Gps Segments
Transcript of Gps Segments
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Introduction to GlobalPositioning Systems (GPS)
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Wh at is GPS?
A satellite based positioning systemThree Segments
Space Segment - 24 satellites Control Segment User Segment
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Satellite Segment
NAVSTAR (NAVigation by Satellite Timingand Ranging)
Use precise clocksVery high frequency 1200-1500 Mhzcommunication (travels 25 cm in .000000083seconds).Satellites orbit in 6 orbital plates to providecomplete coverageSatellites orbit at 10,900 miles (2 orbits per da
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GPS Satellite
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GPS Constellation
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Ground Track
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Control Segment
Run by the US MilitaryGround based
Used to track and adjust satellitesComputes exact orbits (for location)
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U ser Segment
Units receive transmission from SatellitesCost between $150 and $15000 (depends
on accuracy)Must be line of sight with satellites
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H ow does it work?
Position of satellites is known. Satellite provides an electronic almanac to the
receiver
The speed of the radio signal from eachsatellite is knownTiming allows the calculation of the distancefrom each satellite.Using triangulation, the position of thereceiver is calculated.
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H ow measurements are
possibleEach satellite sendsinformation on its location andthe time code is sent.It sends a code that allows themeasurement of the distanceto the satellite.If four satellites are visible, asingle receiver can measurelocations to within 100 meters
95% of the time.
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GPS Signals
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GPS
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Triangulation
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Geometric Dilution of
Precision (GDOP)
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Assume tim i e rr r f m i r second
speed of i t in cuum m i essec
186,000 m i essec
sec1,000,000
! 0.186m i es 5280 ftm i e
! 982 ft.
N ote t h e importance of accurateclocks
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Differential GPS (DGPS)
Used to correct for errorsUses a ground station at a known location
Ground station measures error of GPSsignalError transmitted to DGPS unit and position
is corrected.
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Differential GPS (DGPS)
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DGPS Signals
US Coast Guard Near navigable waters
FreeSubscription Services FM (line of sight) Satellite
Most of the Valley gets Coast Guard signal
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1 Meter Correction sources -Coast Guard
http://www.ngs.noaa.gov/CORS/Cors_coverage.html
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CA Coast Guard Coverage
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True Range vs.
PseudorangeErrors in range (distance) result in positionerrors. Satellite Clock Error Signal Transmission Error Error in receiver clock
Carrier-phase tracking Used for very precise location $$$
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Improving Accuracy -Corrections
Real time vs. Post Process correctionsReal Time - Know your location in real time -
radio link neededPost process - Process the data in the officeto find real location
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Accuracy
GPS with Selective Availability on: 50 mGPS with SA off: 25 m
DGPS 1-3 m to 1-2 cmBetter accuracy is achieved by samplingmore points (statistical)
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Improving Accuracy -
CorrectionsTwo receivers required - error about thesame at each
Put a receiver over a known locationThe difference between know location andmeasured location is the error
Apply this error correction to the movingreceiver to improve accuracy
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Code p h ase vs. Carrier
ph
aseThe accuracy of the measurement is relatedto the length of wavelength used.
Code phase - 300 meter wavelengthCarrier phase - 0.19 meters
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Code P h ase
Vs. Carrier Ph
aseAdvantages of CodePhaseOver Carrier Phase Since the pseudo
random code has aknown pattern it isquicker to obtain aposition.
It is less susceptible tointerference such astrees.
Advantages of carrier phase over codephase The accuracy possibly
increases from 1 meter to 1 cm.
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GPS Output
Commonly Latitude and LongitudeLatitude
North/South positionLongitude East/West Position
Given in decimal degrees. Ex.-119.727289 36.822758 West Lon. North Lat.
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U sing GPS Data
GPS Unit stores data then download tocomputer
GPS Unit has direct interface to computer GPS Unit is interfaced to: Yield monitor Variable rate applicator Navigation guidance (parallel swathing)
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1cm - base station on site