a method and means for evaluating transponder reply signals in a secondary radar system to differentiate valid replies from false replies . transponder reply signals received by the interrogating radar are decoded to establish conformity with an established format . all decoded replies to a succession of n interrogation signals are stored in their order of reception in an array of n shift registers . a reply filter is associated with each of the n shift registers . the reply filters operate in parallel to scan the stages of each associated shift register . a unary adder provides the number m of replies detected by the reply filters at each scan step . reply decision means selects a threshold value that is dependent upon operating conditions of the radar and generates a valid reply signal whenever the value m equals or exceeds the selected threshold value .