GPS (Introduction) References. Terms

Transcription

1 GPS (Introduction) MSE, Rumc, GPS, 1 Terms NAVSTAR GPS ( Navigational Satellite Timing and Ranging - Global Positioning System) is a GNSS (Global Navigation Satellite System), developed by the US-DoD in 197x and fully operational since Other GNSS under development : Glonass (Ru), Galileo (EU), Beidou/Compass (China) References [1] Jean-Marie Zogg [HTW Chur], GPS, Essentials of Satellite Navigation, Compendium, Document: GPS-X D, February 2009, Chapter 1.1: The principle of measuring signal transit time Chapter 2.3.4: WGS-84 Chapter 4: GNSS technology: the GPS example Chapter 7.2: Sources of GPS error Chapter 8.2: Data interfaces [2] GPS SPS Signal Specification, 2nd Edition (June 2, 1995), [3] beautiful visualisation of the satellites positions by HSR / ICOM [4] Parkinson, Spilker, Global Positioning System: Theory and Applications, Volume I/II, Progress in Astronautics and Aeronautics, Volume 163/164, 1996.

2 GPS-Principle MSE, Rumc, GPS, 2 Determination of positions via Time-of-Fly measurements Source: [1] Assumptions 1. distance A between Tx is known. 2. Tx transmit synchronously, Rx can only measure TDOA (time difference of arrival). Conclusions x-position (and time) with 2 Tx and x,y,z-positions (and time) with 4 Tx determinable!

3 GPS-Principle MSE, Rumc, GPS, 3 TDOA measurement by code correlation D = (Δt c+a)/2 A Tx1 Code s 1 with N chips Rx Tx2 Tx1 t DSSS-modulation Tx2 t (small peak-power supports CDMA) Code s 2 with N chips τ 1 τ 2 Rx t N chips τ after correlation with code s 1 T chip N chips with code s 2 T chip

4 GPS-Principle MSE, Rumc, GPS, 4 Worldwide Reference Ellipsoid WGS-84 Ellipsoid approximates true (complex) shape of the earth there are many different reference systems GPS works with geocentric WGS-84 reference system Source: [1] conversion into CH-1903 coordinates required cartesian coordinates [1] 1 Grad = 60 Bogenminuten. 1 Bogenminute Breite = 1 Seemeile bzw. 1 nautischen Meile (NM) = km. 1 Bogenminute Länge = km mal cos(breitengrad). ellipsoidal coordinates (longitude, latitude, altitude) used for further processing

5 GPS-Principle MSE, Rumc, GPS, 5 Basic equations x,y,z,t coordinates and time of user x i,y i,z i,t i coordinates and time of 4 satellites (x 1 -x) 2 + (y 1 -y) 2 + (z 1 -z) 2 = [c (t 1 -t)] 2 (x 2 -x) 2 + (y 2 -y) 2 + (z 2 -z) 2 = [c (t 2 -t)] 2 (x 3 -x) 2 + (y 3 -y) 2 + (z 3 -z) 2 = [c (t 3 -t)] 2 (x 4 -x) 2 + (y 4 -y) 2 + (z 4 -z) 2 = [c (t 4 -t)] 2 4 equations (c: speed of light) and 4 unknowns Source: [1]

6 GPS-Subsystems MSE, Rumc, GPS, 6 Source: [1] (orbital data) 1 Master Control Station (Colorado) 5 Monitor Stations world wide 3 Ground Control Stations (with Satellite Uplink)

7 GPS-Space Segment MSE, Rumc, GPS, 7 24 to 32 Satellites at a height of km 6 different orbital planes (4-5 satellites per plane) time of circulation 12 h 55 always 4 satellites visible everywhere on earth

8 GPS-Space Segment MSE, Rumc, GPS, 8 Orbit and coverage area coverage area [1]

9 GPS-Space Segment MSE, Rumc, GPS, 9 Link budget Coarse/Acquisition (C/A-) Code for civil use L1 ( MHz) km (border of coverage area) [1] min. sensitivity specified in [2]

10 Link Budget MSE, Rumc, GPS, 10 Spectral power density of received signal and (thermal) noise floor -160 signal after despreading + 14 db <= thermal noise + noise figure F signal before despreading <= -130 dbm / MHz bandwidth 1 MHz 1/T chip source [1] f f L1

11 Satellite-Signal MSE, Rumc, GPS, 11 Source: [1] MHz 1023 T chip T bit C/A-code C/A-code C/A-code t / ms T chip 1 / Bandwidth

12 GPS-Coarse/Acquisition-Codes MSE, Rumc, GPS, Gold- / PRN-codes with N = 1023 chips Generation with 2 LFSR, chip rate Mchip/s satellite identified by PRN-number => CDMA

13 GPS User Segment MSE, Rumc, GPS, 13 Correlation receiver Source [1] (Doppler-Shift ± 5000 Hz) Process-Gain 10 log 10 (1023) 30 db SNR = -16 db before despreading => SNR = +14 db after despreading correlation time for data demodulation is 20 times longer Gain

14 GPS Navigation Message MSE, Rumc, GPS, 14 Source: [1]

15 GPS Navigation Message MSE, Rumc, GPS, 15 Navigation message contains 25 frames and lasts 12.5 minutes a GPS-frame has 5 x 300 = 1500 bits and lasts 30 s Subframes 1-3 are identical for all the 25 frames subframe 1 contains clock data of transmitting satellite subframes 2 and 3 contain ephemeris data of transmitting satellite ephemeris data are highly accurate orbital data a receiver has the complete clock values and ephemeris data from the transmitting satellite every 30 seconds Time-To-First-Fix (cold start autonomous) at least s => slow start-up is a system-inherent limitation of GPS Subframe 4-5 are different for all the 25 frames subframe 5 contains almanac data of first 24 satellites plus health almanac data are less accurate than ephemeris data subframe 4 contains almanac data of satellites and difference between GPS and UTC time

16 Accuracy without Selective Availability MSE, Rumc, GPS, 16 Deactivation of SA in the year %- or 2σ-accuracy: 100 m 95%- or 2σ-accuracy: 13 m 68% or σ-accuracy: 6.5 m Source: [1]

17 Improved GPS MSE, Rumc, GPS, 17 Main sources of GPS errors effect of the ionosphere (counter measure: two frequency receiver) multipath (mainly in urban areas) effect of the satellite constellation (DOPs [Dilution of Precision]) Accuracy 90% < 10 m, artifical degradation switched off since 2000 Differential GPS Source: [1] transmission of correction factors

18 Improved GPS MSE, Rumc, GPS, 18 EGNOS (European Geostationary Navigation Overlay System) 34 ground stations calculate correction signals (à la DGPS) for GPS correction in a radius of about 200 km around the reference station broadcast of correction signals via 3 geostationary satellites (C/A-Codes >32) 1-3 m accuracy Source: [1]

19 Improved GPS MSE, Rumc, GPS, 19 Achievable accuracy with DGPS and SBAS SBAS: satellite based augmentation systems [1]

20 Improved GPS MSE, Rumc, GPS, 20 Some Location Based Services are based on satellite navigation GPS-Rx not always on, e.g. because of current consumption time to first fix (cold start): s (missing orbital data) Assisted GPS (A-GPS) delivery of missing orbital data via fast channel, e.g. GSM/GPRS [1]

21 Data Interface to Peripherals MSE, Rumc, GPS, 21 NMEA-0183 data interface standardized by National Marine Electronics Association (NMEA) data telegram for serial interface Example: GGA data set (GPS fix data) $GPGGA, , ,N, ,E,1,08,0.94,00499,M,047,M,,*58<CR><LF>

22 Time Pulse MSE, Rumc, GPS, 22 Most GPS-Rx generate 1-4 time pulses per s time puls is synchronized to UTC-time Accuracy 5-60 ns [1] GPS-time-pulse is often used to synchronize devices in a «large» area as e.g. base stations, gliders,

23 Performance Data of a GPS-Rx MSE, Rumc, GPS, 23 NEO-M8 series: 12.2 x 16.0 x 2.4 mm

24 Modernization: BOC-Modulation MSE, Rumc, GPS, 24 Advantages higher interference robustness and bandwidth efficiency [1]

25 Modernization: BOC-Modulation MSE, Rumc, GPS, 25 BOC(1,1) and BPSK(1) have minimal impact on each other Source: [1]

26 GPS-Modernization MSE, Rumc, GPS, and 3. frequency for civil applications compensation of ionosphere errors! after 2013 integrity-signals, Search-and-Rescue-Functions Source: [1]

27 GPS-Simulator: An Example MSE, Rumc, GPS, 27 GPSG-1000 from Aeroflex / Cobham antenna coupler validation and test of GPS receivers as well as navigation and tracking systems 3D position may be user entered or 3D position may be dynamically simulated simultaneous GPS/Galileo simulations

28 GNSS-Update: Frequency Bands see Navipedia and some comments, MSE, Rumc, GPS, 28 T. Kouwenhoven, "Gnss navigational frequency bands.png",, Jan 2011, also available at

29 Availability of GPS civil signals (Sep 2016) MSE, Rumc, GPS, 29

30 Availability of Galileo civil signals (Sep 2016) MSE, Rumc, GPS, 30

31 GNSS-Update: Signal-Spectra MSE, Rumc, GPS, 31 Source: Stefan Wallner,

32 GNSS-Update: Signal-Spectra MSE, Rumc, GPS, 32 Source: Stefan Wallner

33 Correlation Matrices of GPS-Satellite 9 MSE, Rumc, GPS, 33 f = 3100 Hz f = 2400 Hz PRN periode = 20 ms doppler shift f = 2300 Hz correlations show expected coherence regarding the doppler shifts ( f is proportional to carrier frequency f c )

34 GNSS-Update: RTK MSE, Rumc, GPS, 34 Real Time Kinematics (RTK) is a differential GNSS technique provides cm-level positioning performance in the vicinity of a base station carrier-based (rather than code-based) positioning see also: resolving-errors/real-time-kinematic-rtk/ complicated process ambiguity resolution is needed to determine the number of whole cycles.

35 Example: GNSS RTK module from ublox MSE, Rumc, GPS, 35 NEO-M8P (1-frequency Rx) RTCM protocol some m to 1-10 km faster with multi-frequency GNSS-Rx u-blox, u-blox bringt GNSS-Technologie mit zentimetergenauer Präzision für den Massenmarkt,