If Receiver Autonomous Integrity Monitoring (RAIM) capability is lost in-flight,

Explanation

RAIM is how the GPS receiver checks the integrity of its own position solution. AIM 1-1-17 states that without RAIM capability the pilot has no assurance of the accuracy of the GPS position, and another means of navigation must then be actively monitored. GPS altitude is no more trustworthy without integrity monitoring. Three satellites cannot even give a 3-D fix, let alone integrity; RAIM itself needs at least five satellites (or four plus baro-aiding).