2) Non-precision Approach Position-fixing
This position-fixing mode also obtains integrity from WAAS message. The satellites to be used for position-fixing must be in the WAAS HEALTHY state. Both fast and long-term corrections are applied to all of the satellites to be used.
When the service message (TYPE27) has been received, δUDREI must be 13 or less at the position of the Receiver. The ionospheric correction is used when it is available by WAAS and if not, the ionospheric correction by GPS us used.
For the protection level, both HPLWAAS/VPLWAAS are calculated and must be 0.3 nm or less.
3) En Route and Terminal Position-fixing
This position-fixing mode also obtains integrity from WAAS message. The satellites to be used for position-fixing must be in the WAAS HEALTHY state. Both fast and long-term corrections are applied to all of the satellites to be used.
When the service message (TYPE27) has been received, δUDREI must be 14 or less at the position of the Receiver. The ionospheric correction is used when it is available by WAAS and if not, the ionospheric correction by GPS is used.
For the protection level, both HPLWAAS/VPLWAAS are calculated and must be 1.0 nm or less.
4) FDE Position-fixing
This position-fixing mode obtains integrity from FDE. Even if any satellite goes wrong, such a satellite is excluded and position-fixing remains continued. The information of WAAS is basically not used even if it is available.
For the protection level, both HELFD/VELFD are calculated and must be 1.0 nm or less.
5) FD Position-fixing
This position-fixing mode also obtains integrity from FD. When an occurrence of any abnormality is found in a satellite, an alert is given. The information of WAAS is basically not used even if it is available.
For the protection level, both HPLFD/VPLFD are calculated and must be 1.0 nm or less.
6) FD Position-fixing using baro-altimeter
Position-fixing is performed using a baro-altimeter when integrity cannot be assured with GPS satellites alone. This position-fixing mode also obtains integrity from FD. When an occurrence of any abnormality is found in a satellite, an alert is given.
For the protection level, both HPLFD/VPLFD are calculated and must be 1.0 nm or less.
7) Position-fixing without Integrity Guarantee
In case of this position-fixing mode, integrity cannot be obtained. Therefore satellite failure cannot be detected. Position-fixing is however maintained when there exist four or more usable satellites.
5.3 SELECTION OF POSITION-FIXING METHOD
The position-fixing methods are checked one by one in order of subclause 5.2. If the conditions of the method of 1) Precision Approach Position-fixing which has the highest priority are met, the method is used for position-fixing computation. If not, the method having the next highest priority is checked. Finally, if the conditions of the method of 7) Position-fixing without Integrity Guarantee which has the lowest priority are not met, position-fixing computation is not performed (The alert for Loss of navigation is given).
6. HANDLING MULTIPLE SBAS SATELLITES
The SBAS satellites that are currently transmitting signals are only two satellites, i.e. POR and AOR-W of Inmarsat III. Between those, only signals from POR are available in and around Japan. However, signals from multiple SBAS satellites are expected to be received soon after the first unit of MTSAT is launched. When such time comes, it will be necessary to consider which correction data of SBAS satellites are to be used. It will also be necessary to consider whether the correction data from the same SBAS satellite is to be used for all satellites to be used for position-fixing or the mixed data from multiple SBAS satellites is to be used for each satellite.
6.1 MIXED USE OF CORRECTION DATA
We have designed the Receiver in such a manner that the correction data from SBAS satellites whose providers are different each other are not mixed. It is because the MOPS specifies “Corrections for different satellites may be extracted from different WAAS broadcasts except when in precision approach mode.” and the MOPS further specifies “If data from multiple WAAS satellites are used, then the equipment shall account for differences in the time reference used to generate corrections (e.g., WAAS network time). For each individual GPS or WAAS satellite, the fast and slow corrections and ionospheric data shall be derived from a single WAAS satellite broadcast.", so it does not prohibit mixing the SBAS satellites. However, it is necessary for us to pay attention to the difference in reference times between providers. The reference time refers to the WAAS network time. Since it is only specified that the WAAS network time does not differ by more than 50 ns from GPS time and the time difference is not broadcast by SBAS, we have decided not to mix correction data from multiple SBAS satellites.
