1. An instrument which signals the exit of a missile from a predetermined trajectory or the path of a flying object.
2. An electronic device that indicates the departure of a missile from a predetermined trajectory or the path followed by an object moving through space.

The term sky screen refers to equipment that provides a posiive indication to the military range-safety officer whenever a missile deviates from its planned trajectory or its movement through space.

One sky screen monitors the flight bearing and the other sky screen monitors vertical programming.