Patent GB-858113-A (GB)
Title: Improvements relating to the control of aircraft
Inventor: CRONIN RICHARD HARRY RHEAD
IPC: G01S1/02
Filed: 1955-10-25
Abstract: 858113 Leader gear for assisting navigation PLESSEY CO Ltd 9 Jan 1957 [25 Oct 1955] 30551/55 Headings G3R R4 and R32BX Relates to a control system employing low frequency alternating magnetic fields produced by two spaced leader cables to define a suitable glide path for use in the landing of aircraft on airfield runways, and facilitates landing either by the pilot or by an automatic pilot which receives information in the form of control signals. The system may be used with existing approach control systems which can bring the aircraft to a specified position in the vicinity of the runway, the control then being taken over by the present system. A theoretical discussion is included in which a leader cable laid on the surface of the earth is shown to have a complex magnetic field comprising several components and which is dependent on the ground conductivity and the excitation frequency. It is shown that with normal values of earth conductivity a virtual cable situated above the normal cable at ground level is formed. The effect on a pick-up coil situated at a convenient height on the centreline between two cables excited at the same frequency (a) in antiphase, and (b) in phase, is also discussed. The cables may also be excited at different frequencies to facilitate the separation of signals. The information obtainable from the composite magnetic field set up by the cables can be used to determine the position of a point in space and may be extracted from the field in three possible ways, i.e. the aircraft detecting equipment may respond to the amplitude of the selected components, to their bearing or direction, or to their phase. In one example, Fig. 4, a cable is laid on each side of a runway and excited with alternating current, and a glide path is defined by an operating function provided by maintaining constant the sum of the amplitudes of two of the magnetic field components as detected by the aircraft equipment. If the two cables were parallel a horizontal path would be defined but the cables are flared in the manner shown to define a glide surface extending from a convenient height of approach to ground level. In a modification, Fig. 5 (not shown) a family of similar surfaces are established such that after a displacement, due to a gust of wind for example, the aircraft follows the new path to which it has been displaced rather than attempting a return to the original one. Changes from any fixed value of the operating function are used for controlling purposes so that only changes from the required direction of travel rather than from a specific path cause error signals to be generated. Two cables are employed as in Fig. 4 and suitably flared to cause the family of curves to be disposed in space. A further arrangement is suggested in which the vertical components of the magnetic field of each cable are combined in one coil in the aircraft and the horizontal components in another, the outputs from the coils being differentially combined and maintained at a constant value. In a further arrangement use may be made of the virtual cable phenomenon. The position of the virtual cable depends on that of the aircraft with respect to the real cable and the aircraft takes bearings on two virtual cables, one on either side of the runway, and maintains the angle between them constant, following glide surfaces established by flaring the cables. Each cable is energized at a separate frequency and the pick-up coil in the aircraft rotates at a frequency considerably lower than those selected for the cables. The two fundamental signals are separated each being modulated in accordance with the rotation of the coil and the direction of the field, and a control signal is obtained which is proportional to the angle between the virtual cable bearings. Fig. 24 shows ground equipment in which flared leader cables L1, L2 are energized via a power amplifier driven by a mixer circuit fed from a pair of oscillators. The oscillators 1A, 1B, are tuned to a first audio frequency and the oscillators 2A, 2B, are tuned to a second audio frequency, but whereas the oscillators 1A, 1B, operate in phase, the oscillators 2A, 2B, operate in antiphase. The aircraft equipment Fig. 25, comprises a pair of fixed coils Hx, Hy, each feeding a selective amplifier, signals of the frequency of oscillators 1A, 1B, being amplified by AMP 1, and signals of the frequency of oscillators 2A, 2B, by AMP 2. The outputs of the two amplifiers are detected and fed to a subtraction circuit SUB and the difference signal is applied to a circuit MAN to derive a control function operating a display arrangement or automatic pilot. In a simplified arrangement, Figs. 22 and 23 (not shown) the ground equipment comprises a pair of flared leader cables each energized at a different audio frequency. The aircraft equipment comprises a single rotating coil feeding a first channel comprising an amplifier tuned to one of the audio frequencies, a demodulator, and, if desired, a converter to convert the signal into pulse time modulation, and feeding a similar second channel in which the amplifier is tuned to the other audio frequency. The two outputs are applied to a phase comparator and the resultant output signal is applied as before to a display arrangement or an automatic pilot. Curves showing the cable flaring, glide paths, and lateral coverage, for various suggested systems are included.