Patent GB-1135627-A (GB)
Title: Terrain-viewing system
Inventor: WOOD ALBERT GEORGE
IPC: G01S13/935
Filed: 1966-03-09
Abstract: 1,135,627. Terrain viewing system. UNITED AIRCRAFT CORP. 9 March, 1966, No. 10311/66. Heading H4D. The invention is concerned with apparatus providing, on a C.R.T., a perspective view Fig. 2 (not shown), of terrain Fig. 1 (not shown) beneath the craft carrying the apparatus. Two embodiments are described, the first, Fig. 4, being an airborne monopulse radar system, and the second Fig. 5, being an underwaterborne e.g. submarine monopulse sonar system. In the embodiment of Fig. 4 reflections from a transmitted pulse are received by two horn antennµ 13 and 15 kept in a vertical plane by means of roll stabilizer 88 and having vertical fan lobes with a very narrow horizontal width. The downward direction (#), Fig. 3 (not shown), from which an echo is received is given by phase comparison of the two received echoes in balanced phase detector 56, the echoes having previously been converted to IF signals and passed through equal level limiters 52 and 54. The output of detector 56, representing (#), is added to the aircraft pitch angle α in adder 60 to give the direction with respect to the horizon, of the incoming echo. The thus produced direction signal is passed via normally open gate 62 to the vertical deflection input 66 of C.R.T. indicator 68. The nominally constant intensity spot is thus vertically deflected by an amount proportional to (α + #). If the slope of the reflecting terrain increases the rate of range of (α + #) will also increase and produce a corresponding increase in the speed of movement of the indicator spot. The dwell time of the spot will thus be reduced, so that the steeper the slope the darker will be the trace, and the tops of hills will be brightened. Each transmitted pulse will thus produce an echo which when used to cause vertical deflection of the indicator spot, will produce a line intensity modulated nominally by an amount proportional to the slope of the terrain. If the antenna and the receiving lobes are scanned in azimuth, and a corresponding horizontal scan is applied to the indicator, then the display of Fig. 2, is produced looking like a real view of the terrain when illuminated by moonlight. Even with a flat terrain however, the rate of change of (α + #) will not be constant and decreases with time. This would result in a darkening of the foreground in the display, and to overcome this the beam is intensity modulated by a signal from circuit 84 comprising a decaying exponential voltage started by the transmitted pulse. A non linear circuit 65 operating on the vertical deflection signal aids said compensation. Intensity modulation pulses are also applied by circuit 76 to produce a series of horizontal range calibration lines. When the far side of a hill is in radar shadow the echoes are below the level of limiters 52 and 54 and the resulting inequality between the limiter outputs is detected by amplitude comparator 74 which then closes gate 62. A boxcar circuit 64 stores the last detected (α + #), signal and the spot remains on its last position. The result is that the tops of hills &c. are brightly outlined. An echo from an aircraft will be relatively large such that a summing circuit 90 adding the two echo signals causes a colour input 79 to the display to be actuated whereby the aircraft is shown in a different colour. In the sonar embodiment, Fig. 5, the vertical deflection of the spot is proportional to the depth (#h) of the terrain, i.e. R Sin (a + #), and thus looks like a view of the sea bed as seen from an infinite distance through a telescope of infinite magnification. The angle (#) is determined by dividing the vector (phase) difference of the received echoes by the vector sum in circuit 157. The resultant (#) signal is added to an (α) signal and a signal from a gyro switch apparatus 161 to give, for application to one input of multiplying circuit 163 a signal representing the angle (α + #) referenced to a true horizontal. For small angles this signal may be taken as representing the sine of itself. A ramp signal from generator 178 started by the transmitted pulse and representing the range R is applied to the second input of the multiplying circuit 163. The output of the circuit thus represents the desired quantity R. Sin (α + #). This output is applied via gate 162 to a storage circuit compressing capacitor 164 and diode 165. This circuit ensures that the far sides of ridges, visible from the submarine but not visible for infinity are not displayed. Such far sides are represented by a decreasing #h voltage which is not passable by diode 65. The vertical deflection signal applied via diode 171 to the display thus represents the previously recorded minimum depth (#h). Flat earth correction and range gate generator circuits may be added, and other submarines may be displayed in colour.