Specifications

XT65/XT75 Hardware Interface Description
4 GPS Application Interface
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XT65_XT75_HD_v01.001 Page 73 of 133 2007-1-8
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4 GPS Application Interface
The XT65/XT75 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 XT65/XT75 module is deregistered from the GSM network.
4.1 Operating Principles
This section contains a brief overview of basic GPS operating principles (see also [20]).
4.1.1 Basic Operation Cycle
When the receiver is powered up, it steps through a sequence of states until it can initially determine position,
velocity and time. Afterwards, the satellite signals are tracked continuously and the position is calculated period-
ically.
In order to perform a navigation solution (3D solution), the receiver needs distances (pseudo ranges) for at least
4 SVs (Space Vehicles or satellites) and ephemeris data for the SVs it will use in the navigation solution.
The initial position calculation is made using a least-squares algorithm. Successive position calculations are per-
formed with a Kalman filter. To generate a position calculation (3D solution) the receiver needs at least 4 mea-
surements to different satellites; to calculate a position (Lat/Long/Height) for a 2D solution with an estimated
altitude, 3 different satellites are required.
Pseudo range and carrier phase information is available to the position determination algorithms once the
receiver has found a SV (acquisition) and can track the signal thereafter.
Ephemeris data for a SV can be decoded from orbit data once the GPS signal has been acquired. Each SV trans-
mits its own ephemeris data, the broadcast lasts for 18 seconds, repeating every 30 seconds.
The receiver stores ephemeris data in battery-backup memory (supplied by VDDLP). This data is valid for 2
hours and can be used in future startups to improve the time to first fix (TTFF). Ephemeris can also be supplied
to the receiver.