second period as shown at 20 the other timers 42 44 and 82 83 are wired as monostables the falling negative going edge of the 20 waveform triggers glideslope pulse generator 42 which responds with a pulse of approximately 100 millisecond duration 21 this pulse is inductively coupled to the glideslope needle circuit 32 by transformer 36 the negative going edge of 21 triggers delay timer 1 43 which generates a delay pulse of approximately 500 millisecond 22 this pulse in turn triggers localizer pulse generator 44 which responds with a pulse of approximately 100 millisecond duration 23 this pulse is inductively coupled to the localizer needle circuit 33 by transformer 37 the negative going edge of 23 triggers delay timer 2 82 which generates a delay pulse of approximately 500 millisecond 24 this pulse in turn triggers flag pulse generator 44 which responds with a pulse of approximately 100 millisecond duration 25 this pulse is inductively coupled to both flag circuits 34 and 35 by transformers 38 and 39 if desired the pulse unit 40 can include the option of a push button or switch 49 whereby the pilot can initiate the bobbing test signals automatic repetitive bobbing signals make the checking effortless and continuous however and do not burden the pilot who can become very busy a push button alone or as a supplement to the automatic pulses as shown could be employed if desired in the indicator connected embodiment shown in fig8 the pulses can be injected on the indicator drive lines as shown dc isolated with a coupling capacitor 99 this method is useful and simple when it is acceptable.