Abstract: An activation signal is generated when an aircraft passes directly overhead and may be employed to record aircraft noise levels or to trigger a zenith camera for producing a photograph of the aircrafr useful in determining aircraft position. When used in connection with a second instrument located at a fixed distance from the first and along the path of the aircraft, aircraft ground speed may be determined. Ambient light, normally passing through an optically defined airspace and then incident upon light sensors, is partially blocked when an object such as an aircraft is present. A signal proportional to the change in ambient light incident upon the sensors and a function of the size and shape of the aircraft is used to sense the passage of a wing section. Upon detection of the wing section an activation signal is generated.

Abstract: An activation signal is generated when an aircraft passes directly overhead and may be employed to record aircraft noise levels or to trigger a zenith camera for producing a photograph of the aircraft useful in determining aircraft position. When used in connection with a second instrument located at a fixed distance from the first and along the path of the aircraft, aircraft ground speed may be determined. Ambient light, normally passing through an optically defined airspace and thence incident upon light sensors, is partially blocked when an object such as an aircraft is present. A signal proportional to the change in ambient light incident upon the sensors and a function of the size and shape of the aircraft is used to sense the passage of a wing section. Upon detection of the wing section an activation signal is generated.