Gps Segments

8/8/2019 Gps Segments

1/29

Introduction to GlobalPositioning Systems (GPS)


2/29

Wh at is GPS?

A satellite based positioning systemThree Segments

Space Segment - 24 satellites Control Segment User Segment


3/29

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


4/29

GPS Satellite


5/29

GPS Constellation


6/29

Ground Track


7/29

Control Segment

Run by the US MilitaryGround based

Used to track and adjust satellitesComputes exact orbits (for location)


8/29

U ser Segment

Units receive transmission from SatellitesCost between $150 and $15000 (depends

on accuracy)Must be line of sight with satellites


9/29

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.


10/29

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.


11/29

GPS Signals


12/29

GPS


13/29

Triangulation


14/29

Geometric Dilution of

Precision (GDOP)


15/29

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


16/29

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.


17/29

Differential GPS (DGPS)


18/29

DGPS Signals

US Coast Guard Near navigable waters

FreeSubscription Services FM (line of sight) Satellite

Most of the Valley gets Coast Guard signal


19/29

1 Meter Correction sources -Coast Guard

http://www.ngs.noaa.gov/CORS/Cors_coverage.html


20/29

CA Coast Guard Coverage


21/29

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 $$$


22/29

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


23/29

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)


24/29

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


25/29

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


26/29

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.


27/29

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.


28/29

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)


29/29

1cm - base station on site

Gps Segments

Documents

Transcript of Gps Segments