ATC

Air Traffic Control. The system of people and equipment designed to help keep you safe in the air.

Per FAR 91.125, light gun signals that ATC can send you in the event of lost comms are:

Color/TypeOn GroundIn Flight
Green, SteadyCleared for takeoffCleared to land
Green, FlashingCleared to taxiReturn for landing
Red, SteadyStopGive way to other aircraft and continue circling
Red, FlashingTaxi clear of runway in useAirport unsafe - do not land
White, FlashingReturn to starting point on airportN/A
Red & Green, alternatingExercise Extreme CautionExercise Extreme Caution