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: ???) :

Cálculo de la posición usando la tecnología GPS ? X X Superficie de la Tierra

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

1. Resection from satellites X A Da Db B Da Db X 2 satellites A + B

1. Resection from satellites Dc X Da Db A B 3 satellites A + B + C

2. Measuring distance to satellites = velocity * travel time Distance is about 22,000 km  We cannot see satellites  We cannot measure exact distance Calculating distance to satellite Using travel time of radio signal Travel time = ? approx. 0.07 sec ! Using radio signal to calculate distance This signal travels with speed of light Speed of light = 299,174 km/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

5. GPS settings units’ settings: position format: hddd.dddddo, map datum: WGS 84, and units: metric ‘interface’ settings: i/o format: NMEA out, and baud rate: 4800.

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