Patent GB-1506925-A (GB)
Title: Radar simulator
IPC: G01S7/40
Filed: 1975-04-23
Abstract: 1506925 Radar simulator GENERAL ELECTRIC CO 23 April 1975 [29 April 1974] 16784/75 Heading H4D A radar land mass simulator comprises a store of data which includes features of the physical relief of a terrain whose radar image is to be simulated and displayed in circular or sectorial manner on a P.P.I. display, plus the radar reflectivity of various classes of terrain surface and an identification of the particular terrain class lying on each side of intersecting boundaries between terrain classes which boundaries are stored as a function of the position of their end points the actual texture of each terrain class being respectively generated using a shift register having a feedback loop which produces a pseudo-random output and modulates the brightness of the display range sweep. The shift register is arranged to be synchronized with each sweep and is always reset to a particular state. In addition each shift register for a particular sweep is provided with two additional outputs which are respectively arranged to provide the shift register output on the preceding and succeeding sweeps which outputs are mixed in attenuated form with the register output for a particular sweep whereby to reduce graininess in the display of the simulated terrain. Several pseudo random registers programmed to reproduce the different classes of terrain to be displayed may be provided or a single fed-back register may be arranged to be reprogrammable to produce several terrain class signals, the positions of the interfaces between which differing terrain regions being stored in the data store. The general radar land mass simulation system described in Fig. 2 comprises a data store 48 which contains the ground and altitude coordinates of the end points of boundary line segments which approximate the peaks and valleys of terrain of interest plus the radar reflectivity and coded identification of the class of terrain existing on each side of a boundary line segment defined by the end points. Computer 50 loads scan memory 52 at each complete azimuthal scan with the pertinent data from store 48 regarding assumed location of the radar set. Scan angle generator 54 produces signals simulating the scanning angle which are fed to phase shifter 57 which receives signals from an associated aircraft dynamics simulator and shifts the phase of the scanning angle in accordance with the position, heading and altitude of the simulated aircraft. Other aircraft data are fed direct to line intersection processor 58 along with phase data from 57, which outputs the computed ground range and altitude of the intersection of each range sweep with the boundary lines defined by the end points, and data specifying the terrain class code and reflectivity of each terrain class within the range sweep, which data are ordered according to range and elevation and stored in a core memory in digital terrain profile generator 60. Video processor 64 reads out this data, the range increments of which are defined by timing units supplied by clock pulses from timing unit 62, which continually addresses generator 60, data, e.g. indicating a terrain class intersection being applied to update the registers of video processor 64. In addition simulated aircraft altitude data from aircraft dynamics simulator 56 are processed in video generator 64 to determine the slant range from the aircraft to ground, and the angle of the incident beam, the sine of which is multiplied by the stored reflectivity value, to produce the simulated return signal intensity, taking account of "blind spots" and terrain hidden from the aircraft. The terrain class code is decoded to provide a gating signal which is applied to one of gates 70A, 70B, 70C to gate the output of one of pseudo-random sources 72A, 72B, 72C to summer and filter 74, the output from which is fed to modulator 76 which also receives the video intensity signal on line 78. The terrain class signal comprising the pseudo-random output signal modulates the video intensity signal at 76 the output of which modulates the intensity of the display scan at 68, the azimuthal deflection control signals for which being controlled by the phase shifted output of scan angle generator 54. The display may therefore simulate an aircraft flying over the terrain. Set pulse generator 80 sets the stages of the fed back registers to a reference state at each scan initiation. Each pseudo-random source may include a shift register 20, Fig. 1, having seventeen stages, the signal from stages 5 and 17 being fed back via modulo 2 adder to stage 1, and an output at terminal 30 is produced to simulate, e.g. farmland. Adder 32 and adder 36 are connected to respective stages of the shift register to provide, respectively, output signals on terminals 34 and 38 which represent the signal on terminal 30 on the previous or subsequent scans. Attenuators 40 and 44 respectively attenuate the signals from adders 32 and 36 to half their amplitude and mixer 46 mixes all three signals to produce an output signal which, it is stated, tends to reduce degrading graininess in the displayed image by smearing together the signal on adjacent scans. Details of the connection arrangement and clock frequency to simulate different classes of terrain are given. In a modification (Fig. 3, not shown) instead of three separately adjusted fed-back registers in each of pseudo-random sources 72A-72C, the signals on terminals 30, 34 and 38 of a single register are frequency divided in dividers (84, 86, 88; 98, 100, 102) by suitable factors M or P to produce pseudorandom signals representing different terrain classes. Alternatively (in Fig. 4, not shown) fedback registers having the same feedback arrangement but different clock frequencies may be employed, and (Fig. 5, not shown) the attenuators 40 &c. may operate digitally and comprise flipflops, and mixer 46 a digital three input adder.