Patent GB-1291741-A (GB)
Title: Infrared target search and track system
IPC: F41G5/00
Filed: 1970-05-08
Abstract: 1291741 Photo-electric missile tracking HUGHES AIRCRAFT CO 8 May 1970 [11 Aug 1969] 22358/70 Heading H4D A search/track apparatus comprises means for applying target position signals to servo means for controlling the position of optical means directing infra-red energy from a target on to a receiver which on receipt thereof provides a signal to cause the apparatus to track the targer, said receiver then providing error signals to the servo-means and wherein there are means for determining the target position from the position of the optical means. Approximate target azimuth and elevation signals 21, 22 Fig. 2 are supplied by the acquisition unit which may be a fan beam (azimuth) and pencil beam (elevation) radar or a manual optical sight. The signals are fed through mode controller 26 (initally set to search) to the servo units 23 controlling the mirror 25 position whose co-ordinates are detected by resolvers 54, 46 and fed back to the units 23. The coordinates are also encoded and fed to five control computer 15 along with the target range which is determined by a known laser  r ange finder using the same optical system as the tracker (Fig. 13, not shown). The receiver, Fig. 3, comprises an array of detectors D1-D11 on a plane parallel to the axis of rotation 61 of an oscillating mirror 60 which directs the target infra-red energy from the mirror 25 on to the detectors. With mirror 25 correctly positioned and mirror 60 in its zero position the image of the target formed by telescope 35, falls on the centre detector D5. The mirror 60 scans the image initially over the coarse field 64, the Y error being determined by the particular detector which detects the image and the X error being determined by the displacement Em of the mirror 60 from its null position at the instant of detection. In the tracking mode these error signals are fed to the servo units 23. When the errors are both small, this state being detected by comparators (Fig. 10, not shown) the scanning is switched to a higher rate over the fine field 65. The X error is determined by feeding each detector output through a separate threshold circuit to an input of AND gate (95), Fig. 6 not shown), whose output feeds a counter (121) which switches the controller 26 from the search mode to track mode on receipt of a predetermined number of pulses. The gate (95) output also feeds a hold circuit (120) which reads the instantaneous value of Em, and thus the azimuth error, when a signal is detected. The Y error is determined by connecting the detector outputs through monostables toa weighted resistor ladder nework which supplies a characteristic voltage indicating the Y error, (Fig. 7, not shown). In the fine scan mode only the output from the centre detector D5 is used to determine the azimuth error. To determine the Y error the detectors D5, 10, 11 are used. If the mirror 25 is correctly positioned the image will be detected by detectors D10, 11 at known values of Em, any deviation from this value gives the Y error. The Em value corresponding to D5 is used to remove any errors in Y due to X errors. (Figs. 8-9 not shown). The apparatus may be used to acquire the target by scanning an azimuth angle around the approximate target azimuth supplied by a fan beam radar, whilst performing a stepped scan in elevation (Fig. 11, not shown).