(12) Coarse Calculation of Satellite Position Module:
is usually activated every minute. And for all satellites, the module carries out the coarse calculation of their positions using almanac Based on the calculated positions, the module also calculates elevation angle, azimuth and estimated Doppler frequency for each satellite which are used when the Satellite Capturing/Tracking Manager selects the satellites to be tracked.
5. CLASSIFICATION AND SELECTION OF METHODS OF POSITION-FIXING
The following is the classification by integrity described in the MOPS and also gives an account of the establishment and selection of methods of position-fixing based on the classification, taking the navigation mode into consideration.
5.1 CLASSIFICATION OF METHODS OF POSITION-FIXING FROM VIEWPOINT OF INTEGRITY
It is thought that there are four methods of position-fixing from viewpoint of integrity as shown below:
(1) Guarantee of integrity by WAAS message
The position is fixed using only satellites in WAAS HEALTHY state. In other words, only satellites WAAS of which is broadcast to be in normal operational state. This should be the primary method of the WAAS-based position-fixing. As the errors of all satellites to be used for position-fixing are derived from WAAS message, the position-fixing accuracy can be guaranteed by calculating HPLWAAS/VPLWAAS based on the errors.
(2) Guarantee of integrity by Fault Detection and Exclusion (FDE)
Where the guarantee of integrity by WAAS message cannot be expected, the integrity must be guaranteed by the Receiver alone. Abnormality of satellite is detected by monitoring residual given by the least square method and the satellite in which such an abnormality is found is excluded. This function is effective when six or more satellites are available and the satellite geometry meets the criteria required for FDE. HPLFD/VPLFD are calculated and output in order to guarantee the position-fixing accuracy.
(3) Guarantee of integrity by Fault Detection (FD)
Where the guarantee of integrity by FDE cannot be expected, abnormality of satellite is detected by monitoring residual given by the least square method and the detected occurrence of such an abnormality is notified. This function is effective when five or more satellites are available and the satellite geometry meets the criteria required for FDE. HPLFD/VPLFD are calculated and output in order to guarantee the position-fixing accuracy.
(4) No guarantee of integrity
Where the guarantee of integrity by FD mentioned in the above cannot be expected, the position-fixing accuracy cannot be guaranteed either.
5.2 DEFINITION OF METHODS OF POSITION-FIXING DESIGNED TAKING NAVIGATION MODE INTO CONSIDERATION
The following seven methods of position-fixing are defined based on the classification of methods of position-fixing from viewpoint of integrity described in the subclause 5.1.
1) Precision Approach Position-fixing
2) Non-precision Approach Position-fixing
3) En Route and Terminal Position-fixing
4) FDE Position-fixing
5) FD Position-fixing
6) FD Position-fixing using baro-altimeter
7) Position-fixing without Integrity Guarantee
The mode in use for position-fixing is output externally together with the result of position-fixing. Each method of position-fixing, integrity type, protection level and conditions of usable satellites are shown in Table 2.
1) Precision Approach Position-fixing
This position-fixing mode that meets the requirements of the PA mode presented in the MOPS. While the integrity is obtained by the WAAS message, the FD processing is executed in parallel to cope with local abnormality. When any abnormality is found in the process of FD, an alert is given.
The satellites to be used for position-fixing must be among those which are in the WAAS HEALTHY state and meet the following requirements: elevation angle ≧ 5°; UDREI ≦ 10; ionospheric correction by WAAS is available.
In addition, TYPE10 message must have been received and when the service message (TYPE27) has been received, δUDREI must be 12 or less at the position of the Receiver.
For the protection level, both HPLWAAS/VPLWAAS are calculated and must be 15 m or less.
Position-fixing computation is performed with the least square method weighed based on the error of each satellite derived from WAAS message.
