Abstract: A system and method for pulse interval modulating signals in accordance with a code wherein the code resides in the spacing between adjacent pulses. The invention has particular utility in connection with laser designators and laser seekers and, while clearly not limited to this application, is disclosed hereinafter mainly in connection with laser designators and seekers. In connection with the laser environment, the system may include a pulse interval modulation encoder for encoding the laser designator output signal and a pulse interval modulation decoder for decoding by the laser seeker of the encoded designator signal. The encoder preferably employs an ordered arrangement of unique pulse interval pairs which cyclically repeats after a relatively long time period. Because of the length of the time period and the apparent randomness of the intervals forming the unique interval pairs, the code appears to enemy countermeasures to be random.

Abstract: A matrix code generating method generates a first predetermined code in binary form. A second step permutes the first determined code according to a non-linear feedback method described as pseudo-random. The code is segmented with linear segments positioned in a two dimensional matrix array. Apparatus generates and permutes the code, and segments the resultant code into matrix array form. Apparatus encrypts such information onto objects, photo-optically reads such encrypted information and deciphers the coded information.

Abstract: The invention provides a surveillance system comprising a number of infra-red detectors in an array, each of which is arranged to view a different portion of a field of view in order to discriminate between targets of different veiwed sizes at a target range. Each infra-red detector has in its output a threshold detector, the threshold level of which is determined by the mean output levels of adjacent detectors in the array. In a preferred example, for a given threshold detector in the output of a given infra-red detector the mean output level of two adjacent, but not immediately adjacent, infra-red detectors on one side and two adjacent, but not immediately adjacent detectors, on the other side are summed and the result divided by four to provide the reference level for the threshold detector concerned.