the detected incoming ils localizer signal is applied via lead 11 to 150 hz band pass filter 10 which extracts the 150 hz localizer navigation tone from the detected ils signal this filter is extremely selective and preferably possesses a bandwidth of only a few hertz next the 150 hz sinusoidal tone is buffered by unity gain buffer amplifier 20 and then applied to the input of squarer 30 squarer 30 implemented using a comparator produces a 150 hz square wave signal at its output this square wave is in turn applied as one input to a phase detector contained within phase locked loop 40 the other input to the phase detector applied via lead 65 is the output of the voltage controlled oscillator vco contained within the phase locked loop but counted down by 100 by decade counters 50 and 60 within phase locked loop 40 illustratively cmos type 4046 the output of the phase detector is applied to the control input of the vco and in turn causes the vco to produce a stream of timing pulses illustratively square waves having a frequency of exactly 100 times that of the 150 hz input square wave i e 15 khz by virtue of the feedback path around the phase locked loop the 15 khz timing pulses produced by the vco are phase locked at a zero phase difference not only to the 150 hz square wave applied as one input to the phase locked loop but also to the 150 hz navigation tone since decade counters 50 and 60 together count 100 successive pulses of the 15 khz timing signal for each of the 150 hz square waves the 25th pulse out of each 100 pulses exactly coincides.