Abstract: A method and apparatus for receiving and correlating coded digital information with increased anti-jamming capability. The apparatus uses a receiver for each pair of frequencies at which the information is transmitted, a synchronizer for synchronizing the transmitter and receives, a correlator and a code sequence generator. The receiver is alternately operated at one of two frequencies over a predetermined period, and each correlator is provided with a number of sections equal to the maximum number of codes to be transmitted at each of the frequencies.

Abstract: In order to provide a system for measuring relative velocity of a surface with the accuracy of a laser velocimeter but avoiding the problems associated with operating down to zero speed and with signal dropout during a run, a tachometer is coupled to measure the relative velocity and the output of the tachometer corrected using the output of the laser velocimeter thereby giving laser velocimeter accuracy over the major portion of the velocity range but still permitting operation down to zero speed and operation when signal dropout occurs.