Patent GB-959159-A (GB)
Title: Terrain clearance and contour mapping system
Inventor: JEROMSON GEORGE
IPC: G01S13/935
Filed: 1963-02-08
Abstract: 959,159. Collision warning radar; gating circuits. HUGHES AIRCRAFT CO. Feb. 8, 1963, No. 5218/63. Headings H3P and H4D. An airborne terrain clearance and/or contour mapping radar system comprises means transmitting electromagnetic energy, and means receiving, with simultaneous lobing on two vertically overlapping beams, energy reflections from detected objects, the receiving means comprising an antenna having a boresight axis, said axis being scanned in azimuth, energydividing means coupled to the output of the antenna to produce sum and difference signals, signal-forming means coupled to the energydividing means and responsive to the amplitude of the sum and difference signals to develop a control signal when reflected energy is received along the boresight axis, and display means coupled to the signal forming means and to the transmitting means and responsive to the control signal to provide an indication of the range and azimuth of objects intercepted by the scanning plane of the boresight axis characterized in that said antenna when scanned in azimuth defines with its boresight axis in space, a range arc in a clearance plane below the aircraft, and that said display means has a screen with a fixed reference arc thereon which is so positioned that the indication of the range and azimuth of objects at a range greater than the distance between said aircraft and said range arc, is displayed on one side of said fixed arc, and the indication of the range and azimuth of objects above said clearance plane and at a range shorter than the distance between said aircraft and said range arc, is displayed on the other side of said fixed arc. The invention is described incorporated in an airborne monopulse system comprising a horizontally scanning antenna 10, Fig. 1, having two vertically disposed overlapping lobes. Pulses, produced by transmitter 50 under the control of pulse generator 56 pass via circulator 38 and magic T-structure 24 to radiators 16 and 18 to be radiated on both lobes towards the ground 256, Fig. 2, 12 being the direction of the boresight axis. Reflections from the ground 256 are received by both radiators 16 and 18, echoes from ground below the point of intersection of the boresight axis being received before those from ground above the said point. The received signals on both branches 20 and 22 are fed to the magic T-structure 24 to produce a sum signal on line 40 and a difference signal on line 58. Assuming the echoes of a single transmitted pulse are received over the whole width of the combined lobes, the sum and difference signals will have waveforms as shown at 32 and 30 respectively, echoes received from objects on the boresight axis being identified in time and range by a point of zero amplitude in the difference signal and one of maximum amplitude in the sum signal. The difference signal is converted to an I.F. signal and detected, and then passed to a pulse generator 84 which delivers a pulse 86 coincident with the point of zero amplitude, which pulse is then passed to one input of a gate 90. Such a pulse however will also be produced when the difference signal falls to zero through lack of signal. The pulses are not therefore used for display unless they coincide with a sum signal of amplitude greater than a datum valve. The sum signal is therefore converted to an I.F. signal and detected and passed to the second input of gate 90. The gate comprises a triode 120, normally cut-off, and a system of diodes 132, 134, 140 and 142 normally conducting and shorting the pulses from generator 84 down to earth at the cathode of diode 132. Application to the grid of the triode of a positive sum signal of amplitude greater than the datum value causes the triode to conduct and apply respective negative and positive going signals to lines 136 and 138 to cut-off the diodes. The signal 86 from the generator 84 is then passed, together with a biasing voltage from source 188 to the grid of a C.R.T. 176. The C.R.T. has X and Y-plates energized in a known manner from sweep signals modified by an antenna coupled sine and cosine generator 200, to produce an off-centre sector P.P.I display on the C.R.T. screen 177, Fig. 5. The system can operate in two modes, a terrain clearance, or collision warning mode, Fig. 6, and a contour mapping mode, Fig. 7 (not shown). In the terrain clearance mode the antenna 10 scans in a plane at a constant depression angle α with respect to the aircraft's direction of travel. A clearance plane 286 is at a vertical distance to below the aircraft and parallel to its direction of travel, and the scanning plane cuts the clearance plan in a reference arc 285. This arc, at a constant distance from the aircraft is marked on the C.R.T. at 294, Fig. 5, such that the pilot knows that objects such as 292, showing within this arc are above the clearance plane and present a potential collision hazard. The output of the system in its first mode can be used to control an automatic terrain or obstacle avoidance system when an above the clearance plane is detected. In its second mode, the clearance plane is a vertical distance h below the aircraft, but is always horizontal, and the depression angle α below the horizontal is kept constant by gyro stabilizer means 248, whatever the inclination of the aircraft.