Definition
A self-checking function built into a GPS receiver that verifies the integrity of the satellite signals it is using for navigation. RAIM compares signals from multiple satellites to detect whether any one of them is providing faulty data. To perform a basic RAIM check, the receiver needs at least five satellites in view (or four plus a barometric altimeter input). If a fault is detected, RAIM warns the pilot that the GPS position cannot be trusted for the current phase of flight.
Plain English
RAIM is the GPS receiver's way of checking its own work. It looks at signals from several satellites and cross-checks them to make sure none of them are lying. If something looks wrong, it tells the pilot the GPS may not be reliable.
Context Anchor
Seen during IFR GPS planning, GPS approach use, and checks of whether satellite navigation is available and reliable enough for a planned route or procedure.
Derivation
Receiver Autonomous Integrity Monitoring breaks down as: Receiver (the GPS unit itself), Autonomous (it does this on its own, without help from outside), Integrity (trustworthiness of the signal), Monitoring (continuously watching). Together: the receiver watches over the trustworthiness of its own GPS signals, by itself.
Why Pilots Care
It warns the pilot if GPS accuracy falls below safe limits, allowing timely switch to an alternate navigation source before position data becomes unreliable.
Grounding Statement
RAIM is the receiver’s way of saying, “I can trust this position,” or “I cannot confirm this position well enough.”
Intuition Check
RAIM does not make GPS more accurate. It checks whether the receiver can trust the GPS position information it already has.
Example Sentence 1
Before departure, the pilot checked RAIM availability for the planned GPS approach at the destination and confirmed it was predicted to be available at the estimated arrival time.
Example Sentence 2
When RAIM was lost en route, the crew reverted to VOR navigation as a backup.