Thales Geo. Solutions Introduction to GPS
OBJECTIVES By the end of this session you will be able to: Ø List the 3 segments of the GPS system Ø Explain how a range to a GPS satellite is obtained Ø List the errors of the GPS system Ø State the given accuracy of the GPS Standard Positioning Service (SPS) Ø Describe the characteristics of GPS signals Ø Name & describe three segments of the GPS system
WHAT IS GPS? Global Positioning System GPS provides us with: Ø Worldwide, continuous, high accuracy, three dimensional position and velocity Ø Precise time transfer
GPS ACCURACY Standard Positioning System (civilian) Single-frequency (L 1) performance: Ø Position accuracy standard Ø Global 95% horizontal error – 13 meters Ø Global 95% vertical error – 22 meters Ø (L 1) user performance given accuracy standard conditions Ø Global 95% horizontal error – 33 meters Ø Global 95% vertical error – 73 meters Tests conducted June 2000
GPS ACCURACY GPS single-FREQUECY (L 1) all-in-view user performance: Ø Horizontal error statistics Ø (95%) global average – 8. 3 meters horizontal error Ø (95%) worst site – 19. 7 meters horizontal error Ø Vertical error statistics Ø (95%) global average – 16. 8 meters vertical error Ø (95%) worst site – 44. 0 meters vertical error Tests conducted June 2000
GPS ACCURACY Cumulative global distribution of 95% accuracy performance: Ø 95% of the earth had better then 16. 4 meters 95% single-frequency horizontal performance Ø 95% of the earth had better then 29 meters 95% singlefrequency vertical performance Tests conducted June 2000
THE HISTORY OF GPS Ø Ø Ø Ø Ø 1957 1964 1973 1985 1986 1989 1993 1994 1995 -2000 USSR LAUNCHES SPUTNIK USNSS TRANSIT SYSTEM OPERATIONAL US DOD NAVSTAR GPS DEVELOPMENT BEGINS GPS USED FOR OFFSHORE TASKS STARFIX DGPS SERVICE LAUNCHED SKYFIX GLOBAL DGPS SERVICE OPERATIONAL FULL GPS 2 -D COVERAGE FULL GPS 3 -D COVERAGE GLONAS MERGE, INMARSAT NAV. SELECTIVE AVAILABILITY TURNED OFF
GPS NAVIGATION
ADVANTAGES OF GPS Ø For military users: Ø User passive, jam resistant, selective access, low detectability, nuclear hardening, mil spec equipment. Ø Receivers for all user classes - fighter aircraft, ships, tanks, bombers, soldiers, etc. Ø For civilian users: Ø Low cost continuous positioning for general use Ø Very high accuracy for industrial / commercial users.
NAVSTAR GPS CHARACTERISTICS Ø Ø Ø 21 operational satellites & 3 active spares 12 HOURS, 20200 km, NEAR CIRCULAR ORBITS Dual frequency: 1575. 42 MHz & 1227. 6 MHz TWO CODES: Ø C/A COARSE/ACQUISITION (civil users) Ø P PRECISE (military & authorized users) Accuracy of c/a code: (1. S. D. ) Ø 10 -15 m (no SA) Ø 50 m (S. A. Applied) Carrier phase applications possible for high precision stationary/low dynamic
GPS SEGMENTS Ø Control segment Ø Monitor stations track all satellites Ø Master control station injects updated orbit and clock data into satellites Ø Space segment Ø 21 satellites plus 3 active spares in 6 orbit planes Ø 12 hour 20, 000 KILOMETRE orbits Ø At least 4 satellites visible to any user at all times Ø User segment Ø Ø User set receive ranging signals from the satellites Compute position and velocity Many different types of receivers Civil and military users
THE GPS SEGMENTS
GPS SYSTEM CONFIGURATION
GPS SEGMENTS Ø User segment Ø User set receive ranging signals from the satellites Ø Compute position and velocity Ø Many different types of receivers Ø Civil and military users
GPS CONTROL STATIONS
GPS CONSTELLATION 21 SATELLITES WITH 3 OPERATIONAL SPARES 6 ORBITAL PLANES, 55 DEGREE INCLINATIONS 20, 200 KILOMETRE, 12 HOUR ORBITS
THE GPS SATELLITE
THE GPS BLOCK 1 SATELLITE
GPS Block IIR SPACE VEHICLE
GPS SYSTEM - SPACE VEHICLES
GPS SATELLITE INSTALLATION
THE MS 750 GPS RECIEVER
GPS SIGNALS (Observables) Ø Signal L 1 L 2 Ø Carrier Frequency 1575. 42 Ø Wavelength 19 cm Ø Codes C/A, P (P 1)P (P 2) Ø NAVDATA (ephemeris, clock corrections, etc) 1227. 60 24 cm
GPS SIGNALS Ø The p-code is clocked at 10. 23 MHz and is 7 days long (apparent wavelength = 30 Meters) Ø The c/a-code is clocked at 1. 023 MHz and is 1 ms long (apparent wavelength = 300 meters) Ø The c/a code is used to acquire a satellite Ø The c/a code provides the means to acquire the p-code Ø Under normal circumstances, the p-code is encrypted and referred to as the y-code Ø The navigation message can be decoded once either code is acquired and tracked
GPS SIGNALS C/A CODE Ø 1023 bit binary sequence with a period of 1 millisecond P CODE Ø 2. 34*1014 bit binary sequence with a period of 38 weeks NAVIGATION MESSAGE Ø Binary format of 1500 bits transmitted at 50 hz includes orbital, clock and ionospheric information
THE GPS NAVIGATION MESSAGE
EPHEMERIS TYPES PRECISE Ø Observed data from tracking stations available after approximately 10 days over the Internet BROADCAST Ø Available from the Navigation Message. Full Kepler elements ALMANAC Ø Available from the Navigation Message. Reduced Kepler elements, used for acquisition and predictions
THE C/A CODE AMBIGUITY
GPS ERROR SOURCES Ø Ø Ø Ø Satellite clock offset from GPS time Satellite ephemeris Atmospheric delays (Ionosheric and Tropospheric) Receiver clock offset from GPS time Receiver Multipath Receiver measurement noise Satellite geometry (-DOP) Selective Availability (SA) – Switched off May 2000
Thales Geo. Solutions Introduction to Differential GPS
OBJECTIVES By the end of this session you will be able to: Ø Explain why Differential corrections are needed Ø Explain the operation of a differential correction system Ø Explain why time is critical in the application of differential corrections Ø Explain how dual-frequency DGPS operations overcome Ionospheric delay Ø Explain how SDGPS differs from DGPS Ø Explain the techniques used in SDGPS & Sky. Fix. XP
DIFFERENTIAL GPS Ø Ø Ø Differential GPS is the use of a correction signal to improve the accuracy of the Standard GPS DGPS utilizes GPS receivers at a fixed reference station and at a mobile vehicle, vessel or user The GPS determined position of a reference station is compared to it’s surveyed geodetic position from which a correction is derived Some DGPS systems use the error in fix position, while others use individual satellite range errors to calculate the correction The correction message is broadcast via a radio or satellite link
DIFFERENTIAL GPS PRINCIPLES
RTCM SC-104 MESSAGE TYPES Radio Technical Commission for Maritime Services
CONSIDERATIONS FOR DGPS CORRECTIONS Speed Ø With SA on correction latency must be under 10 seconds and with SA off must be under 50 seconds Accuracy Ø Must be free of errors Range Ø Must be capable of transmitting corrections from the reference station to the work area Distance Ø Distance from reference station effects accuracy
INMARSAT COMMUNICATION SATELLITES
SKYFIX USES THE INMARSAT SYSTEM
CORRECTIONS ARE MONITORED 24/7
SKYFIX - NETWORK Ø Operational since 1989 Ø Global DGPS system using Inmarsat for broadcast links Ø Also available over high-power Spot. Beam links Ø World-wide network of >80 reference stations Ø Two control centers at Aberdeen and Singapore provide 24 hour monitoring and quality control Ø Fully redundant equipment and links
SKYFIX CO-ORDINATIONS Ø Existing control ü GPS carrier phase data ü Broadcast Ephemeris ü Local/ITRF/DMA control used Ø Accuracy considered better than 2 meter usingle reference station and better then 1 -meter using multiple reference station network solution
SKYFIX CO-ORDINATION Ø CURRENT AND FUTURE CONTROL ü LOCAL GPS DATA REPROCESSED WITH IGS ü PRECISE EPHEMERIS (JPL) ü ADJUSTMENTS CONSTRAINED TO IGS EPOCH 1992. 5 CO-ORDINATES Ø ACCURACY CONSIDERED TO BE BETTER THAN 5 cm
SKYFIX - REFERENCE STATIONS Ø Ø Ø Ø Dual equipment, redundancy of communications Typically 12 channel Trimble 4000 DS receivers with RS 4000 software High level of remote access and control capabilities (change of parameters, initialization of logging etc) Both sets of RTCM data returned to control center Multipath audit every 6 months Coordinated relative to ITRF 91 - WGS 84 (G 730) 24 hour local support at each station
BASIC REFERENCE STATION LAYOUT
THALES SKYFIX INFRASTRUCTURE GPS Satellites VSAT Satellites High Power and Low Power satellite links 2 MCC facilities • Aberdeen and Singapore • 2 remote MCC facilities at Perth and Reston • Monitor and Control • Archive • Maintain and Plan • Manage external entities • Interface Multi. Fix III RIMS A 19 inches rack RIMS A LES facilities 19 inches rack • Uplink Sky. Fix messages • Including Sky. Fix Premier Messages RIMS A 80+ Reference Stations • Dual-Frequency Stations • Single Frequency • Generate, process and transmit messages X 25 NETWORK
THE IONOSPHERE – A SUMMARY Two distinct problems: 1. Differential Ionospheric delay errors Ø Propagate into the navigation solution causing position biases 2. Scintillation effects Ø Loss of DGPS corrections from satellite DGPS links Ø Intermittent tracking of GPS satellites
SOLAR DISTURBANCE AND THE IONOSPHERE
WHERE: AREAS AFFECTED Operational ‘Hot Spots’ Scintillation Severe Disturbance Geomagnetic Boundaries
GLOBAL IONOSPHERIC TEC MAP
SOLAR ACTIVITY CYCLE
SKYFIX PREMIER Ø Ø Ø Introduced to mitigate ionospheric disturbances Use dual-frequency GPS reference stations Sky. Fix premier messages (type 55’s) Dual-frequency GPS user setup Thales’ Multi. Fix III or Multi. Fix IV software Calculated Iono-free DGPS positions: unbiased and consistent performance around the clock
SKYFIX XP SDGPS What is Sky. Fix XP? Ø SDGPS – Satellite Differential GPS Ø High Accuracy Ø 10 cm – Horizontal (1 - 68%) Ø 15 cm - Vertical (1 - 68%) Ø Global Coverage from a single set of corrections Ø No Station Range Restrictions – seamless worldwide coverage Ø Ø Available on all Sky. Fix Beams (excluding local SPOT services) Compatible with all existing Sky. Fix Hardware
SKYFIX XP SDGPS – HOW DOES IT WORK? Ø Ø Ø Global Network of stations used to track all satellites simultaneously Each individual error source on each satellite is identified and uniquely corrected for All satellite corrections are combined into a single global correction message Ionospheric delay and other local error sources are measured using a dual frequency GPS receiver and Multi. Fix 4 software. The user software corrects the standard GPS pseudorange based on the global XP corrections and the local ionospheric observations “Error Free” GPS position calculation formed
SKYFIX XP – WHAT ARE THE ERRORS? Satellite Clocks Orbits Ionospheric Delay Multipath Tropospheric Delay
SKYFIX XP – CORRECTING THE ERRORS Orbits ØBy tracking each satellite throughout its orbit a precise orbit correction can be generated. ØRegularly updated in case of satellite re-tasking (deliberate movement of SV within the orbit) Clocks ØThe global network of reference stations is also used to generate a highly accurate clock corrections
SKYFIX XP – CORRECTING THE ERRORS Ionosphere ØDual Frequency GPS observations used to correct for ionospheric error Troposphere ØTropospheric modeling and GPS delay measurements used to correct for Tropospheric delay Multipath ØHandled using the Strobe Edge correlator technology in the Thales ZX-Sensor Dual Freq. GPS receiver
SKYFIX XP - SDGPS vs. DPGS 24 Hour Plots for Norwich, England
SKYFIX XP HARDWARE n Compatible with all Existing Sky. Fix Hardware n Mk. 5 Sky. Fix Decoder n 90938 Sky. Fix Decoder n 2403 Sky. Fix Decoder n Mini. Dome n Inmarsat Taps n SPOT beam antennas
TYPICAL SKYFIX XP INSTALLATION Ships Inmarsat Dual Frequency GPs Antenna and/or Mini. Dome and/or SPOT Antenna or or Dual Frequency GPS Rx Sky. Fix Decoder Multi. Fix 4 PC
SKYFIX XP + SKYFIX PREMIER COMBINED INSTALLATION LOW POWER BEAM Ships Inmarsat or Mini. Dome HIGH POWER BEAM Dual Frequency GPs Antenna Sky. Fix SPOT antenna Dual Frequency GPS Rx Sky. Fix Decoders – RTCM + Sky. Fix XP Multi. Fix 4 PC Sky. Fix. XP Multi. Fix 3 PC Sky. Fix (Premier)
SKYFIX XP l Multi. Fix 4 Software n Based on Multi. Fix 3 n Full UKOOA based QC Statistic testing and displays n Sky. Fix XP, Premier and Standard DGPS calculations available n Automatic switching between calculations for optimum solution
SUMMARY OF DGPS SERVICES Standalone DGPS (or single reference DGPS) Ø Using a single reference station to provide corrections to a user’s GPS receiver Ø Maximum effective range from reference station is approximately 2000 km Ø Accuracy is range, constellation, and system dependent – approximately 2 -3 meters of horizontal error within 1000 km
SUMMARY OF DGPS SERVICES Multiple reference DGPS (and/or network adjusted) Ø Uses a network adjusted, multiple reference station correction and user system for a more accurate and reliable position Ø Maximum effective range from reference station is approximately 2000 km Ø Accuracy is range, constellation, and system dependent – approximately 1 meter of horizontal error within 2000 km
SUMMARY OF DGPS SERVICES High Precision DGPS or SDGPS Ø Corrects clock & orbit of each satellite Ø User software & hardware corrects for Ionosphere, Troposphere, Multipath, receiver noise and Earth Tides Ø Reference stations are actually tracking stations observing satellite orbit and clock Ø No range limitations – Global correction Ø Accuracy is constellation dependent – approximately 10 centimeters of horizontal error - Globally
Thales Geo. Solutions Introduction to GPS & DGPS Test Session
QUESTIONS 1. 2. 3. 4. 5. What is the stated 2 -sigma (95%) position accuracy standard of the single-frequency (L 1) service as of June 2000? What are the 2 -sigma (95%) horizontal & vertical global averages for L 1 all-in-view user performance for 95% of the earth’s surface? State the L 1 GPS frequency? State the L 2 GPS frequency?
QUESTIONS How many active GPS satellites are in orbit? 7. GPS satellites orbit the earth in how many orbital planes? 8. What is the minimal number of GPS satellites that must be in view above the GPS receiver’s elevation mask in order to calculate a 3 D position? 9. What is the wavelength of the L 1 signal? 10. What is the C/A code used for? 11. When can the navigation message be decoded? 12. What information is contained in the navigation message? 6.
QUESTIONS What are three types of ephemeris information? 14. What are the GPS error sources? 15. What is differential GPS? 16. What does RTCM stand for? 17. Name two methods used to calculate corrections? 18. What is the approximate maximum effective range from a reference station that a user can be for the corrections to be valid? 13.
QUESTIONS Name the two most common methods of receiving corrections? 20. What are the two types of ionospheric effects that plague DGPS operations? 21. Where are ionospheric delay and scintillation most often experienced? 22. About how long is the solar activity cycle? 23. What does SDGPS stand for? 24. What are three types of DGPS services? 25. What horizontal accuracies can be obtained utilizing SDGPS? 19.
ANSWERS 1. 2. 3. 4. 5. 6. 7. 8. 9. 13 meters horizontal & 22 meters vertical 8. 3 meters horizontal & 16. 8 meters vertical 16. 4 meters horizontal & 29 meters vertical 1575. 42 MHz 1227. 6 MHz 21 operational and 3 active spares 6 4 19 centimeters
ANSWERS Acquire GPS satellites 11. After the C/A code is acquired and tracked 12. Orbital, Clock and Ionospheric information 13. Precise, Broadcast, and Almanac 14. Satellite clock, Satellite orbit, Ionospheric delay, Tropospheric delay, Receiver clock, Multipath, Receiver noise, and satellite geometry 15. The use of a correction signal to improve the accuracy of a standard GPS 16. Radio Technical Commission for Maritime Services 10.
ANSWERS Calculate the error in the fix position or individual satellite ranges 18. 2000 Km 19. Satellite and radio broadcast 20. Ionospheric delay errors and Scintillation 21. Along the Magnetic Equator (or in Equatorial regions) 22. 11 years 23. Satellite Differential GPS (or Satellite-corrected Differential GPS) 17.
ANSWERS Standalone or Single Reference DGPS, Multiple Reference and/or Network Adjusted DGPS, and SDGPS 25. 10 centimeters horizontal 24.
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