Local Area Differential GPS (DGPS) Mitigation of Correlated Measurement Errors

Transcription

1 Local Area Differential GPS (DGPS) Mitigation of Correlated Measurement Errors Differential Corrections DGPS Positioning Accuracy: 1-3 m 53

2 Sendai Earthquake of 11 March 2011 Displacement of the IGS Station MZU at Mizusawa Source: Simon Banville, University of New Brunswick 54

3 55

4 Receiver Functions Condition input signal Bandpass filter to suppress OOB interference Down-convert Digitize (A/D conversion) Separate signals from individual SVs Acquire and Track Signals Demodulate navigation data Calculate position, velocity, and time (PVT) Report results through user interface 56

5 GPS Receiver Functional Diagram Courtesy: Prof. Per Enge, Stanford University 57

6 Code and Carrier Phase Measurements A Conceptual Exercise Suggested by Frank van Diggelen 58

7 Code and Carrier Phase Measurements Precision vs. Accuracy Carrier phase can be measured with a precision of millimeters, code phase with decimeters Pseudoranges from each are affected by the same error sources, and the error in each can be several meters 59

8 Satellite Navigation Position Estimation by Trilateration ρ K ρ k (x k, y k, z k ) ρ 1 Error Sources Ephemeris Satellite Clock Propagation through Ionosphere Troposphere Multipath Receiver Noise b (x, y, z) Measurements: Pseudoranges {ρ k } Given: Satellite Positions {(x k, y k, z k )} ρ k = (x k x) 2 + (y k y) 2 + (z k z) 2 b, k = 1, 2,, K Unknown: User Position (x, y, z) Receiver Clock Bias b 60

9 Typical GPS SPS Positioning Accuracy 95% horizontal positioning error (HPE) ~ 2.7 m 95% vertical positioning error (VPE) ~ 4.0 m GPS position accuracy data from 28 sites distributed throughout North America for 3 month period (April June 2011) Source: July 2011 Federal Aviation Administration SPS Performance Analysis Report. 61

10 GPS Augmentations Why augment? For better accuracy: Mitigate measurement errors For robustness: Mitigate effects of RFI (intentional or not) signal attenuation due to blockage (e.g., by foliage or building), or temporary loss of signal (e.g., going under a bridge or through a tunnel) How augment? Transmit corrections for errors that are correlated spatially and temporally Local Area Differential GPS Space-Based Augmentation Systems (SBAS): WAAS, EGNOS, MSAS Assist GPS receiver with complementary technologies (e.g., inertial), signals of opportunity (e.g., eloran), or by offloading some functions (e.g., to a cell tower in E911) 62

11 National DGPS Coverage (2005) In In the the Lower Lower 48, 48, single single coverage: 87%, 87%, dual dual coverage: 55% 55% 63

12 Positioning Accuracy Hierarchy GPS and Its Augmentations Differential Autonomous Surveying & Geodesy Relative Navigation SPS ( ) SPS (2000 ) PPS WADGPS DGPS 1 mm 1 cm 10 cm 1 m 10 m 100 m Position Error Carrier Phase Measurements Code Phase Measurements 64

13 100 Real-Time Position Estimates from GPS (1997) 10 Position Error (m) Misra/1997 GPS / SPS (1997) DGPS Code L DGPS 3Carrier-Smoothed Code L1 4 DGPS Carrier Phase L1 & L2 Time (hours) 65

14 Performance Metrics Accuracy How good are the estimates? RMS error Error Bounds Your error is no worse than x (with probability ) Integrity of Signals The signals on which your estimates are based are genuine. (Probability) Availability of Service Consistent with your requirements. (Probability) Continuity of Service For the next x seconds, consistent with your requirements. (Probability) 66

15 GPS Has Raised Expectations n Instantaneous, meter-level position estimates EVERYWHERE n Performance metrics: accuracy, continuity, integrity, availability n Intense research focus on multi-sensor navigation 67

16 Assisted GPS (AGPS) Assistance : Time, Navigation data 68

17 Current Navigation Challenges n Multi-Sensor Navigation Open interface standards for plug-and-play sensors Software architectures to accommodate reconfiguration of sensors Dynamic calibration/characterization of sensors of diverse quality Kalman-type filters to process asynchronous data from changing sensors of diverse quality n New Technologies Better MEMs sensors, Atomic reference mass accelerometers & gyros ( cold atoms interferometry ) Chip-scale atomic clocks (CSACs) Inertia of elastic waves/self-calibration/3-d fabrication Ohio State Univ. photo 69

18 GPS Modernization First announced in 1998 by Vice President Gore Additional civil signals More and More Capable Signals Civil signals: from 1 to 4 Military signals: from 2 to 4 Classic GPS Modernized GPS GPS III 2005 was the Year of Transition First GPS Block IIR-M satellite launched An additional civil signal: L2C Additional military signals on L1 and L2: M-codes GPS III to be completed ~

19 GPS Modernization Space Segment Cross links among satellites, flexible power, spot beam, integrity monitoring Additional, more capable military signals: M-code on L1 and L2 Additional, more capable civil signals: L2C, L5, L1C Control Segment Block II OCS OCX to fly new spacecraft Distributed architecture, security User Segment MGUE (Mil. GPS User Equip t): Enhanced A-J, A-S, A-T New security architecture: SAASM Pronav New paradigm: from form-fit military receivers to GPS chipsets 71

20 > $1b award to LMC in 2008 > $1b award to Raytheon in

21 GPS Signal Modernization Civil Signal (Starting 2010) MHz (L5) MHz (L1) C/A Code Civil Use Degraded (2 May 2000) MHz (L2) P(Y) Code Encrypted L1C (2014) P(Y) Code Encrypted M-Code (Starting 2005) L2C-Code (Starting 2005) M-Code (Starting 2005) Misra

22 Common GPS Module (CGM) Commercial Method (GPS engines enable multiple applications) Enablers Build Integrators Build Engines + Applications = Global GPS Use Proposed MGUE Approach (Emulate commercial, Build the engine!) Common GPS Module (CGM) GPSW Builds Enabling Engines Integrators Build + Applications = Global Military GPS Use

23 Evolution of GPS Signals C/A P(Y) P(Y) Block II / IIA / IIR M L2C M Block IIR-M Starting in 2005 L5 Block IIF Starting in MHz MHz MHz 75

24 GNSS Fundamentals Outline Principles of Satellite Navigation GPS Overview System Signals & receivers Measurements & Performance Modernization Potential GPS Partners/Rivals: GLONASS, Galileo, BeiDou/Compass, 76

25 GNSS Spectral Allocations in L-Band (1 GHz 2 GHz) L5, E5A Galileo GPS E5B L2 G2 E6 E2 L1 E1 Galileo GPS GLONASS Galileo Galileo GPS G1 GLONASS ~

26 History GLONASS Developed by Soviet Union, first launch: 1982 Similar to GPS: Passive, one-way ranging Full constellation in 1996, declined to 7, revived and now back to 24 No significant user base Constellation 24 satellites in 3 orbital planes, 64.8º inclination 19,100 km altitude, 11 ¼ hour period Signals 3 allocated bands: G1 (1602 MHz), G2: (1245 MHz), G3 (?) C/A-like code: 511 chips, 1 ms code period, 50 bps data All SVs use same PRN with frequency division multiple access (FDMA) using 16 frequency channels, reused for antipodal SVs 78

27 Soviet-built GLONASS Aviation Receiver & Tool box (circa 1990) ASN-16 79

28 GLONASS Modernization GLONASS: 3-year design life GLONASS-M: 7-year design life, civil signals on L2, higher clock stability GLONASS-K: 10-year design life, new CDMA signals (at 1202 MHz) GLONASS-M Group launch by PROTON GLONASS-K 80

29 GPS+GLONASS Satellite Visibility GLONASS-21 GPS+GLONASS (2x21) Probability (%) Satellites Visible Probability (%) HDOP VDOP Dilution of Precision (DOP) GLONASS-21 GPS+GLONASS (2x21) Misra/

30 GPS & GLONASS Position Estimates* 1-Minute Samples, 15 June

31 GPS & GLONASS Position Estimates* 1-Minute Samples, 15 June

32 L5 GNSS Signal Plans L2 L1 GPS (US) GLONASS (Russia) Future CDMA signal Galileo (Europe) COMPASS (China) IRNSS (India) QZSS (Japan) SBAS (US Europe India Japan) Frequency (MHz) Compass & IRNSS In S-band 84

33 Summary: Take-Away Points Satellite navigation systems exploit basic properties of radio waves: Transit exploited the Doppler effect, GPS exploits the known speed of propagation GPS is based on the old idea of trilateration, but implemented with the technology of the second-half of the 20 th century: space-based radio transmitters, ultra-stable clocks, and spread spectrum signals A GPS receiver measures pseudoranges to the satellites by measuring pseudo-transit times of radio signals. It takes 4 satellites (i.e., 4 pseudoranges) in order to estimate position (x, y, z) and time t With a clear view of the sky, it s easy to get positioning accuracy of several meters with a $100 GPS receiver, or relative positioning accuracy of millimeters with a pair of $1000 receivers. GPS satellites are 20-watt transmitters 20,000 km away, so the signals reaching the earth are very weak and, therefore susceptible to interference. The success and breadth of GPS applications is attributable largely to the chip. The VLSI revolution was well-timed for GPS 85

34 GPS-based Position Estimates (SPS)* Sampled over 24 hours (2002) 10 5 $100 Receiver (L1 C/A-code only) (1-minute samples) N<5 5<N<25 25<N $1000 Receiver (L1 C/A-code, L2 codeless) (1-second samples) North Error (m) East Error (m) 10 N: # points in a cell East Error (m) 86

35 Triangulation Method of determining the position of a fixed point from the angles to it from two fixed reference points a known distance apart from Trigonometry Surveying and Navigation by G.A.Wentworth 87

36 Triangulation Method of determining the position of a fixed point from the angles to it from two fixed reference points a known distance apart Trilateration Measure lengths of the sides of a triangle rather than angles from Trigonometry Surveying and Navigation by G.A.Wentworth Surveyor s chain from A chain = 100 links = 66 feet long, 80 chains make a mile. A "rod" or "pole" is 1/4 of a chain, or 16-1/2 feet long. Thus "40 rods" is 10 chains, or 1/8 of a mile. 88

37 Pseudorange Measurement 89

38 Pseudorange Determination Simplified 90 Source: Prof. Per Enge, Stanford University

39 WAAS Accuracy Improvement Source: Dr. Todd Walter, Stanford University

40 Common GPS Module (CGM) Commercial Method (GPS engines enable multiple applications) Enablers Build Integrators Build Engines + Applications = Global GPS Use Proposed MGUE Approach (Emulate commercial, Build the engine!) Common GPS Module (CGM) GPSW Builds Enabling Engines Integrators Build + Applications = Global Military GPS Use

41 Unmodulated Carrier Spread Spectrum Signaling Source: Prof. Per Enge, Stanford University Two repeats of a code with 4 chips per data bit x(t) Two bits from data Stream D(t) 1st data bit = +1 2nd data bit = 1 Carrier modulated by code and data 93 Transmitted Signal s () t 2 P D() t x()cos(2 t f t) T T L

42 Typical GPS SPS Positioning Accuracy 95% horizontal positioning error (HPE) ~ 2.7 m 95% vertical positioning error (VPE) ~ 4.0 m GPS position accuracy data from 28 sites distributed throughout North America for 3 month period (April June 2011) Source: July 2011 Federal Aviation Administration SPS Performance Analysis Report. 94

43 Direct Sequence Spread Spectrum T 0 carrier T c spread spectrum waveform = T d data waveform modulated spread spectrum signal f = 1/T 0 0 = carrier frequency (Hz) R c = 1/T c = chipping rate (chips/s) R d = 1/T d = data rate (bits/s) 95 Source: Dr. Chris Hegarty, MITRE

44 Binary Offset Carrier Modulation Carrier Spreading code = F sq = 1/T sq = subcarrier frequency (Hz) T sq Square wave Data BOC signal* *Shown at baseband (i.e., without carrier) Source: Dr. Chris Hegarty, MITRE 96

45 Frequency Plan 97

46 Frequency Plans* *Adapted from T. Grelier et al., Inside GNSS, May/June

47 99

48 From SPS PS, 2007 A GPS Satellite footprintt covers about one-third of the erth s surface 100

49 Hand-Held Military Receiver Suffers in Comparison to Civil Counterpart Defense Advanced GPS Receiver (DAGR) 101

50 Amplitude Spectrum of GPS Signals 1 November 2005 P(Y) C/A P(Y) f L2 : MHz f L1 : MHz Main lobe width: 2 x chip rate; sidelobe width: chip rate GPS signals are extraordinarily weak(~ watts), but they make up for this weakness with their ingenious design Courtesy: Prof. Per Enge, Stanford University 102

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