Patent GB-1437961-A (GB)
IPC: G01S13/933
Filed: 1973-06-28
Abstract: 1437961 Radar RCA CORPORATION 28 June 1973 [7 July 1972] 30801/73 Heading H4D A radar system of the type in which coded interrogation signals result in corresponding replies is provided with a tracking system comprising essentially a digital tracker, a track gate movement limiter and a range rate computer. The claims of the present Specification are directed to the digital tracker which comprises a range gate 16 for receiving the replies, a detector 20 coupled to the range gate output for converting the replies to digital pulses, means 52 for counting the time interval between the interrogation signals and said reply signals and means responsive to said counting means output for determining the range to the replying target. A correlator 10 is coupled to the tracking system of Fig. 1 the correlator determining if the reply signals are valid by comparing transmitted and reply codes. The correlator provides a signal To each time that an interrogation is transmitted and a signal Tr and a strobe indicating a valid target. Replies from receiver 12 are fed through a range gate 16 and a detector 20 for determining the centre of the reply signal, a pulse 28 being generated whose leading edge (time measured from left to right) corresponds to the centre of the reply signal. A counter 52 is enabled by each To pulse via a delay 44 and is stopped either by a pulse 28, a T R  pulse or when the counter reaches maximum range and no reply is received. These signals are fed to gate 42 via OR gate 37. Track gate movement limiter.-The range count in counter 52 is fed to device 54 which compares the previous count in register 56 to the present count. If the counts are the same no change in the register count is effected. If the register count is greater or less than the present count then its count is reduced or increased respectively by one only. The limiting of the count change is stated to reduce the probability that the gate will be pulled off a valid target by fake target or other noise. The register count is transferred to down counter 58 and a zero register 60 detects when the contents of counter 58 has been down. The register 60 then generates a gate pulse 79 which is applied to AND gate 18 to enable the range gate 16. Range rate computer.-A tracking period is defined by a certain number of To pulses and these are counted in device 104. A short track or a long track mode may be selected by switch 115, the former comprising fewer To pulses than the latter. Device 104 divides each track period into three portions and activates counter 100 to count up the range counts in the first portion, disables the counter in the second portion, and activates the counter to count down in the third portion. The remaining count is then the difference in the range counts in the first and third periods. The difference is converted by means of a divider 101 to indicate range rate i.e. target velocity, on display 172. The relative difference between the first and third period count depends upon the direction of target movement and this is indicated at the  output of counter 100. The use of the three periods is stated to smooth the range rate and give increased accuracy. Further tracker components.-A counter 102 counts up all the range counts in the tracking period and, via a converter divided 103, indicates the range on display 174. A device 184 divides the range by the range rate and indicates the time to impact of an approaching target. If this is less than a predetermined value then a light 192 is energized. A target assessor 24 is programmed to give a track signal on line 108 if three consecutive replies are received within eight interrogation intervals (the time between To pulses). This represent the transition from requisition to trackniode. The track signal is maintained if at least six replies are received out of any ten consecutive probes. If not, line 310 is activated and illuminates light 313. A signal indicating non-acquisition of the target is provided on line 312 to light 316. After losing the target or acquisition has failed an indication on the tracker station line to correlator 10 informs the correlator that a new target may be located. Detailed circuitry of the tracker system is provided.