91 vernier range oscillator 86 is controlled by a range gate generator 87 and coarse range selector 88 range gates from range gate generator 87 are coupled to receiver 82 and the vernier range oscillator 86 in response to trigger pulses from the output terminal of or gate 29 while commands for coarse range settings are coupled to the coarse range selector 88 via line 89 from the processor 26 when pulses from short generators 83 85 coalesce at a tap on transmission line 84 as for example tap 93 a vernier module 94 couples a signal to a locking generator 95 wherefrom a signal indicating that coalescence has occurred is transmitted via line 96 to a processor 26 vernier module 94 additionally supplies a signal to the processor 26 via line 97 to identify the tap at which coalescence has occurred after lock on and the tap at which it has occurred has been noted by the processor 26 a reset signal is coupled therefrom via line 98 to the vernier module 94 and a signal to select the subsequent position signal with which the next range measurement is to be made is coupled to position pulse selector 25 via line 99 a circuit that may be used as the position pulse selector 25 is shown in fig6 toggle flip flops 102 and 103 are set initially in the high state as shown in fig7 prior to t 1 on curves 102a and 103a when a high to low transition of a trigger pulse is coupled via line 99 from processor 26 to the clock terminal of the flip flop 102 it toggles from a high state to a.