INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION The Global Positioning System Bart Krol / Jeroen Verplanke

The Global Positioning System

Acknowledgements  GPS Overview by Peter H. Dana, Department of Geography, University of Colorado, USA.  GPS Tutorial by Trimble Navigation Ltd.

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 segmentUser segment

How GPS works 1. The basis for GPS is resection from satellites 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

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 distance This signal travels with speed of light Speed of light = 299,174 km/sec Calculating distance to satellite Using travel time of radio signal Travel time = ? approx 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 4 th satellite to correct for clock error

3. Exact timing Correcting for clock errors in 2D: At least: 3 satellites for 2D fix 4 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 somewhere in this box At close angles the box gets bigger

5. Correcting for errors GDOP, continued GOOD COMPUTED GDOP AND BAD VISIBILITY RESULTS 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 (

How GPS works To sumarize 1. Resection from satellites2. 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: Accuracy Units 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