Error signals and discrimination in optical trackers that see several sources

A typical optical tracking system is described in which incoming radiation is amplitude modulated by a chopper disk in such a way that the photosensitive detector output is an FM signal with the frequency excursion and phase of the modulation dependent on the angular coordinates of the source in the eye field-of-view. The generation of error signals from the modulation caused by several sources, and the effects of system frequencies and bandwidths, are considered. For the particular case of two sources, error signal formulae are obtained in the form of series that converge rapidly for configurations of sources and eye axis for which the effective brightness ratio β is not close to one. Methods are set out for calculating error signals for the case of n collinear sources, and one of these methods is specialized to two sources to yield results valid for any value of β. The discrimination of the tracking system between two sources is shown to depend strongly on the form of the angular sensitivity function of the optical system.