4.1 OUTLINE OF PROCESS OF EACH MODULE
(1) Signal -tracking Processing Module:
in accordance with the signal processing command generated by the Satellite Capturing/Tracking Manager, controls the signal processing hardware to capture the satellite signal. After capturing, the module tracks code and carrier.
The raw satellite tracking data is output with a cycle of 200 ms in synchronization with GPS time. The module also controls the signal processing hardware to output the synchronized pulse with UTC.
(2) Extracting WAAS Message Module:
performs detection of 500 SPS messages broadcast by WAAS satellites, synchronization of symbol, decoding of convolution code, synchronization of frame and CRC check, and outputs one message (250 bits) every second. The module detects the occurrence of four consecutive CRC errors which is one of the factors of WAAS UNHEALTHY conditions.
(3) Time Management Module:
is activated every 200 ms to manage internal GPS time and effective time in the WAAS message.
(4) Position Fix Calculation Module:
is activated every 200 ms and carries out the following processing to calculate the position and the speed:
a) detects step error based on pseudo-range observed and satellite position calculated;
b) for 12 satellites allocated to the channels, judges that the WAAS HEALTIIY / WAAS UNMONITOR / WAAS UNHEALTHY and GPS HEALTHY / GPS UNHEALTHY;
c) calculates HPLWAAS/VPLWAAS or HPLFD/VPLFD;
d) establishes the method of position-fixing based on availability of correction information, protection level, integrity type, etc.;
e) calculates error for each satellite and weight based on the error;
f) applies fast, long term and ionospheric corrections to the observed data and the estimated position with weighted least square position-fixing calculation.
