User's Manual

XT55/56 Hardware Interface Description
Confidential / Released
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XT55/56_hd_v02.06a Page 70 of 125 17.12.2004
4 GPS application interface
The XT55/56 module integrates a GPS receiver which offers the full performance of GPS
technology. The GPS receiver continuously tracks all satellites in view, thus providing
accurate satellite position data.
The GPS block can be used even if the XT55/56 module is deregistered from the GSM
network.
4.1 Theory of operation
Figure 26: Theory of operation
The XT55/56 GPS part is designed to use L1 Frequency (C/A Code) GPS receiver and
performs the entire GPS signal processing, from antenna input to serial position data output.
The processing steps involved are:
• RF section
In the RF section the GPS signal detected by the antenna is amplified, filtered and
converted to an intermediate frequency (IF). An A/D converter transforms the analogue
intermediate frequency into a digital IF signal.
• GPS channels
The received digital IF signal bit stream is passed to the baseband section, where it is fed
into the correlators. The function of the correlators is to acquire and track the satellite
signals. There are 12 channels used in parallel, with each correlator looking for a
characteristic PRN code sequence in the bit stream. Once the correlator has found a
valid signal, pseudo range, carrier phase and orbit information can be extracted from the
GPS signal.
• Navigation
The on-board processor is running an algorithm that calculates the position, velocity and
time. This calculation is called navigation solution. Once the navigation solution is
calculated, it can be transformed into the desired coordinate system, e.g. Latitude/
Longitude/ Altitude. For further details refer to [3].
• Interface
The data of the navigation solution are available at the serial RS-232 interface.