(5) Data Input / Output Module:
outputs the result of position-fixing through the serial port. It receives altitude data from a baro-altimeter and memorizes it.
(6) WAAS Message Compiler Module:
is activated each time that the message is collected (every one second if message is received stably), compiles the collected message. The module identifies the types of the received messages and stores them in the specified data area respectively. The message types to be processed are 0/1/2-5/6/7/9/10/12/17/18/24/25/26/27.
The most of the WAAS messages require management of effective time and therefore, the corresponding effective time counter is initialized whenever the message is received.
(7) WAAS Ionospheric Calculation and Determination of δUDREI Module:
has the following two functions. One is to calculate the ionospheric correction and the error using the ionospheric correction parameter given by the WAAS satellite in the TYPE 18/26. The other is to determine δUDREI from a service message given in the TYPE 27 and the position of the Receiver.
(8) FD/FDE Processing Module:
is activated by the Position Fix Calculation every second, assures the integrity with the Receiver alone.
The module judges if there exists any abnormality in the satellite used for position-fixing and if any, the relevant satellite is identified and registered in the exclusion satellite list so that it may not be used for position-fixing calculation.
When any satellite is listed in the exclusion satellite list, the module monitors such a satellite if its abnormality is removed and is usable for position-fixing, including step error to be used for position-fixing.
The module calibrates the baro-altimeter using the result of GPS position-fixing to minimize the error due to the effect of weather when the baro-altimeter is used.
(9) Satellite Capturing/Tracking Manager Module:
determines the appropriate satellites to be allocated to the signal tracking channels. That is that the module generates the signal processing command specifying the satellites to be tracked and gives instruction to the Signal-tracking Processing module.
The module is activated when satellite tracking fails and carries out the above processing.
(10) Calculation of Position Fix Parameters Module:
calculates the following parameters beforehand among parameters to be used for position-fixing to reduce the calculation load when the position-fixing computation is performed:
a) interpolation parameter for satellite position;
b) satellite clock offset parameter,
c) ionospheric correction of each satellite by GPS message;
d) tropospheric correction of each satellite.
[Note] The orbit of satellite, for short period of time, can be approximated by quadratic curve of variation of ,X, Y and Z of ECEF with necessary and sufficient accuracy. Therefore, this module calculates the coefficients of the quadratic equation of each axis. The clock offset of a satellite is approximated by a linear equation.
(11) GPS Message Compiler Module:
is activated every 6 seconds. If the GPS message is output from the Signal-tracking Processing module, the following information is extracted and memorized:
a) ephemeris;
b) almanac;
c) GPS ionospheric correction parameter and UTC correction parameter;
d) satellite health status.
Ephemeris and, GPS ionospheric and UTC correction parameters are stored as the valid data and used for position-fixing, provided that the message is received twice and the information contents of these two messages are identical.
The module monitors the following items which become factors of GPS UNHEALTHY:
i) that six-bit health information in the subframe 1 has MSB "1" except for "111101";
ii) that IODC in subframe 1 is different from IODE in subframes 2 and 3;
iii) that User Range Accuracy (URA) included in subframe 1 is 128 m or more;
iv) that bit 18 of Hand Over Word (HOW) in each subframe is "1";
v) that the data of either subframes 1 to 3 is all "0".
