INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

Global Positioning Systems (GPS)

Jeroen Verplanke

Department of Urban and Regional Planning and Geo-Information Management

What is GPS ?

A world wide radio-navigation system

Uses satellites as reference points to calculate positions

Three components: Space segment Control segment User segment

GPS components

Space segment

GPS components

Control segment User segment

How GPS works

1. The basis for GPS is resection from satellites(triangulation)

2. For resection a GPS receiver calculates distance to satellites

using travel time of radio signals

3. To measure travel time, GPS needs exact timing

4. Along with distance you need to know the satellite position

5. For accurate positioning you must correct for errors

What GPS accuracy do we need?

Soil map (1:250,000) : Road map (1:100,000) : Topographical map (1:50,000) : Ecological map (1:25,000) : Cadastral map (1:10,000) : Architectural plan (1:1,000) : Sketch map (1: ???) :

Superficie de la Tierra

Cálculo de la posición usando la tecnología GPS

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Resection from satellites: determination of an unknown location using 1, 2 and 3 distances to known locations of satellite(s).

1. Resection from satellites

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2 satellitesA + B

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1. Resection from satellites

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3 satellitesA + B + C

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1. Resection from satellites

2. Measuring distance to satellites

Distance is about 22,000 km We cannot see satellites We cannot measure exact distance

Distance = velocity * travel time

Using radio signal to calculate distanceThis signal travels with speed of lightSpeed of light = 299,174 km/sec

Calculating distance to satelliteUsing travel time of radio signalTravel time = ? approx. 0.07 sec !

3. Exact timing

How to measure travel time satellite and receiver generate radio signal at the same time travel time = phase difference between signals

1 msec

Satellite’s signal

GPS receiver’s signal

3. Exact timing

Very precise clocks for exact timing satellites : highly accurate ‘atomic’ clocks

(about USD 100,000 each)

receivers : moderately accurate quartz clocks

Clock error due to difference in clock accuracy

use a 4th satellite to correct for clock error

4. The satellite position in space

Using satellites as reference points for positioning also requires that you know the exact position in space of each satellite, at any place and at any time.

The GPS control segment monitors the satellite position in space.

All details of satellite orbits is available in an ‘almanac’

This satellite status information can be downloaded to the GPS receiver

5. Correcting for errors

Main GPS error sources

Clock errors Signal errors (noise) Interference in ionosphere and troposphere Multipath error Satellite position (“ephemeris”) error Geometrical error (Geometric Dilution of Precision - GDOP) Intentional errors (Selective Availability - SA) Human errors Receiver errors (hardware, software, antenna)

5. GPS Signal – optimal

5. Correcting for errors

Geometric Dilution Of Precision (GDOP)

GOOD GDOP ( 2) POOR GDOP (2-6)

5. GPS Signal – good GDOP, poor visibility

5. GPS Signal – beware of yourself

5. GPS Signal - solution

5. Correcting for errors

Selective Availability (SA)

SA on (before May 2000) SA off (after May 2000)

5. Correcting for errors

Some typical errors

Satellite clock error 2 meter

Receiver noise 0.5 meters Interference in ionosphere and troposphere 5 meters

Multipath error 1.4 meter Satellite position (“ephemeris”) error 2 meters

poor GDOP up to 200 meters

Human errors up to hundreds of meters Receiver errors (hardware, software, antenna) any size possible

• units’ settings: position format: hddd.dddddo, map datum: WGS 84, and units: metric

• ‘interface’ settings: i/o format: NMEA out, and baud rate: 4800.

5. GPS settings

About GPS accuracy

Accuracy can be from 100m to 1mm Accuracy depends on purpose

Soil map (1:250,000) : Road map (1:100,000) : Topographical map (1:50,000) : Ecological map (1:25,000) : Cadastral map (1:10,000) : Architectural plan (1:1,000) : Sketch map (1: ???) :

Using a handheld GPS receiver

Typical accuracy: 10 m Horizontal( civil use,good GDOP)

Results for a Month (Garmin 12XL)

Horizontal Accuracy (50%) 3.9 meters Vertical Accuracy (50%) 9.6 meters

Horizontal Accuracy (95%) 9.3 meters Vertical Accuracy (95%) 21.9 meters

Source: GPS ACCURACY MONITOR by Dennis Milbert (http://mywebpages.comcast.net/dmilbert/handacc/accur.htm)

In conclusion

1. Resection from satellites 2. Distance to satellites 3. Exact timing

4. Position in space 5. errors