Definition
A GPS signal error caused when a satellite signal reaches the aircraft's GPS antenna by more than one path — the direct path from the satellite plus one or more reflected paths off surfaces such as the ground, water, buildings, or parts of the aircraft. The reflected signals arrive a fraction of a second later than the direct signal, distorting the receiver's measurement of distance to the satellite and reducing position accuracy.
Plain English
The GPS signal bounces off something (like the ground or a building) and arrives at the antenna twice — once directly and once delayed. The receiver gets confused by the two copies and its position fix becomes slightly less accurate.
Context Anchor
Seen in GPS error discussions, especially when operating near hangars, buildings, terrain, or other surfaces that can reflect GPS signals.
Derivation
From 'multi-' meaning many, and 'path' meaning the route a signal travels. The name describes the problem directly: the signal took more than one route to reach the receiver.
Why Pilots Care
Position errors from multi-path can reach several hundred feet, affecting the accuracy of approaches, terrain awareness, and situational awareness in instrument conditions.
Analogy
It's like hearing someone shout in a canyon — you hear their voice directly, then a slightly delayed echo off the wall. If you tried to time exactly when they shouted using both sounds, you'd be a bit off.
Intuition Check
Do not read multi-path as “several navigation routes.” In GPS use, it means one satellite signal arriving by more than one signal path because of reflection.
Example Sentence 1
Parking near a large metal hangar can introduce multi-path errors as you start up and acquire satellites.
Example Sentence 2
In mountainous terrain, multi-path can cause the receiver to display an erroneous altitude during an IFR arrival.