In order to maintain an aircraft or other moving craft on a direct course towards its destination despite any tendency to drift, it is steered automatically in accordance with a magnetic or gyroscopic compass associated with a directional antenna which receives radio‐frequency signals from any broadcasting station near the destination, the antenna being maintained at a constant angle to the earth's magnetic field. The polepicces 2 of an earth‐inductor compass are initially adjusted to give a zero current through the armature when the craft starts out on a set course, and a frame aerial 24 is set at the required angle, relative to the compass pole‐pieces, to lie in a plane at right‐angles to the direction of propagation of the radio‐frequency waves. The two are geared together, so as always to maintain this setting, by the worm gear 12, 13, gears 11, 59, differential gear 57, gears 58, 62 and worm gear 63; the initial relative setting being effected by turning a shaft 60, on which the planet pinion spindles of the differential gear are mounted, through gearing 68, 69 from an indicator 70. The gears 58, 59 are loose on the shaft 60 and are fixed to the two other pinions of the differential gear. The output from the armature of the compass is connected across a polarized relay 4 through a resistance 21 across which a voltage is introduced by a known gyroscopic device 11 to correct for the vertical component of the earth's magnetic field whenever the craft is turned. When the craft yaws the relay 4 is actuated to reduce the bias on one or other of the grids of two thermionic valves 5, 6, causing anode current to flow and energize one or other of two clutches 15, 16, whereby a shaft 14 is rotated in one or other direction by a continuously running motor 18. This shaft 14 is geared to the wheel 11 whereby the polepicces 2, and the frame aerial, are caused to follow up the yaw. A dial 77 indicates the position of the craft with respect to the north. Simultaneously a contact arm 51, which is a loose friction fit on its spindle, makes contact with one or other of two contacts 52, 53 to apply a voltage from one of two oppositely polarized batteries 55, 56 and a resistance 42 in a circuit including a polarized relay 41. This relay is similar to the relay 4 and acts through similar valves and gearing to rotate a gear 45 to actuate a rudder 37. The rudder is connected also to a pointer 47 moving over a potentiometer 48 in bridge formation with two resistances 50 whereby a voltage is fed back to the relay circuit tending to de‐energize it. The ends of the frame aerial winding are connected to the grid cathode circuit of a valve 26 which includes also a source 29 of low frequency oscillations. A non‐directional aerial 25 is connected to the grid of a valve 27. The plates of the two valves are in parallel and the combined amplified output is fed to a demodulator 28 connected to the primary 30 of a transformer 31. As long as the frame 24 is at right angles to the direction of the incoming waves no low frequency oscillation will appear at the winding 30, but should the craft drift or yaw off the course, oscillations appear which are fed in opposite phase to the grids of two valves 32, 33. A transformer 36 also transmits the oscillations from the device 29 to the two grids in the same phase. The anode currents of the two valves flow in opposite directions through the two halves of a resistance 34 and the valves are so biased that the two currents are equal. Consequently there is normally no potential difference between the two ends of the resistance 34 and no current flows through a resistance 43 across a portion of which the relay 41 is connected. Whenever modulations appear in the primary 30, however, the balance is upset and the relay 41 is actuated to energize the rudder mechanism. An instrument 73 shows the amount of deviation. Should there be no forces tending to cause the craft to drift it may be steered by the compass alone, in which case a switch 79 is moved to put the relay 41 into communication with the compass instead of the resistance 43, and a switch 80 is moved to cut out the relay 4.
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