The Ground Observer Corps (GObC) was a civilian organization within the Royal Canadian Air Force formed in October 1950 to help identify intruder aircraft in the era before the Pinetree Line radar network was fully operational.
Members were required to report any sighting of an aircraft with four or more engines via Forces-supplied radios.
[1] Its formation prompted the USAF to introduce its own Ground Observer Corps the next year, and the two organizations operated in concert until both were deactivated in the late 1950s.
In one 1955 test, USAF Strategic Air Command bombers conducted a mock attack on North America to test the defensive network.
The GObC's reports beat the radar systems by three hours.