Ionosphere Troposphere GNSS antenna(s) Data broadcast antenna(s) Ref. Stn. 20 September 2011 Augmented GNSS: Integrity and Continuity 18 Local Area DGNSS: The Basic Concept (2) (2) (1) RS RS (N) RS “baseline” – separation (vector) between reference reference and user antennas receiver(s) a correction user transmitter 20 September 2011 Augmented GNSS: Integrity and Continuity 19 GPS Range Error Sources Error Source Approx. 1 Error for Approx. 1 Error for Standalone GPS LADGPS Users Users (a = 50 km) SV Clock 1 – 2 m 2 – 3 cm Ref. – User Correlation Ref. – SV Ephemeris 1 – 3 m 1 – 5 cm Troposphere 2 – 3 m (uncorrected) 1 – 5 cm 0.1 – 0.5 m (corrected by atmospheric model) Ionosphere 1 – 7 m (corrected by 10 – 30 cm Klobuchar model) Multipath (ref. and PR: 0.5 – 2 m(*) PR: 0.5 – 2 m(*) user receivers) : 0.5 – 1.5 cm : 0.5 – 1.5 cm Receiver noise (ref. PR: 0.2 – 0.35 m(†) PR: 0.2 – 0.35 m(†) and user receivers) : 0.2 – 0.5 cm : 0.2 – 0.5 cm Antenna survey N/A 0.2 – 1 cm error/motion (*)In obstructed scenarios with many large reflectors, multipath errors can be significantly larger.
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