INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

The Global Positioning System

Bart Krol / Jeroen Verplanke

The Global Positioning System

The Global Positioning System

Acknowledgements

GPS Overview by Peter H. Dana, Department of Geography, University of Colorado, USA.

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

GPS Tutorial by Trimble Navigation Ltd.

http://www.trimble.com

Why GPS ?

Finding out where you are and where you’re going

Positioning is crucial in handling geographical data

A world wide system for positioning

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 satellites2. For resection a GPS receiver calculates

distance to satellites using travel time of radio signals

3. To measure travel time, GPS needs exact timing4. Along with distance you need to know the satellite position 5. For accurate positioning you must correct for errors

1. Resection from satellites

3 satellites for a position fix

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

3. Exact timing

Correcting for clock errors in 2D:

At least:3 satellites for 2D fix4 satellites for 3D fix

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. Correcting for errors

Geometric Dilution Of Precision (GDOP)

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

5. Correcting for errors

GDOP, continued

We’re somewherein this box At close angles

the box gets bigger

5. Correcting for errors

GDOP, continued

GOOD COMPUTED GDOP AND BAD VISIBILITYRESULTS IN POOR GDOP

5. Correcting for errors

Selective Availability (SA)

The US military can introduce intentional errors to limit accuracy for civil GPS users

SA introduces an artificial clock error into the radio signal and writes an error in the satellite status information

If SA is ‘on’ a potential horizontal accuracy of ± 30 meters will be reduced to ± 100 meters.

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

Using a handheld GPS receiver

Typical accuracy: ±10 m Horizontal

( civilian use, good GDOP)

Results of measurements over one 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)

How GPS works

To sumarize

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

4. Position in space 5. errors

How to operate the Garmin

Basic buttons:

change up/down

Enter

Next page

Power

How to operate the Garmin

Navigation screen:

AccuracyUnits

Horizon

Skyline at 45º

Zenith (90º)

Signal strength

Satellite number

Battery level

Mode

Expected sat position

Known sat position

Set up the GPS and go outside

In the classroom Familiarize yourself with the GPS Set-up the GPS Enter Waypoints

Outside the main entrance Check position Try using the compass in the GOTO function Go and find the waypoints