GLOBAL POSITIONING SYSTEM

1. GLOBALPOSITIONING SYSTEM • GNSS – Global Navigation Satellite System • US GPS System (Navstar) • Russian GLONASS system • European Galileo System

2. GPS SYSTEM COMPONENTS • (Fully operational since 1993) • The Space Segment • 24 satellites in six near circular orbits orbits • 24 hour coverage anywhere on earth’s surface between Lat. 80°N and 80°S • Altitude approx. 20 200km • Orbital period approx. 12hrs (speed of satellites about 14000km/hr) • Satellites equipped with very precise (and expensive!) atomic clocks • Satellites transmit signals with extremely stable frequencies • The Control Segment • Five monitoring stations (Col. Springs, Hawaii, Ascension, Diego Garcia, Kwajaleni) • Satellites monitored and tracked at control stations • Data relayed to Master Control Station (Colorado Springs) • Orbital parameters and clock corrections computed and uploaded to satellites for transmission to system users (broadcast vs rapid (24 hrs) vs precise ephemeris (2wks)) • The User Segment • GPS receivers • Passive devices that record and analyze satellite signals for positioning • Various receiver types for different levels of accuracy and applications • Equipped with less precise (less expensive) clocks than satellites

3. WGS 84

4. CORE IGS TRACKING NETWORK – late 1998 Source: http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap12/1224.htm#fig1

5. A THE GPS MEASUREMENT PRINCIPLE ·Based on the basic physical relationship: distance = velocity * time ·Observations (pseudo-ranges) from 4 satellites provide 3 dimensional position (3 positional and 1 time unknown) ·Coordinate system realized by the satellite orbits (ephemeris data) and by the coordinates and physical locations of the control and tracking stations Trilateration

6. A: Geodetic (carrier phase with resolved ambiguities), real-time/post-processed B: Carrier smoothed C/A Code Phase, post-processed C: Real-time (RTCM SC104), post-processed C/A Code D: Real time P-Code (Precise Positioning Service [PPS]) E: Real time C/A-Code (Standard Positioning Service [SPS]) GPS TECHNOLOGY CLASSIFICATION mapping geodetic navigation/ 100 m civilian (SPS) recreational grade grade (prior to 05/02/00) grade 20 m civilian (SPS) – post 05/02/00 APPROXIMATE ACCURACY 10 m military (PPS) 5 m 1 m 0.5 m dm cm mm E C A B D POINT (ABSOLUTE) RELATIVE POSITIONING POSITIONING Selective Availability switched off – see http://geography.about.com/library/weekly/aa050400a.htm

7. The Geocentric Cartesian Coordinate System Z Satellite P Greenwich Meridian N ZP A Y XP YP Equator S X AP = √(XP-XA)2 + (YP-YA)2 + (ZP-ZA)2

8. THE GPS SIGNALS • Each Satellite transmits two carrier waves • L1 - frequency of 1575.42 MHz and a wavelength of approx 19cm • L2 - frequency of 1227.60 MHz and a wavelength of approx 24cm • The following satellite-specific signals, called the pseudo random noise (PRN) codes are modulated on the carrier waves: • On L1: C/A (Coarse/Acquisition) code λ = approx 300m • - Accessible to civilian users • - Consists of a series of 1023 binary digits (called chips) that are unique to each satellite. • - The chip pattern is repeated every millisecond • P (precise) code λ = approx. 30m • - Accessible only to military equipment • On L2: P code only Coming on-line: L2C and L5

9. Code Signal Positioning Subframe of message Receiver Signal TimeDelay Matching Subframe DelayedSatellite Signal The ‘mis-match’ between the code patterns is a measure of the time the signal has taken to travel from satellite to receiver.

10. Geometric Dilution of Precision - Measures the effect of geometry on the precision of the observations - Multiply GDOP by the Std Error to get actual uncertainty - Also HDOP, VDOP Position Dilution of Precision (PDOP) - This is positional part of GDOP

11. COMMON MISTAKES • Logistical weaknesses • battery power, memory overruns, no inter-party communications, no • contingencies in observation schedule • Operator mistakes • incorrect antenna heights, careless centering, incorrect receiver settings • (epoch interval), accidental deletion of raw observations, inadequate field • records, careless handling of antenna and power cables • Processing mistakes • insufficient or incorrect datum definition (e.g. incorrect base station • coords), no checks on centering and antenna heights, inclusion of trivial • base lines, insufficient redundancy and quality checks

12. Precautions to minimize errors • Schedule your survey to fall within periods of good satellite geometry (i.e. low PDOP) • Eliminate satellites at low elevation to reduce the length of the signal path through the atmosphere • Avoid multi-path conditions near the GPS antenna • For precise positioning use differential corrections and/or phase observations of the carrier waves 15° (Mask Angle) Earth Atmosphere Multipathing

13. GPS POSITIONING ERROR CLASSIFICATION

15. Post-processing vs Real Time Correction

16. Base Stations Connected via cable Tirana, Albania Antenna on Tripod Receiver and Laptop logging base station measurements Base Station over Known Point – Cajamarca, Peru

17. Differential GPS (Static) Single Differencing: One satellite observed from two receivers Satellite clock error is eliminated Double Differencing: Two satellites observed from two receivers Receiver clock error is eliminated Triple Differencing: Two satellites observed from two receivers at two different epochs. Eliminates integer cycle ambiguity Epoch 2 Epoch 1

18. Real Time Kinematic (RTK) Differential corrections are broadcast via radio Base station over free point Base station over known point Data latency = 0.05 – 1.0 secs Radio limits range between base and rover

19. THIRD PARTY DIFFERENTIAL CORRECTION SERVICE • Service available commercially (e.g. Omnistar) • Sub-meter accuracies possible when used in combination with L1 • User needs only one receiver GPS satellites Geostationary Communication Satellite Differential Base Station Rover Footprint of Communication Satellite coverage See http://www.omnistar.com/

20. Eccentric Points Geostationary Communication Satellite Useful when Canopy prevents direct occupation of point or when Communication Satellite is blocked

21. STATIC SURVEYS FOR CONTROL NETWORK IN NAMIBIA Source: Walter Volkmann

23. Fiducial Points for defining GPS datum in the country

24. Surveying river boundaries with GPS – Belize River, Belize

25. Cadastral survey using handheld GPS - Ecuador

26. Field testing rapid GPS cadastral surveying methodology - Peru

27. Calibrating sub-meter GPS receiver – Tirana, Albania

28. Testing GPS methodology for surveying rural properties - Nicaragua

29. Measuring Control Points - Zaire