A320 MMR FDE

[underfire]

Fault detection, and fault detection and exclusion are dependent on the number of satellites.

Units need a minimum of 5 satellites for RAIM monitoring and fault detection to operate. Therefore, the unit will need a minimum of 6 satellites to use fault detection and exclusion.

8400.33

"(a) Global Navigation Satellite Systems (GNSS) as primary navigation. Aircraft having GNSS as the primary LRNS for oceanic and remote operations approval must meet performance requirements. AFMs or airworthiness documentation should indicate if the GNSS system installation meets these requirements. Dual independent GNSS equipment is required and an approved dispatch fault detection and exclusion (FDE) availability prediction program must be used. The maximum allowable time for which FDE capability is projected to be unavailable is 25 minutes. Maximum outage times will be included as a condition of the operational authorization."

EDIT: I have checked a few sources, and for the FDE prediction function to work properly, you would still have to de-select the sats that are known to be out. This would be an interesting discussion topic on SOP.