Definition
An advisory term used by ATC to alert pilots that the integrity of the GPS or WAAS signal cannot be guaranteed in a specified area, at a specified altitude, or for a specified period. When GPS/WAAS is reported unreliable, pilots should anticipate degraded navigation and approach capability and be prepared to use alternate means of navigation. The term does not mean the system has failed; it means the signal cannot be trusted to meet required performance standards in that area or time.
Plain English
ATC is warning you that GPS or WAAS signals in this area may not be accurate or dependable right now, so don't rely on them and have a backup plan ready.
Context Anchor
You may hear or see this in ATC advisories, notices, or preflight information when GPS or WAAS service could be affected by an outage, test, or signal problem.
Derivation
“Unreliable” comes from “rely,” meaning to depend on something, with “un-” meaning “not.” In this aviation use, it does not mean the equipment is always bad; it means the service may not be dependable for that area or time.
Why Pilots Care
A pilot must immediately switch to an approved alternate navigation source or terminate the IFR operation if this status appears.
Grounding Statement
If the system cannot be counted on to give the expected navigation service, treat it as uncertain and use a safe backup.
Intuition Check
Do not read “unreliable” as “completely failed.” It means GPS or WAAS service may be degraded, unavailable, or not dependable enough for the planned use.
Example Sentence 1
The ATIS reported GPS unreliable below 5,000 feet within 200 miles of the testing area, so we briefed the ILS as our primary approach.
Example Sentence 2
During the RNAV approach the receiver flagged Unreliable (GPS/WAAS), so the crew executed the missed approach procedure.