modulation frequencies f 1 f 2 f 9 respectively of the individual transponders ommission of signals and circuits corresponding to f 3 thru f 8 is to be understood to be for simplification however these signals and circuits are of course present in the practical equipment the output signals of the band pass filters 32 33 and 34 are rectified in corresponding rectifiers 35 36 73 each of the rectified signals is then fed to a corresponding threshold circuit 42 43 44 which delivers a trigger pulse when the signal amplitude exceeds or falls below the threshold value s fig3 thus at the instants t nhv t nhr t nrv and t nrr n threshold circuits provide trigger pulses which are fed to n counting devices counters 38 39 40 in the first counting device 38 the trigger pulse provided at the instant t 1hv resets the counter to zero and the trigger pulse provided at the instant t 1hr stops it analogous remarks apply to the two other counters and to the trigger pulses provided at the instants t 1rv and t 1rr to stop the desired counter by a given trigger pulse the counters 38 39 and 40 are to be understood to be preceded by conventional logic circuits all counts are transferred to the comparator 41 with addressable memory since all counts are proportional to a given period of time instead of comparing the time differences u03b4t n with the nominal time differences.