Patent GB-1233651-A (GB)
IPC: G01S13/58
Filed: 1969-07-01
Abstract: 1,233,651. Pulse radar. INTERNATIONAL BUSINESS MACHINES CORP. 1 July, 1969 [10 July, 1968], No. 33041/69. Heading H4D. A pulse radar for measuring target range rate comprises means for generating range marker pulses, means for producing an output pulse each time the target passes a range mark, and means for measuring the interval between output pulses. Accuracy for low radial velocity targets is thereby enhanced compared with a known system in which the number of range marks passed in a predetermined interval is counted. The system may be employed for docking a ship. For very low speed targets an artificial velocity can be added. In the embodiments the transmitted pulse repetition rate is sufficiently high to ensure that echo pulses occur at only slightly different ranges relative to range marker pulses representing e.g. ten-foot range intervals. In general the range error #V feet is: where V is target radial velocity in feet/second, F is the number of radar pulses transmitted per second and L is the range marker interval in feet. In one embodiment a clock pulse generator 32 generates range markers and synchronizes transmitter pulse generator 34, the pulses from which (ignoring delay 62) are applied via delay 38 (which compensates for system delays) to the start input of counter 36 to reset the counter to zero and start a new count. A received echo pulse stops counter 36. A comparison circuit 42 provides an output pulse, if its digital inputs are unequal, representing a range mark crossover; that pulse causes the existing count in counter 36 to be entered into store 40 and triggers a flip-flop 46 which controls identical counters 48, 50 such that one is stopped whilst the other is counting: the counters are fed with pulses from divider 44. At the end of the interval between a pair of cross-overs one of counters 48, 50 contains a count proportional to that interval and this is passed via a switch 52 (to connect the appropriate counter and reset it) to digital divider 54. For introducing an artificial velocity, a sawtooth generator 60 provides a direct or an inverted ramp voltage to control delay means 62, the maximum artificial range added or subtracted corresponding to one range marker interval, and a subtractor 56 is utilized. To add a negative velocity if the target is moving away from the radar the direct ramp voltage is fed to delay means 62 and vice-versa; this is achieved automatically by arranging for circuit 42 to provide positive output on line 43 if the input from counter 36 is greater and a negative output if the revorse. A detector 72 generates a pulse when the range rate from subtractor 72 reaches zero: when line 43 is positive the direct ramp voltage is utilized and vice-versa. Ranging.-To afford a range measurement, A/D converter 66 gives a digital one output if its input amplitude is above the middle value of the ramp voltage from generator 60 and this is subtracted from the output of counter 36. In an alternative embodiment a single counter counts pulses between cross-overs and an analogue divider is provided, but means are not included for adding an artificial velocity, Fig. 3 (not shown).