Definition
TSO C-129 is the FAA Technical Standard Order that sets the minimum performance standards for GPS receivers used as a supplemental means of navigation under instrument flight rules. A GPS unit certified to TSO C-129 may be used for IFR navigation provided the aircraft also carries, and the pilot monitors, the appropriate conventional navigation equipment required for the route or approach being flown.
Plain English
It is the FAA rule that says what a GPS receiver must be able to do before it can be approved for instrument flying. A GPS approved under this rule can be used in the clouds, but the aircraft must still carry traditional navigation equipment as a backup.
Context Anchor
Seen in discussions of GPS equipment approval, instrument navigation, and whether a specific aircraft GPS unit is legal to use for a planned operation.
Derivation
A Technical Standard Order is an FAA-issued minimum performance specification for a particular piece of aircraft equipment. The letter and number (C-129) simply identify which TSO in the series. Knowing it is a performance standard helps explain why one GPS unit is legal for IFR use and another is not -- it comes down to which TSO the unit was built and certified to.
Why Pilots Care
Determines whether the aircraft's GPS may be used legally as a supplemental navigation source during IFR enroute flight and certain approaches.
Intuition Check
TSO C-129 is not a GPS brand or model number. It is an FAA approval standard that tells you what kind of use the equipment was built and approved for.
Example Sentence 1
The avionics shop confirmed the panel-mount GPS was certified to TSO C-129, so the pilot could use it for the en route portion of the IFR flight as long as the VOR receiver remained operational.
Example Sentence 2
Because the unit met TSO C-129 standards, the pilot could rely on it as a supplemental navigation aid when the primary system was unavailable.