Wow, interesting thread.
RAIM Prediction Tools
There is a significant amount of confusion between RAIM prediction and real time RAIM monitoring.
When you file a flight plan that requires GPS, and especially RNP AR, you must check the RAIM prediction to see if it is available for approach. Note, this is NOT the RAIM onboard, one must check the approved State Source. If the prediction says no, then you cannot use that flight plan.
Because RAIM operates autonomously, it requires redundant pseudorange measurements. To obtain a 3D position solution, at least four measurements are required. To detect a fault, at least 5 measurements are required, and to isolate and exclude a fault, at least six measurements are required, however often more measurements are needed depending on the satellite geometry.
This is what the unit on the aircraft is doing to provide the HIL/HPL/HAL.
The horizontal integrity limit (HIL) or horizontal protection limit (HPL) is a figure which represents the radius of a circle which is centered on the GPS position solution and is guaranteed to contain the true position of the receiver to within the specifications of the RAIM scheme. (Note: this is NOT prediction, but real time RAIM measurement).
The HPL is calculated as a function of the RAIM threshold and the satellite geometry at the time of the measurements.
The HPL is compared with the horizontal alarm limit (HAL) to determine if RAIM is available.
Always remember, due to latency in the GPS signal (a single GPS transmission from each sat lasts about 3 seconds) and latency in the unit, the GPS system tells you where the aircraft WAS, not where it IS. The Kalman filter estimates where the aircraft IS. The unit will alarm if HIL is beyond HAL for your selected level. This is real-time.
Even though the RAIM prediction site may have told you that there was sufficient coverage, the terrain where you are going may affect the number of sats that your unit can see. As an example, many procedures in China involve flying down "valleys". If you note that the unit must see at least 6 sats for fault protection, terrain may mask enough where the unit will start degrading HIL.
That is why RAIM is monitored real-time by the unit. Note the Terminal/Approach check. This is usually more accurate, and will given outages on a time schedule, which is important to review before departure, to determine if RAIM is available/unavailable during certain time periods.
RNP AR operators must always check RAIM avail prior to departure for the RNP level on final. As noted in China, there are many RNP 0.1 finals that are not available due to RAIM.
Of course, if the HIL/HPL on your unit go to HAL, you have to use another method of navigation.
What I don't understand is beyond the FAF, if you lose FD RAIM, i.e. from 5 satellites down to 4, (and I mean only 4 satellites in sight of the receiver), and the RAIM warning is "inhibited" for 5 mins, how does the unit monitor integrity at all?
Units on commercial aircraft have the GPS connected thru the IRU. The inertial unit, with its predictions thru the filter, provide a certain 'pseudo RAIM' should RAIM drop for a brief period of time (usually about 15mins)
EDIT: Given the latentcy in the system, 5 mins is used as an acceptable drift rate for the approach to stay within the defined level.
EDIT:
Hey, I do a RAIM prediction as well, because SOPs say so; the point is that its pointless - the NOTAM is more up to date than the almanac in the FMS.
RAIM prediction by almanac prior to flight plan would not be acceptable. The State source should detail RAIM prediction for the route, and RAIM prediction for individual approach (as noted on the website above). The boys in the back room should have checked all this prior to sending the flight plan. If approach was not available, then the flight plan cannot be filed.
Curious about a RAIM NOTAM...I havent ever written one, so not sure what that would be about