User's Manual

XT55/56 Hardware Interface Description
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XT55/56_hd_v02.06a Page 78 of 125 17.12.2004
Figure 30 above shows the block diagram of the XT55/56 GPS receiver architecture. The
GPS module can be separated into four major parts: RF frequency down-converter, digital
baseband demodulation, embedded ARM microprocessor and internal GPS software stored
on-board (1 MByte) Flash-Memory. The RF frequency conversion and the baseband
demodulation are executed by hardware while the embedded ARM processor computes the
GPS position, velocity and time solution employing the internal GPS software.
The purpose of the RF circuitry is to reinforce the very weak (-130dBm nominal) GPS signal,
to filter it and to down-convert it to an Intermediate Frequency (IF) of 9.45MHz for digital
processing. The SiRFstarII architecture relies on the high level of integration in the GRF2i to
significantly reduce part count and circuit complexity. A IF filter is built-in as well.
• The digital baseband demodulator takes the quantified GPS signal and detects the
individual satellites serial data bit stream, along with the associated pseudo range. This
action consists of removing spread spectrum and Doppler frequency components of the
signal to obtain the serial data messages.
• The embedded ARM processor monitors channel allocation, extracts the raw satellite
tracking data, computes the position and time solution and sends it on a serial port for
high level applications to use or processes it locally. Support functions for the
microprocessor include real time clock and reset pulse generator circuits. The internal
GPS software monitors and allocates channels, computers the position, velocity and time
using the pseudo-range of the satellites and reformats the data to be output or used
locally. The internal GPS software is a tasking based architecture driven by the 100ms
interrupt generated by GPS2e internal hardware.
4.9 Operation procedure
When the receiver is powered up, it steps trough 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 periodically.
In order to perform a navigation solution (3D solution), the receiver needs.
• Pseudo-ranges for at least 3 satellites
• Ephemeris data for the satellites it will use in the navigation solution.
Note: If almanac navigation is enabled, the receiver can calculate a position without
downloading ephemeris data (with a significant position error compared to an
ephemeris based solution).
The initial position calculation is made, using a Least-Squares Algorithm. Successive position
calculations are performed with a Kalman Filter. To generate a position (3D solution)
calculation the receiver needs at least 4 measurements to different satellites. In order to
calculate a position (Latitude/Longitude/Height), as a 2D solution with an estimated height
value, then 3 different satellites are required. Pseudo-range and carrier phase information
are available to the position determination algorithms if 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 transmits its own
ephemeris data, the broadcast lasts for 18 seconds, repeating every 30 seconds.
The receiver stores ephemeris data in battery-backed memory. This data can be used in
future startup’s to improve the time to first fix (TTFF). The ephemeris can also be supplied to
the receiver via the serial port.