GPY 206.1

Computers and GIS in Geography

Fall 2002

3 December 2002

Global Position Systems

1.      What is it?  The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.  It is a  product of the US military.  Cost:  approximately $12 Billion.

2.    GPS Basics.

·        Satellites and their orbits http://www.colorado.Edu/geography/gcraft/notes/gps/gif/orbits.gif and http://www.colorado.Edu/geography/gcraft/notes/gps/gif/svs27.gif

·        Ground Stations http://www.colorado.Edu/geography/gcraft/notes/gps/gif/gpscont.gif

·        Receivers

3.    Finding your location using GPS (http://www.colorado.Edu/geography/gcraft/notes/gps/gif/figure09.gif  the intersection of signals from 3 satellites will give an approximate earth location.  Intersection of signals from four satellites will give precise location.  An explanation of finding location using GPS technology is given at http://www.trimble.com/gps/how.html

4.  The importance of time in GPS.

·        Time = distance = location

·        distance from satellites is measured in terms of the time it takes for the signal from the satellite to reach the receiver.  The signal travels at the speed of light (186,000 miles/sec or about298,000 kms/sec).  The time taken x the speed = distance.

·        in reality the time taken is very short, of the order of 0.06 seconds if the satellite is directly overhead.  Consequently, the clocks in the satellite must be very accurate.  It is not so important that the clock in the receiver be so accurate.  Instead, by measuring positions relative to four satellites, inaccuracies in receiver clocks are eliminated.

·        the US military has positioned each satellite into a very precise orbit.  That orbit is programmed into each GPS receiver’s almanac so that the receiver knows where (and when) each satellite’s position will be.

4.    Errors in Timing.  There are inherent errors in the operation of the system.

·        Signals from satellites are slowed by their passage through the ionosphere and the atmosphere (water vapour).

·        There may be “reflection” error (referred to as multipath error) on earth as the signals bounce off local obstructions.

·        Atomic clocks on satellites are very precise but they aren’t perfect.  They must be monitored by the earth station.

·        The US D of D had programmed in selective availability error (SA).  The idea behind it is to make sure that no hostile force or terrorist group can use GPS to make accurate weapons (this capability is reserved for the US!)  NOTE:  GPS Selective Availability (SA) has not been used since its deactivation by the US President on May 1, 2000.  At that time, the US Government said that it had no intention to ever use SA again.

·        Table 1 shows the typical errors that must be corrected for. It also shows how differential GPS (next point) corrects for much of these errors.

* eliminated May 1, 2000

5.    Differential GPS http://www.colorado.Edu/geography/gcraft/notes/gps/gif/difnav.gif

Involves the cooperation of two receivers, one that is fixed and the other that is roving around.  The fixed or stationary receiver ties all the satellite measurements into a solid local reference.  If the stationary receiver is positioned at a known location then any errors in the GPS operation can be corrected to the known location.  The stationary receiver can be accurately plotted (located) and the corrected information is passed onto the roving receiver(s).

6.    Applications of GPS http://www.trimble.com/gps/work.html This site gives the following applications

·        Finding location

·        Navigation

·        Tracking

·        Mapping

·        Timing

Sources:

http://www.trimble.com/gps/how.html
Don’t worry about downloading the plug-in. Trimble is a commercial developer of GPS receivers.
http://www.colorado.Edu/geography/gcraft/notes/gps/gps.html