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

2 Basic Concepts zGPS yU.S. government yNAVigation System with Time And Ranging –NAVSTAR 24 satellites yRussian syste xGLONASS

3 Geography zLocation yindex space: coordinates xLatitude-longitude xUTM yabsolute v’s relative coordinates x121 33’ 12” x 47 24’ 15” = absolute x120km east and 40km north of Seattle

4 Position v’s Location zPosition : GPS ycoordinates that specify “where” zLocation: maps y“where” with respect to know objects

5 Why use GPS(1) zAvailability: y1995, DoD NAVSTAR, civilian use foreseeable future zAccuracy: Factors ywork with “primary” data sources yHigh inherent accuracy (2.5m medium- quality properly corrected receiver) yTime Corrected to 1/1 billionth of a second

6 Why use GPS(2) zEase of use ystop and read a single coordinate = 20m accuracy (+/- 5m) real-time z3-D data yhorizontal (x & y) and altitude (z) yvariances in z = horizontal * 2

7 Satellite Transmitter Specifications(1) zRadio wave transmission (~20cm) zNot good without direct view of sky (i.e. inside, underground, under canopy, precipitation z24 solar-powered radio transmitters, 3 spares z“middle altitude”, 20,200km, below geosynchronous orbit GPS SV

8 Satellite Transmitter Specifications(2) zNeither polar nor equatorial zeach execute a single 12 hour orbit z4 satellites in each of 6 orbital planes zspeed of 3.87 km/sec ( 8,653 mph) zweigh ~ 1 ton with 27 feet of solar panels zOrbit tacks monitored by 4 base stations yMaster control station in Colorado Springs zEach satellite monitored twice a day

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

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

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

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

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

14 zSatellite location The point can be located on the earth’s surface

15 zSatellite location The precise location is determined

16 zMore on timing: Setting receiver clock 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-estimated for each signal

17 zMore on timing If the receiver’s clock is behind, the position will be under-estimated for each signal

18 zMore on timing If the receiver’s clock is correct, the position will be properly estimated for each signal

19 zMore on timing 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

20 zSources 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 zSatellite clock errors< 1 meter zEphemeris errors (satellite position)< 1 meter zReceiver errors< 2 meters zIonosphere errors (upper atmos.)< 2 meters zTroposphere errors (lower atmos.)< 2 meters zMultipath errors (bounced signals)??? z“Selective Availability” signal transmission0 - off (< 33m if on)

22 Error zAtmospheric yLight travels at 299,792,458 m/s only in a vacuum yIonospheric effects: ionizing radioation yTropospheric effects: water vapor yLight is “bent” or reflected zClock yReceiver clock errors, mostly corrected by software in receiver ySatellite clock errors ySatellite time stamp errors yTime stamp errors are not correctable ySV timing & clocks are constantly monitored and corrected zReceiver yPower interrupts yOn-board microprocessor failure yFirmware ySoftware yBlunders (user error)

23 zSources of error: Selective availability (S/A) 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

24 Recording Data z180 fixes needed for maximum accuracy for a receiver and constellations z1 fix every 3 seconds zYou’ll need ~ 9 minutes

25 zImport into GIS Uncorrected data

26 zImport into GIS Differentially post-processed

27 zImport into GIS Real-time corrected