What are the rules for this?
If it did suddenly remove navigation, that would be a really bad thing. Having no RAIM is not wholly uncommon.
I think an EGNOS/WAAS unit doesn't need RAIM because of the extra check, but I am not sure...
I don't know about general rules, and since it never happened to me all my knowledge is from training / manuals.
WAAS is one level up from RAIM - if you have WAAS, RAIM coverage (additional sat in view) is not required.
The Garmin units downgrade gracefully. The below is for the GNSx30W and G1000 which I am familiar with.
If WAAS fails, the unit falls back to RAIM.
If no RAIM is available, The unit displays a warning flag ("INTEG") in its display, and I believe also a message you can get using the MSG button (which is good because I have a MSG light in my primary instrument scan) but continues normally.
If you want to fly an approach that requires WAAS and WAAS has failed, you get a message before the FAF, and the glideslope is not shown
All approaches can be loaded, and the lateral profiles are all shown and flyable.
The G1000 even has a DR mode, where even in a complete GPS failure it uses heading and speeds to estimate the aircraft position, and still displays lateral navigation tracks.
RAIM unavailable means that there are insufficient satellites viewable with satisfactory geometry to perform the RAIM calculation. If RAIM is unavailable, the NAV flag will display and you should use other means of navigation. This does not mean your position is inaccurate, it means that RAIM can't be used to determine the current integrity value.
A totally different issue is that although RAIM is available, meaning the calculation can be performed to determine the integrity, but the RAIM calculation determines that the integrity is unsatisfactory for the current operation. On final approach, this limit is 0.3 NM.
My understanding is that if RAIM is unavailable at 1 minute prior to the FAF, the approach mode won't activate and you will get a message. If RAIM becomes unavailable while on the final approach segment, the approach may be continued. However, if the RAIM calculation determines an over limit calculation while on the final approach segment, then you will get a message to abandon the approach and commence a missed approach.
A TSO C146/145 GPS when within the service volume of the SBAS, does not use RAIM unless the SBAS signal is unavailable. It uses the integrity values transmitted by the SBAS geo satellite to determine integrity and therefore a RAIM check is not required while in the SBAS service volume. Vertical guidance is not supported by RAIM and the SBAS integrity requirements are quite stringent. A VPL of 50 meters is required to support vertical guidance down to a DH of 250 feet, if the DH is below that it must be 35 meters. For lateral guidance on a LPV or LP procedure, the HPL must be lower than 40 meters. By comparison, the lateral integrity required for a LNAV or LNAV/VNAV procedure is 556 meters (0.3 NM).