G. P. S.

1. G. P. S. Miles Logsdon, College of Ocean and Fishery Sciences Phil Hurvitz, College of Forest Resouces The Global Position System

2. Basic Concepts • GPS • U.S. government • NAVigation System with Time And Ranging • NAVSTAR • 24 satellites • Russian syste • GLONASS

3. Geography • Location • index space: coordinates • Latitude-longitude • UTM • absolute v’s relative coordinates • 121 33’ 12” x 47 24’ 15” = absolute • 120km east and 40km north of Seattle

4. Position v’s Location • Position : GPS • coordinates that specify “where” • Location: maps • “where” with respect to know objects

5. Why use GPS (1) • Availability: • 1995, DoD NAVSTAR, civilian use foreseeable future • Accuracy: Factors • work with “primary” data sources • High inherent accuracy (2.5m medium-quality properly corrected receiver) • Time Corrected to 1/1 billionth of a second

6. Why use GPS (2) • Ease of use • stop and read a single coordinate = 20m accuracy (+/- 5m) real-time • 3-D data • horizontal (x & y) and altitude (z) • variances in z = horizontal * 2

7. GPS SV Satellite Transmitter Specifications (1) • Radio wave transmission (~20cm) • Not good without direct view of sky (i.e. inside, underground, under canopy, precipitation • 24 solar-powered radio transmitters, 3 spares • “middle altitude”, 20,200km, below geosynchronous orbit

8. Satellite Transmitter Specifications (2) • Neither polar nor equatorial • each execute a single 12 hour orbit • 4 satellites in each of 6 orbital planes • speed of 3.87 km/sec ( 8,653 mph) • weigh ~ 1 ton with 27 feet of solar panels • Orbit tacks monitored by 4 base stations • Master control station in Colorado Springs • Each satellite monitored twice a day

9. u B A X Finding distance by measuring time • Almanac: predicted position of satellites • Constellation: set of satellites used • DOP: Dilution of Precision • PRN: Pseudo random noise code • Electromagnetic radiation (EM) 299,792.5 Km/sec << 7/100 of a second after 4:00 4:00 p.m. >> Receiver: Satellite: G J K E T Y U O W V W T D H K … G J K E T Y U O W ...

10. We can locate our position on the surface of a sphere • Satellite location Given 1 satellite …

11. We can locate our position on the intersection of 2 spheres (a circle) • Satellite location Given 2 satellites …

12. We can locate our position on the intersection of 3 spheres (2 points) • Satellite location Given 3 satellites …

13. We can locate our position on the intersection of 4 spheres (1 point) • Satellite location Given 4 satellites …

14. The point can be located on the earth’s surface • Satellite location

15. The precise location is determined • Satellite location

16. After the correct position is determined, the receiver’s clock is adjusted Adding or subtracting time will make the location more or less precise If the receiver’s clock is ahead, the position will be over-estimatedfor each signal • More on timing: Setting receiver clock

17. If the receiver’s clock is behind, the position will be under-estimatedfor each signal • More on timing

18. If the receiver’s clock is correct, the position will be properly estimatedfor each signal • More on timing

19. The receiver adds and subtracts time from simultaneous equations until the only possible (correct) position is located. The receiver’s clock becomes virtually as accurate as the atomic clocks in the SVs • More on timing

20. Sources of error: Dilution of precision (DOP) The best spread of satellites makes the best trilateration We want low DOP Satellites that are close to each other result in higher DOP: HDOP: horizontal DOP VDOP: vertical DOP PDOP: positional DOP (combination of HDOP & VDOP) TDOP: time DOP GDOP: geometric DOP (combination of PDOP & TDOP)

21. Major Factors of error • Satellite clock errors < 1 meter • Ephemeris errors (satellite position) < 1 meter • Receiver errors < 2 meters • Ionosphere errors (upper atmos.) < 2 meters • Troposphere errors (lower atmos.) < 2 meters • Multipath errors (bounced signals) ??? • “Selective Availability” signal transmission 0 - off (< 33m if on)

22. Error • Atmospheric • Light travels at 299,792,458 m/s only in a vacuum • Ionospheric effects: ionizing radioation • Tropospheric effects: water vapor • Light is “bent” or reflected • Clock • Receiver clock errors, mostly corrected by software in receiver • Satellite clock errors • Satellite time stamp errors • Time stamp errors are not correctable • SV timing & clocks are constantly monitored and corrected • Receiver • Power interrupts • On-board microprocessor failure • Firmware • Software • Blunders (user error)

23. Clock timing error factor introduced by the DOD Standard operation on the satellites. S/A changes the time stamp of the outgoing signals Calculated positions are erroneous SA causes locations to be in error up to 100 m Each satellite encrypts its own data separately Encryption keys shift frequently In the event of warfare, enemy forces cannot use the same accuracy as the US armed forces Military-grade have the ability to decrypt the time dithering,which lowers error to about 15 m from ~100 m uncorrected • Sources of error: Selective availability (S/A)

24. Recording Data • 180 fixes needed for maximum accuracy for a receiver and constellations • 1 fix every 3 seconds • You’ll need ~ 9 minutes

25. Import into GIS Uncorrected data

26. Import into GIS Differentially post-processed

27. Import into GIS Real-time corrected