Global Positioning System: Description and Operation

GF82.61-P-3007C Global Positioning System Location Finding, Function
- with CODE (353) Audio 30 APS







Basic representation of the GPS location finding shown on model 210

General information
The global positioning system (GPS) is a location finding system established by the US Defense Department and is satellite supported. It is toll-free and is available throughout the world. A lot of satellites are located on different orbits. The GPS satellites constantly transmit time and position data. If data from at least 3 satellites is received it is possible to calculate the position of the receiver.

GPS position finding is irreplaceable for journeys outside the digitized map area and after vehicle transport (e.g. train or ferry journeys and towing). A new GPS location finding (after interruption of satellite reception) can last several minutes (up to 20 minutes when first commissioning the equipment).

Function
The navigation processor that is integrated in the radio (A2) calculates the location of the vehicle and the accuracy of positioning from the satellite signals received and amplified.

The following components can receive satellite data:
- Telephone and GPS roof antenna (A2/49) (model 163)
- GPS antenna (A2/49a2) (model 168, 208.3, 210)
- GPS rear antenna (A2/23a2) (model 170, 208.4)
- GPS antenna (A2/23) (model 203, 209)

The actual calculation is essentially based on the measurement of the travel times of the signals sent synchronously by the various satellites. Here, an important prerequisite is that the satellite and receiver clocks are synchronized.

For this purpose, the receiver has an automatic time correction feature which again is possible with the aid of the satellite signals. If the navigation processor determines that the basic location finding is less accurate than the GPS location finding, the position currently calculated is corrected using GPS data.

In addition, the vehicle's course of travel is determined with the aid of the sequence of the locations calculated with GPS.

Possible interference when receiving GPS signals
Since GPS operates in the GHz range (microwaves) and the signals are weak, reception interference can occur as the result of:
- atmospheric interference,
e.g. weather, water vapor, fog
- multiple reception resulting from reflecting of signals,
e.g. from building walls
- Signal shadows,
e.g., in built-up areas, tunnel, in the proximity of higher buildings and trees

GPS reception therefore necessitates a clear view of the sky. The location finding accuracy of the GPS system is 100 m or better.