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 .