Definition
Baro-aiding is a technique that uses barometric altitude information from the aircraft's altimeter to supplement GPS satellite data, allowing the GPS receiver to perform integrity monitoring (RAIM) with one fewer satellite than would otherwise be required. It substitutes a known altitude reference for one of the satellite signals normally needed to verify position accuracy.
Plain English
The GPS uses the aircraft's altitude reading as if it were an extra satellite. This lets the GPS keep checking that its position fix is reliable even when fewer satellites are available.
Context Anchor
Seen in IFR GPS and advanced avionics discussions, especially when the handbook explains how GPS checks the reliability of its own position information.
Derivation
Baro' is short for barometric, from the Greek baros meaning weight or pressure -- the altimeter measures altitude using air pressure. 'Aiding' means helping. Together: pressure-altitude information helping the GPS do its job.
Why Pilots Care
It increases the accuracy and integrity of GPS vertical information, supporting safer use of GPS approaches.
Grounding Statement
Baro-aiding gives the GPS one more piece of information: how high the airplane appears to be based on air pressure.
Intuition Check
Baro-aiding does not mean the GPS is fixing or calibrating the altimeter. It means the altimeter’s pressure-based altitude is helping the GPS receiver with its own calculation.
Example Sentence 1
With baro-aiding enabled, the GPS retained RAIM coverage for the approach even though only four satellites were usable.
Example Sentence 2
Baro-aiding allowed the system to maintain reliable vertical guidance when satellite geometry was marginal.