Patent GB-1048991-A (GB)
Title: Improvements relating to automatic avoidance of ground obstacles by an aircraft or missile
IPC: G01S13/935
Filed: 1963-05-15
Abstract: 1,048,991. Pulse radar. ELECTRONIQUE MARCEL DASSAULT. May 15, 1963 [May 18, 1962], No. 19335/63. Heading H4D. In a system for automatically flying an aircraft (or missile) substantially at a predetermined height h 0  above the ground-eg. " hedge-hopping "-means on the aircraft define a "guard" " curve which extends below and in front of the aircraft and concavely upwards, and the evolutions of the aircraft in the vertical plane are controlled such as to maintain the guard curve substantially tangential to obstacles lying ahead. In the embodiments the aircraft carries a pulse radar which surveys the ground profile and the ensuing height and elevation data are fed into a computer together with electrical factors which define the guard curve; the latter may be the arc of a circle, centred on a point O a distance R c  + R p  - h 0  vertically above the aircraft and of radius R c  + R p  where R c  and R p  are the minimum tail-down and nose-down radii of the aircraft respectively; or it may be a parabola. Considering an aircraft A, Fig. 9 (not shown), with a velocity of magnitude v at an angle # above the horizontal, measured by Doppler radar, a vertical guard surface consists of divergent lines surrounding the guard curve and the beam (p) of a monopulse radar explores the elevation of the terrain ahead; the automatic pilot alters the attitude of the aircraft if there are any obstacles within the guard surface. The guard surface may be replaced by a three-dimensional zone limited by the surface of an incurved cone. When there are no such obstacles the aircraft follows the nose-down path on radius R p  so as to bring it into hedge-hopping flight as quickly as possible. The pitch command signal to the automatic pilot is made substantially dependent on the clearance e between the guard curve and the nearest point thereto on the obstacle, and to ensure stability of control is also made dependent on the temporal rate of change of e, i.e. de/dt. The pitch command signal is of the form: where G and T are constants, for each obstacle encountered by the guard surface. The angle b of the horizontal control surface of the aircraft, relative to the longitudinal axis, is made, by the command signal, equal to (v/Kg)d#/dt where K is a parameter dependent on the aircraft specification and g is gravitational acceleration, or equal to (#v/Kg)d#/dt + cos # - 1. Radar ranging is effected by a gating pulse (" window ") of duration decreasing with range to correspond to the divergence of the lines surrounding the guard curve. In its initial position the radar aerial emits a beam along an elevated line of sight, the beam then being depressed at maximum speed; a fixed distance D, e.g. 15 kms. corresponding to the maximum range of the radar is defined by a distancemeasuring gate pulse F as long as no obstacle echo is received. A strobe pulse F 1 , Fig. 23 (not shown), oscillates rapidly between range D and a minimum range d, e.g. 600 metres. When an echo appears in window F 1 , the aerial movement ceases and window F moves to meet the echo; window F then continues to move in the same direction at constant speed and the aerial is controlled to maintain that window in contact with the terrain. The pitch command signal may be generated by an analogue computer, Fig. 16 (not shown), in which the factor de/dt is computed as v cos (# - #) where # is angular depression from O to the point P, Fig. 7 (not shown), at which the radar beam crosses the guard circle. The Specification includes a mathematical treatment on the results of which the computer is designed. The radar aerial may perform a cross-like scan, Fig. 21 (not shown), whereby it is available for azimuthal detection purposes. In a further embodiment, Figs. 21 . . . 24 (not shown), a rapid radar survey of the ground contours is effected and the contour data is quantized and stored as a succession of pairs of numbers X i , Z i  representing the ground profile in abscissa and height and the pitch command signal function (d#/dt) i  is calculated by a digital computer for each point X i , Z i , the signal retained being the maximum value of (d#/dt). The calculation is repeated, the programme for which is elaborated in the Specification.