Patent GB-1007966-A (GB)
Title: Improved system for simulating land mass radar reception
Inventor: BENAMY DANIEL ALEXANDER
IPC: G01S7/40
Filed: 1962-04-03
Abstract: 1,007,966. Radar simulation. CURTISS-WRIGHT CORPORATION. April 3, 1962 No. 12821/62. Heading H4D. In a radar simulator whereby ground mapping, contour mapping and terrain avoidance modes of an airborne pulse radar can be simulated, shadows may be simulated by means comprising a shadow computer arranged to operate on flight computer signals representing aircraft altitude and intantaneously scanned ground range and on video signals representing reflectivity and terrain elevation data, to produce signals representing the slope of the line of sight from the assumed radar position to a scanned point on the terrain, and the slope of the terrain at that point, means for storing the line of sight signal when the two slopes become equal and for blocking the video signals to represent the start of a terrain shadow condition, and means for comparing the stored signal with the line of sight signal, for unblocking the video signals to represent the end of the shadow condition, when the line of sight alone become equal to the stored slope. The general system is shown in Fig. 1 and comprises two transparencies 1 and 2, respectively containing information as to the radar reflectivity and elevation of the simulated terrain, and scanned by a flying spot CRT scanner FSS to reproduce corresponding reflectivity and elevation video signals at the outputs of two photomultipliers PM. The two transparencies are movable together to simulate movement of the aircraft and the intensity of the flying spot is controlled by a monitor photomultiplier PM and a monitor amplifier. The reflectivity signal is fed to a radar return computer and the elevation signal is fed to a shadow computer, the radar return computer and a gating control. A flight computer aircraft altitude signal Zf is also fed to both computers and gating control, and a signal representing the elevation of the antenna is fed to the gating control. The reflectivity signals are passed through the radar return computer, a shadow gate, a vertical beam gate, a video mixer, and a receiver simulator, to a display CRT. The shadow computer, Fig. 3A, is fed with an elevation signal representing the elevation e of the terrain at a range X, Fig. 3, a signal representing the said range X, and a signal representing the aircraft altitude Zf. The line of sight angle # to any point B, is given by tan # = e-ZF/X and the slope of the de de terrain at point B is given by - (or - since the dx dt elevation transparency is scanned at uniform velocity). At the commencement of a shadow e-ZF/X = sz/dt = (e-Zf<SP>1</SP>/X) whilst at the end of the shadow e-Zf/x once more equals (e-Zf1)/x. To enable these conditions to be determined the e signal is time differentiated to produce de/dx which is then multiplied with the X signal to de produce a signal representing x-. This is then dx compared with a signal representing (e - Zf ), produced by adding the e signal to the Zf signal, to produce a start pulse when the two signals are equal. The start pulse is fed to a logic sequencing means which in turn produces a signal closing the shadow gate and a storage command signal which causes the storage of the (e-Zf<SP>1</SP>/x) signal. The stored (e-Zf<SP>1</SP>/x) signal is   then compared with the continuing e-Zf/x signal   until a stop pulse is produced when equality is reached. The stop pulse is fed to the logic sequencing means which in turn produces a signal opening the shadow gate and a signal emptying the storage means. In the ground mapping mode an antenna beam, Fig. 4, is simulated extending from vertically beneath the aircraft to 9À6 degrees above the antenna axis, producing an extent of displayed terrain from A to B. Such a limitation of the display is produced by subtracting a signal representing R Sin (#e-9À6 degrees) where R is the range and #e the antenna depression angle, from the aircraft altitude signal, Fig. 4A (not shown, and generating a signal closing the vertical beam gate, Fig. 1, when the two are equal. In the contour mapping mode which is to be simulated a beam is used having a central axis of a depression angle of #e, Fig. 5, and intercepting a clearance or reference plane at a distance ho below the aircrafts flight path, at a point P, and having a width #. The lower edge of the beam will therefore cut the terrain at a point T 1 , having an elevation e and horizontal distance from aircrafts vertical x, such that x tan (#e + #/2) = (h - e) where h is the aircraft altitude. Similarly the upper edge of the beam will cut the terrain at point T 2  when x tan (#e e - #/2) = (h - e). The arrangement of Fig. 6 is introduced to produce this simulation, and comprises a function potentiometer 11, having an earthed centre tap, potentials respectively representing + ho max/60,000 and - ho max/60,000   applied to its ends, and a distributed winding such that the voltage produced at wiper 11<SP>1</SP>, which is positioned in accordance with the chosen valve of ho, represents #e = tan <SP>-1</SP>ho/60,000 A linearly wound potentiometer 12 similarly produces a voltage representing ho. The signal representing #e is fed to an antenna tilt servo to produce a corresponding angular movement controlling the position of wipers 13<SP>1</SP> and 14<SP>1</SP> of tangent potentiometers 13 and 14. These two potentiometers are supplied at either end with sweep signals of opposite sign representing x = ct andx = - ct, and are earthed at points on opposite sides of their mid points, such that the voltages at the wiper 131 and 14<SP>1</SP> represent   x tan (#e - #/2) and x tan (#e + #/2) respectively. These two voltages are then compared in stop and start beam position comparators respectively with a voltage representing (e - h) such that the comparator only produce outputs   simultaneously when x tan (#e - #/2) #(e - h)   #x tan (#e+#/2), i.e. if the point of the terrain 2 being scanned is within the antenna beam. To further ensure that the point of terrain is above the reference plane, a third comparison is made between the signal representing h 0  and that representing (e - h), an output being produced when h 0 # (e - h). A three input gate A produces a signal opening a gate T to pass the video signals to the display means, when the three comparisons produce outputs simultaneously. In simulating the terrain avoidance modes, Figs. 7 and 8 (not shown), wherein the aircraft is supposed to have a flight vector inclined at an angle # to the horizontal, the arrangement of Fig. 6 is modified such that the comparisons # are between (e - h) and x tan (#e--+#), (e - h) 2 # and x tan (#e+-+#) and between (e-h) and 2 (h 0  Sec #+x tan #). Means are described, whereby the sweep of the scanner CRT, FSS, Fig. 1, is delayed by an amount t 1  - t 0 , Fig. 9, from the sweep of the display CRT, to compensate for the fact that the display range scale is along the line of sight, whilst the scanner range scale is horizontal.