Definition
The slight delay that a GPS or other satellite signal experiences as it travels through the layers of Earth's atmosphere on its way to a receiver. The signal slows and bends slightly as it passes through the ionosphere and troposphere, which adds a small but measurable error to the calculated position if not corrected.
Plain English
Satellite signals slow down a tiny bit as they pass through the air and upper atmosphere on their way to the aircraft. That delay can throw off the position fix unless the system corrects for it.
Context Anchor
Seen in GPS and instrument flying discussions about navigation accuracy and position errors.
Derivation
Propagation comes from the Latin propagare, meaning 'to spread or extend forward.' Here it refers to a radio signal travelling forward from the satellite to the receiver. The 'delay' is the extra time that travel takes because the atmosphere is not a perfect vacuum.
Why Pilots Care
This delay is one source of GPS position error; understanding it helps pilots know when and why corrections such as WAAS are needed for reliable instrument flight.
Grounding Statement
A GPS signal would arrive a fraction of a second sooner if Earth had no atmosphere; that small extra travel time is the propagation delay.
Intuition Check
Do not read “delay” as a late message or a slow pilot response here. It means a tiny extra travel time in the navigation signal itself.
Example Sentence 1
WAAS improves GPS accuracy by broadcasting corrections for atmospheric propagation delay and other error sources.
Example Sentence 2
Without WAAS, uncompensated atmospheric propagation delay can increase cross-track error during a GPS overlay approach in humid conditions.