Abstract: An aircraft navigation system uses an infra-red scanner to provide an image of the ground, and a laser doppler system to measure the speed of the aircraft relative to the ground. The apparent movement relative to the aircraft of features in the image is extrapolated to find the point in the image at which this movement converges. This point corresponds to the direction in which the aircraft is moving. Knowing the original position, the direction and motion of the aircraft, its new position can be calculated.

Abstract: By placing a mirror in the field of view of an infrared imager it is caused to produce an image of its own detector assembly which is correctly focussed when the lens system is focussed at infinity. This phenomenon is used either manually or automatically to focus the system.

Abstract: Passive target detectors which employ infra-red sensors may have difficulty in distinguishing between a cool target near the detector and a warmer one further away.A detector detects a source of infra-red radiation and if its size indicates that it may be of interest, a point source detector operates a gate to allow information regarding the direction of the target to be transmitted to a store and to a calculator.This information is combined in the calculator with stored directional information from the previous scan to give the range of the source. The real intensity of the source is then calculated from its range and apparent intensity. If the real intensity indicates that the target is of interest, an alarm or other indication may be operated.

Abstract: The invention provides a range responsive apparatus primarily intended for operation at infra-red. A detector is arranged to receive electro-magnetic energy simultaneously from different spaced portions of a field of view which is caused to rotate so as to sweep across a target. The electrical output of the detector includes a frequency component which is dependent upon the range of the target. Two detectors are normally provided each receiving energy from alternate portions of the field of view, the outputs of the two detectors being combined in anti-phase. Use may be made of frequency filters to create one or more range windows and frequency responsive means provided either simply to give an indication of target presence within a range window or the actual range of the target.

Abstract: When monitoring the velocity of a moving surface an indication of the direction of movement is given by providing, on the moving surface, a pattern or shape. This is viewed by a detector which, because of the nature of the pattern or shape, produces a signal which identifies the direction of movement of the surface.