Location Finding, Global Positioning System, Function

Basic illustration on model 210


General
The global positioning system (GPS) is a satellite-supported location finding system set up by the US Department of Defence. It is not subject to fees and is available worldwide. It presently consists of 26 satellites (including one in reserve) in 6 different earth orbits. The GPS satellites permanently transmit time and position data. When the data from at least 3 satellites is received, the position of the receiver can be calculated (for two-dimensional location finding).
GPS location finding is of particular importance during trips outside of the digitized map area and following transport of the vehicle (e.g. by train, ferry or towing).
New calculation of the GPS position (following an interruption in the satellite reception) can require several minutes (up to 20 minutes when unit is first put into operation).

Function
The navigation process or integrated into the radio and navigation unit (A2/56) calculates the vehicle location and the accuracy of the position from the amplified satellite signals received by the
GPS antenna (A2/49a2) (on models 163, 168, 208.3, 210) or rear GPS antenna (A2/23a2) (on models 170, 208.4) antenna amplifier module (A2/71) (on model 203)

The actual calculation is based primarily on the measurement of the transition times of the signals transmitted synchronously from the various satellites. An important prerequisite here is that the satellite docks and the dock in the receiver be set synchronously. For this purpose, the receiver has an automatic time correction feature which again, is possible with the aid of the satellite signal.
When the navigation processor determines that the basic location finding
is not as precise as the GPS location finding, the currently calculated position is corrected with the aid of the GPS data. Moreover, the course of the vehicle is determined from the sequence of the locations calculated with the GPS data.

Possible interference for GPS signal reception.
Since the GPS data is transmitted in the Giga-Hz range (microwaves) and the signals are weak, reception interference can occur as the result of:

Atmospheric interference,
e.g. weather influences, water vapor, fog

multiplex reception resulting from reflection of the signals,
e.g. from building walls

signal shadows,
e.g. in built-up areas, tunnels, high-rise buildings, trees


A free view of the sky is therefore required for GPS reception. The accuracy of the position calculated with the GPS system is within 100 m or better.