Definition
Periods during which one or more Global Positioning System satellites are unavailable for navigation use, either because the satellite is out of service, broadcasting unreliable data, or temporarily not visible to the receiver in a way that degrades position accuracy. Predicted outages are published in advance through NOTAMs and through GPS receiver manufacturer prediction tools, and they may cause an approach or route segment to become unusable for a specific time window at a specific location.
Plain English
Times when some GPS satellites are not working or not usable, which can make GPS navigation less accurate or unavailable in a particular area for a period of time.
Context Anchor
Seen in IFR GPS planning, GPS NOTAM checks, and discussions of whether GPS navigation or a GPS approach will be available for a flight.
Derivation
An 'outage' comes from 'out' plus the suffix '-age,' meaning a period during which something is out of service. Originally used for electrical and telephone service interruptions, the term was carried over to describe periods when GPS satellites are unavailable.
Why Pilots Care
Outages can reduce the number of usable satellites, lowering navigation accuracy or availability and requiring pilots to switch to other methods to stay IFR-legal.
Analogy
It is like losing one or two streetlights on a road at night. The whole road may not go dark, but your view can become less reliable in that area.
Grounding Statement
Before an IFR flight, a pilot may find that a GPS satellite outage is scheduled along the route or near the destination during the planned arrival time.
Intuition Check
Do not assume a GPS satellite outage means the entire GPS system is down. It usually means one or more satellites are temporarily not usable, and the effect depends on where and when you are flying.
Example Sentence 1
During preflight, the pilot checked GPS NOTAMs and found a predicted satellite outage that would affect the RNAV approach at the destination near the planned arrival time.
Example Sentence 2
When a GPS satellite outage was reported the crew monitored the remaining satellites and prepared to use VOR navigation as backup.