When you say RAIM check, that is an internal function of the system. For the system to determine horizontal and vertical position, at least 4 sats are needed.
To provide error detection in those measurements, at least 5 are needed.
To be able to detect, error detect, and isolate an error, at least 6 are needed.
The RAIM system will look at all of the avail data, and compare the required level of accuracy (ex 0.15 RNP) with the actual accuracy level of the system.
For GPS PRIMARY...
ESTIMATED is the measure of position accuracy.
If ESTIMATED value exceeds the REQUIRED value, NAV ACCUR DOWNGRAD is displayed.
FM position is reliable but not sufficient to continue.
GPS PRIMARY and HIGH accuracy must be present on BOTH MCDU PROG pages.
If GPS PRIMARY is lost, ESTIMATED position will increase with time.
FM position is reliable but not sufficient to continue.
Last edited by FlightPathOBN; 13th May 2013 at 21:37.