Definition
An advisory term used by Air Traffic Control to inform pilots that GPS or WAAS-based navigation signals may be degraded, intermittent, or unavailable in a specified area, altitude block, or time period. The advisory does not prohibit GPS or WAAS use, but warns that the system may not provide reliable position, course, or approach guidance and that the pilot should be prepared to use alternate means of navigation.
Plain English
ATC is letting you know that GPS or WAAS signals in this area might not be trustworthy right now. You can still try to use them, but be ready to switch to another way to navigate if they act up.
Context Anchor
Seen in preflight briefings, NOTAM information, and cockpit navigation messages when GPS or WAAS service may be reduced, unavailable, or not approved for a planned route or approach.
Derivation
GPS means Global Positioning System, the satellite-based system that gives position. WAAS means Wide Area Augmentation System, which improves GPS accuracy and checks its trustworthiness for aviation use. Unreliable comes from rely, meaning to depend on; unreliable means not safe to depend on.
Why Pilots Care
Continuing to use an unreliable signal can produce large position errors leading to airspace violations or terrain conflicts; pilots must immediately switch to an approved backup navigation source.
Intuition Check
Unreliable does not always mean the GPS unit in the airplane is broken. It means the service or signal may not be trustworthy enough for the operation you want to use it for.
Example Sentence 1
The controller advised, "GPS/WAAS unreliable below 5,000 feet within 50 nautical miles of the testing area; advise intentions."
Example Sentence 2
Preflight planning included checking current GPS/WAAS — Unreliable NOTAMs to confirm WAAS availability along the route.