Patent GB-1356678-A (GB)
Title: Method of and apparatus for determining the maximum or minimum relative velocity between two bodies
Inventor: BROWN D J L
IPC: G01S13/58
Filed: 1971-08-13
Abstract: 1356678 Doppler speed measurement AUSTRALIA COMMONWEALTH OF 13 Aug 1971 [14 Aug 1970] 38198/71 Heading H4D [Also in Division G4] A method of measuring the relative velocity -between two bodies comprises producing a first signal, in a transmitter-receiver arrangement by means of the Doppler principle, whose frequency is proportional to the relative velocity wherein there is counted the numbers of cycles of a second signal occurring during successive occurences of a fixed number of cycles of a third signal only the greatest of said numbers being stored in a memory and wherein one of said second or third signals is said first signal and the other represents a timing function. A radar transmitter/receiver 1 is mounted adjacent a gun 3 and receives Doppler shifted signals from a shell fired therefrom. The Doppler signal is fed to phase comparator 8 where it is compared with a signal derived by division by 100 from a voltage controlled oscillator 6. The error voltage produced is used to vary the phase and frequency of the oscillator such that the signal fed to gate 10 is exactly 100 x the Doppler frequency. Clock 9 produces a pulse which enables gate 10 during a measuring time interval and counter 11 holds the greatest count over a number of measuring times. When monitor logic 12 detects that the shell velocity is decreasing gate 17 is opened and the counter 11  content is read into counter 16 and accumulator 19. Counter 16 has a BCD output which drives a decimal display 18. Accumulator 19 holds the sum of the maximum velocities measured during previous firings and a variable modulus divider 21 produces the average maximum velocity which is supplied to counter 20 and display 25. The modulus of divider 21 is provided by a round counter 23 which may be reset by button-22 and also has a display 24. If phase lock between the reference and Doppler signals is lost, logic 12 after a delay 13 indicates by means of lamp 14 that the measurement is invalid and the content of counter 11 is not passed on.  Maximum counter 11, Fig. 3.-At the start of the measuring cycle counter A is set to all zero and counter B to the complement thereof. When input 36 is enabled by the measuring time signal Doppler pulses on line 35 pass via gates 38 and 37 (since counter B is at all ones) to counter A. At the end of the first measurement period counter B is set to the complement of counter A, this disables gate 37 and enables gate 39. Thus during the next measuring period the Doppler pulses are fed to counter B until it reads all ones (i.e. has received the same number as A in the first period). This switches gates 37 and 39 again and the excess pulses are fed to counter A which thus always reads the maximum number received during any measuring period. When the shell velocity falls, counter B will not reach an "all one" state during a measuring period and subsequently the output of counter A is read out via counter B. In an alternative arrangement the measuring time interval is made equal to the length of a fixed number of Doppler cycles and the input 35 is provided by timing pulses. The maximum count is then representative of the minimum velocity reached. Monitor logic 12, Fig. 5.-Counter 56 has a capacity of three and is enabled only if a phase lock signal is present an input 50 from the phase comparator 8, Fig. 1. If a count exceeded signal is detected (31, Fig. 3) at the end of a measuring period, pulse input 51 is supplied to the counter. If three consecutive count exceeded pulses are received output E of the counter enables gate 57 which then produces a pulse after the first measuring period during which a count exceeded pulse is not received (i.e. when the shell velocity falls). Thus a pulse F indicates that phase lock was retained during two periods prior to and one period after the period during which the maximum count was recorded.