How does the receiver predict RAIM will be available for 5 minutes after passing the FAF without knowing where the satellites will be in the constellation?
I thought it would use the almanac?
I don't think this is the way it works. The GPS doesn't go to approach mode/approach active based on a RAIM prediction and doesn't concern it self as to whether there is going to be RAIM 5 minutes after passing the FAF.
My understanding is RAIM warnings are inhibited after the FAF and the GPS will provide a DR nav solution for 5 minutes after the loss of RAIM after passing the FAF, meaning an approach can be completed and if necessary missed approach can be carried out in DR mode.
How do you know RAIM was available at the time?
The RAIM prediction from Airways said RAIM would be available. The other box didn't have a RAIM warning.
I still don't see any point in doing a RAIM prediction just prior to the approach so long as you have one from ASA or Airways. You are not going to learn anything that you don't already know.
Remember we are talking about a prediction based on an almanac, what happens in real life can be and is different.
While there may be enough satellites to ensure RAIM there can be other factors that will cause a GPS unit to not have RAIM available.