Interesting thread with a few arms and legs to it.
When in 3D Mode, the Altitude displayed on the 430 and 530 is GPS-derived. Only when degraded to 2D mode by inadequate satellite coverage does the GPS fall back to display externally serialised altitude (such as from a transponder or blind encoder). It is this need for a fallback under conditions of poor satellite reception that drives the BRNAV requirement.
There are plenty of references to this in the Flight Manual Supplement, the Flight Guide and the Installation Guide.
RAIM and RAIM prediction on the 430 and 530 does not appear to rely on any form of external altitude signal (that I can find). A RAIM error is caused by actual or predicted integrity problems caused by the anglular position of GPS satellites in the sky. It is absolutely not generated by disparity in external versus derived altitude.
RAIM is computed based on the Satellite Almanac contained within the GPS and occasionally downloaded from the satellites. It is calculated with respect to the position which is being FPLed/DIRECTed to and the ETA, or the current position at the current time as appropriate.
Barring external factors such as hangars, a RAIM error will of course tend to occur when the satellite reception downgrades to 2D anyway, so that RAIM and the reliance on the external altitude source are occasionally linked... but not quite in the way being implied above.
Hope this helps
2D
Last edited by 2Donkeys; 8th January 2004 at 20:39.