Patent GB-1309381-A (GB)
Title: Method and apparatus for data correlation
IPC: G01S13/28
Filed: 1968-09-03
Abstract: 1309381 Radar SANDERS ASSOCIATES Inc 3 Sept 1968 [1 Sept 1967] 41934/68 Heading H4D A pulse doppler radar for resolving range and doppler of received target signals transmits a signal comprising three pseudo-random codes. The first code comprises 10 bits each consisting of a 10 nanosec pulse, the 10 bit code comprising, for example, three successive positive pulses followed by two negative pulses, four positive pulses and a negative pulse. This code repeats continuously. Since each pulse is nanosecs wide it is equivalent to five feet of range and because there are 10 pulses, each pulse is unambiguous for 5 feet within 50 feet. This first code is coded by a second code comprising 50 bits each bit consisting of a 100 nanosec pulse, this code resolving range ambiguities of 50 feet in every 2500 feet. The resultant of the first code as modulated by the second code is modulated by a third code having 240 bits each comprising a 5000 nanosec pulse. The third code resolves 2500 feet ambiguities in a range of 600,000 feet or 100 miles. The drawing shows a processor in which the signal reflected from the target is received at 10 and subsequently processed to derive the range and doppler of the target. In the processor the first code is removed by passive correlation techniques by feeding the received signal through a series of delays 20 ... 36 and a plurality of inverters 38, 40, 42. Each delay 20 ... 36 is of 10 nanosec duration corresponding to the time between successive pulses of the first code. The inverters 38, 40, 42 correspond to the negative bits of the first code. Thus, at point 16 which receives the signal directly through line 18 and after delay through the delays 20 ... 36, is presented a signal of substantial magnitude corresponding to the reflection of the first code by the target. This signal is fed to range gates 44 ... 62 via delays 64 ... 80 each of 10 nanosecs. The gates 44 ... 62 are opened by a generator 82 for a period of 10 nanosecs within each 100 nanosec interval, an output from any one of the gates indicating detection of a target which is to be resolved within a specific five feet within a 50 foot range interval. The outputs from the gates are stretched by a factor of ten by networks 84 ... 102 to produce 100 nanosec pulses. Reducing the bandwidth in this manner permits the use of active correlation techniques to remove the second code. To this end the outputs of the stretch networks 84 ... 102 are connected to fifty groups of phase inversion switches 108 1 -108 10  ... 208 1 -208 10 , the ten switches of each group being connected respectively to the fifty stages of a shift register 104. The shift register is driven by a clock 106 operating at the rate of the second code, viz, ten megacycles per sec., and the number of stages of the register equals the number of bits in the second code. This operation produces at the output of the switches 108 ... 208 an intermediate range resolution equivalent to five feet within 2500 feet. The output of each phase inversion switch is connected to a corresponding one of 500 horizontal wires H1 ... H500 of an orthogonal storage matrix 114. The matrix has 2000 vertical wires V1 ... V2000 and a magnetic core 116 is located at each intersection thus permitting the storage of one million bits of data. A signal is stored in a core 116 when the core is simultaneously excited by an input on a horizontal wire H and by a write pulse from a sequencer 118 on a corresponding vertical wire V. The sequencer 118 provides write pulses on the vertical wires in series fashion at the rate of the incoming signals. The information written into the cores 116 represents resolution of range of five feet within 2500 feet. Each horizontal wire of the matrix 114 represents a five foot range gate; thus wire H1 represents the first five feet in 2500 feet, wire H2 represents five to ten feet in the same 2500 feet and so on up to 600,000 feet. A correlator 132 coupled to the vertical wires V of the matrix comprises a multiplicity of stitch networks 122 of magnetic cores 124 so stitched by wires 126 as to recognize signals applied thereto. The horizontal wires H of storage matrix 114 are sequentially read out by a sequencer 128 and the signals applied to the correlator 132. When a large read-out pulse is applied to a horizontal wire, the cores 116 in that particular row which have been previously energized and placed in a 1 state are de-energized and placed back in an 0 state. This is a destructive readout which causes pulses to appear on appropriate vertical wires, these pulses being summed by stitch networks 122. Should the characteristics of a signal derived from horizontal wires coincide with a particular stitch a maximized signal from the particular stitch network is obtained. A large pulse appears at output B0 for a correct signal (in range, doppler and doppler phase) and noise appears for all other targets. The stitch networks are arranged so that the cores simulate the third code modified by time delay for range and by phase inversion at the proper intervals for the doppler effect in the code. The number of stitch networks required depends upon the expected number of range and doppler combinations. In this example 240 range increments of 2500 feet are required to cover 600,000 feet and this 240 must be multiplied by 100 to obtain the necessary number of stitch networks if one hundred different dopplers are to be recognized.