MakeitHappenCaptain:
The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.
I don't believe this statement is correct.
Whether or not the unit scales to approach mode/approach active has nothing to do with whats in the almanac. If the GPS has the required satellite coverage for RAIM, it will scale, if it doesn't it won't scale, simple as that. It doesn't matter what the almanac says.
I've seen units fail to go to approach mode/approach active even though RAIM was available. Poor satellite geometry relative to the ships GPS aerial might cause this.
All the almanac does, is provide data to predict if RAIM will be available. Nothing more nothing less.
Baldeangel:
The receiver will not sequence onto appch (or LNAV) mode if RAIM won't be available, so the idea of doing the manual check is so that you can select a different approach before you even begin, rather than waiting until it's about to transition and being unable to continue.
Exactly! Why wouldn't you do one in flight? Finding out on a 7nm final in IMC is prob not "best practice".
Why would you bother if you had the RAIM prediction from ASA or Airways. The RAIM prediction from the ships GPS unit isn't going to tell you anything that you don't know. As has been already said the ASA or Airways prediction is going to be more accurate anyway.