ground-communications outlet (GCO)
An unstaffed, remotely controlled, ground or ground-communications facility. Pilots at uncontrolled airports may contact ATC (air traffic control) and FSS (flight service station) via VHF (very high frequency) through a telephone connection to obtain an instrument clearance or close a VFR (visual flight rules) or IFR (instrument flight rules) flight plan. They may also get an updated weather briefing prior to takeoff. Pilots will use four “key clicks” on the VHF radio to contact the appropriate ATC facility or six “key clicks” to contact the FSS. The GCO system is intended to be used only on the ground.