Plain English Meaning

Before flying an instrument approach using a non-WAAS GPS, the pilot checks ahead of time whether enough GPS satellites will be visible at the destination, at the expected arrival time, for the GPS to trust its own position. If the check passes, the GPS approach can be flown. If it fails, the pilot must plan a different approach.

Full Definition

A pre-flight check performed by a GPS receiver, or by an external service, that forecasts whether enough satellites will be available and properly positioned at the destination — at the planned arrival time — to allow the receiver to verify the integrity of its own position information. RAIM stands for Receiver Autonomous Integrity Monitoring. A successful RAIM prediction is required before a may rely on a non-WAAS GPS as the means of for an instrument approach.