GPS (Introduction) References. Terms

Transcription

1 GPS (Introduction) WCOM2, 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, 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 WCOM2, GPS, 2 Determination of positions via Time-of-Fly measurements 1 Δτ2 D A c (Δτ 2 ) Source: [1] Assumptions 1. distance A between Tx is known. 2. Tx transmit synchronously, Rx can only measure TDOA (time difference of arrival). Conclusions with 2 Tx, x-position (and time) and with 4 Tx, x,y,z-positions (and time) can be determined!

3 GPS-Principle WCOM2, GPS, 3 TDOA measurement by code correlation D = (Δt c+a)/2 Tx1 Rx Tx2 Code s 1 mit N Chip s 1 Tx1 S1 Code S1 Code t Tx2 S2 Code S2 Code t s 2 s τ 2 1 Rx S1 Code S2 Code τ τ 2 t after correlation s 1 with code s 1 T chip s 2 with code s 2 T chip

4 GPS-Principle WCOM2, 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] Swiss Terrestrial Reference System CHTRS95, CH1903+/LV95 (CH1903) 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 WCOM2, 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 WCOM2, 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 WCOM2, 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 WCOM2, GPS, 8 Orbit and coverage area coverage area [1]

9 GPS-Space Segment WCOM2, 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 WCOM2, 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 WCOM2, 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 WCOM2, GPS, Gold- / PRN-codes with N = 1023 chips Generation with 2 LFSR, chip rate Mchip/s delayed PN-code due to tap-selection (=> cyclic shift) seed = [1 1 1] satellite identified by PRN-number => CDMA

13 GPS User Segment WCOM2, 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 WCOM2, GPS, 14 Source: [1]

15 GPS Navigation Message WCOM2, 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 WCOM2, GPS, 16 Deactivation of SA (artificial degradation) in the year %- or 2σ-accuracy: 100 m 95%- or 2σ-accuracy: 12 m 68% or σ-accuracy: 6 m Source: [1]

17 Improved GPS WCOM2, GPS, 17 Main sources of GPS errors effect of the ionosphere (counter measures: DPGS and in the near future 2 frequency receiver) multipath (mainly in urban areas) effect of the satellite constellation (DOPs [Dilution of Precision]) Differential GPS (DGPS) Source: [1] transmission of correction factors

18 Improved GPS WCOM2, 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 WCOM2, GPS, 19 Achievable accuracy with DGPS and SBAS SBAS: satellite based augmentation systems [1]

20 Improved GPS WCOM2, GPS, 20 Some Location Based Services are based on satellite navigation GPS-Rx not always on, e.g. because of power 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 TTFX: 1-2s [1]

21 Data Interface to Peripherals WCOM2, 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 WCOM2, 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,