Definition
TSO C-115a is an FAA Technical Standard Order that sets the minimum performance standards for airborne area navigation equipment that uses multi-sensor inputs, including GPS receivers integrated with other navigation sources such as VOR, DME, or inertial systems. Equipment certified to TSO C-115a is approved for supplemental navigation use only and is not authorized as a stand-alone means of navigation under IFR; it must be used alongside other approved navigation equipment.
Plain English
TSO C-115a is an FAA rule that says what a certain class of GPS-based navigation box has to be able to do before it can be installed in an aircraft. Equipment built to this rule is allowed to help with navigation, but the pilot still has to back it up with other navigation equipment.
Context Anchor
Seen in instrument flying and GPS equipment discussions when checking what kind of navigation equipment is installed and what approval standard it was built to meet.
Derivation
A Technical Standard Order is the FAA's formal minimum performance standard for a piece of aircraft equipment. The 'C-115a' is just the catalog number for this particular standard, with the 'a' indicating the first revision.
Why Pilots Care
Confirms the equipment meets required accuracy and reliability levels for instrument flight operations.
Intuition Check
Do not read TSO C-115a as the name of a GPS receiver. It is an FAA equipment standard; a unit built under it may use GPS, but GPS is only one possible input.
Example Sentence 1
The aircraft's GPS is certified under TSO C-115a, so the pilot kept the VOR tuned and monitored as the primary IFR navigation source.
Example Sentence 2
Before an IFR flight the pilot verified the receiver carried TSO C-115a approval.