Definition
The Course/Acquisition (C/A) code is the civilian GPS signal transmitted by satellites on the L1 frequency (1575.42 MHz). It is a unique pseudo-random digital code, repeating every millisecond, that allows a GPS receiver to identify each satellite and measure the time it took the signal to travel from the satellite to the receiver. The receiver uses these timing measurements from multiple satellites to calculate position, altitude, and time.
Plain English
The C/A code is the standard GPS signal that civilian receivers use. It is a unique pattern broadcast by each satellite, and your GPS uses it to figure out which satellite it is hearing and how far away that satellite is. Combine that information from several satellites and the receiver knows where you are.
Context Anchor
Seen in explanations of how GPS receivers determine aircraft position from satellite signals.
Derivation
Course/Acquisition reflects the original design intent. 'Acquisition' means the receiver uses this code first to find and lock onto a satellite. 'Course' refers to coarse — meaning lower-precision — because the C/A code provides standard civilian accuracy, as opposed to the encrypted, higher-precision P (Precise) code reserved for military use.
Why Pilots Care
It is the starting point for GPS receivers to provide reliable position data during instrument flight.
Intuition Check
Do not read “Course” here as the course you fly over the ground. This term is about a GPS signal code used to acquire satellite signals, not a heading or route.
Example Sentence 1
Civilian GPS receivers use the C/A code on the L1 frequency to determine position.
Example Sentence 2
During preflight checks the GPS unit quickly locks onto the CA code and begins displaying position.