Patent GB-916431-A (GB)
Title: Radar simulation apparatus
IPC: G01S7/40
Filed: 1960-01-21
Abstract: 916,431. Radar simulators. ACF INDUSTRIES Inc. Jan. 21, 1960 [April 7, 1959], No. 2213/60. Class 40 (7). [Also in Group XL (b)] Apparatus for simulating the display produced by an airborne radar of echoes from the ground beneath the aircraft comprises a small-scale three-dimensional model of the ground viewed by a television camera which is moved over the model by a flight computer in accordance with the simulated movement of the aircraft, the output from the camera being applied to a cathode-ray tube (C.R.T.) to provide a display simulating a known form of radar display, e.g. a PPI sector display. The display may be limited to objects protruding above a clearance plane at a selectable perpendicular distance h o , Fig. 13, from the aircraft, said plane being parallel to either the longitudinal axis of the aircraft (i.e. terrain avoidance operation) or the earth's plane (i.e. contour mapping operation). Photo-electric means are also described for generating a voltage representing the height above sea level of the terrain immediately below the aircraft and said voltage is combined with a voltage representing the barometric height of the aircraft to give a voltage representing the height of the aircraft above the surface of the ground as would be determined by a radar altimeter. The three dimensional ground model 8, Fig. 1, may be constructed of plastic material cast from a master mould, specific targets on the ground being represented by detachable blocks, Figs. 6, 7, 8 (not shown). The camera assembly 60, Fig. 1, is moved over the model by mounting it on a carriage which is movable in one direction along a member which is itself movable in a perpendicular direction, Figs. 19a, 19b and 19c (not shown). The camera tube 61 may be vertical, but as shown in Fig. 1, it is horizontal and the image of an area on the model corresponding to the radar coverage is focused on to the camera by means of a prism 4 and lens 6, the camera assembly 60 including the optical system 4, 6 being rotatable about a vertical axis Y through the prism 4 in accordance with the simulated heading of the aircraft. The model is illuminated by light from a low-power source 2 directed through a transparent plastic light guide mounted on the camera carriage, Fig. 19b (not shown), the end of the guide being adjacent the rotation axis Y and being lower than the camera assembly 60. The heights of the camera assembly 60 and light source 2 are independently adjusted in accordance with the simulated aircraft height and the light guide is resiliently mounted, Fig. 22 (not shown), so that if it comes into contact with the model it is deflected and actuates a relay which stops the carriage and actuates the crash simulation warning system. When the camera is lowered, the prism 4 is tilted about its lateral axis to simulate the different aspect of the ground at low altitudes and the camera tube 61 is moved back to maintain the same coverage, Figs. 24 and 25 (not shown). The photo-cathode of the camera tube is scanned by a rotating radial scan, Fig. 4 (not shown), produced by applying a timebase from a synchronizer 18 to X and Y deflection members through a revolver 44 rotated in accordance with the simulated aerial rotation. The indicator CRT 16 is scanned in a similar manner by a corresponding revolver 48 and an electronic bearing cursor is produced by a manually-adjustable revolver 56. The amplified video output from the camera tube 61 is combined in a mixer 20 with range calibration signals from the synchronizer 18 and with signals simulating jammings noise or air targets from a generator 70. The range of the display is changed by changing the lens 6, a number of such lenses being mounted in a turret; when a new lens is selected the prism 4 is tilted about its lateral axis to maintain the rear edge of the field of view of the vertical rotation axis Y, Fig. 21 (not shown). Terrain avoidance display, Fig. 13 and Fig. 14 (not shown). As shown in Fig. 13, if the flight path α of the aircraft 171 is directed downwards at an angle # to the ground plane b then the height of the clearance plane R above the ground plane b is given by he = h - h e  directly below the aircraft and by h tr  = he - h ta  = he - e i  tan # at a plan range distance e i  in front of the aircraft. A computer, Fig. 14 (not shown), responsive to h, h o , cos # and tan # produces a voltage h tr  and the video signals are applied to the display through a gate which is opened only when the voltage h tr  is less than a voltage representing the height h t  of the terrain at the plan range e i . The voltage h t  is generated by scanning in synchronizm with the display a terrain height transparency, Fig. 15 (not shown), or opaque print, Fig. 16 (not shown), having a light transmittance or reflectance at any point which is proportional to the height of the surface of the model at that point. The transparency or print may be mounted above or below the ground model, Fig. 18 (not shown), and scanned by a flying spot-scanner or additional camera tube which is moved in correspondence with the camera 60. The transparency or print may be prepared as claimed in Specification 916,430 by projecting radiation through a three-dimensional model of the ground on to a photographic plate, Fig. 17 (not shown), or by photographing the model while it is graduually covered or impregnated with lightabsorbing or reflecting liquid so that the exposure time of any point on the model is a function of its height. A contour mapping display is produced in a similar manner to the terrain avoidance display but in this ease the height h - h o  of the clearance plane is independent of the altitude # of the flight path. Simulating radar altimeter. The voltage representing the height above sea level of the terrain immediately below the aircraft may be produced as claimed in Specification 916,432, (1) Fig. 28 (not shown), by gating the height voltage produced by scanning the terrain height transparency or print by means of a gating pulse coincident with the initiation of each range scan and smoothing the gated signal, or (2) Fig. 29 (not shown), focusing a photo-cell on the point on the transparency or print corresponding to the simulated aircraft position and moving the photo-cell in correspondence with the camera viewing the threedimensional ground model. In either case the resultant voltage is subtracted from a voltage from the flight computer representing the barometric height of the aircraft to give the required radar altimeter voltage. The resultant voltage may also be displayed directly to indicate terrain altitude.