XT55 AVL User’s Guide
Confidential/Released
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Page 14 of 37
28.06.2004
3 Tracking
The GPS satellites in orbit are continuously transmitting radio signals. Each satellite
transmits a unique code, allowing the GPS receiver to identify its position, time and speed.
The main purpose of these received signals is to carry out the necessary calculation to
determine the current position of the vehicle.
3.1 History function
The embedded GPS receiver continuously calculates the current position data. In order to
save these data to the XT55 Flash memory the required NMEA command has to be sent,
see [4] for details.
When receiving valid GPS messages, the module XT55 is capable of saving up to 100000
GPS protocols in its history memory.
The XT55 is capable of saving up to 100.000 GPS messages (GGA, GSV, VTG, GLL,
RMC, depend on the user configuration) in its history memory.
When the memory space has been used up, the oldest message will be overwritten
automatically.
Figure 11 shows a logical flow chart, for a better understanding how the module XT55 saves
history data.
