Patent GB-518881-A (GB)
Title: System for determining the altitude of an aircraft in flight utilizing electromagnetic waves radiated from a pair of radio transmitters
IPC: G01S1/30
Filed: 1938-09-06
Abstract: 518,881. Wireless signalling. HAMMOND, J. H. Sept. 6, 1938, No. 26036. Convention date, Sept. 16, 1937. [Class 40 (v)] A pair of radio transmitters 11, 12, energized at the same frequency produce an interference field 17 having maxima and minima the spacing of which along a path of constant altitude between the two transmitters is a function of the altitude. According to the invention, the altitude of an aircraft flying along such a path at an altitude greater than the spacing of the transmitters is determined by means of a receiver which indicates the distance between successive maxima or minima. The energy in the field is picked up at the receiver, Fig. 2, by the aerial 21, amplified in the radio stage 22 and the audio stages 25, 27, and is applied to the phones 35. The number of maxima heard in a certain time corresponding to a given distance, the necessary time being deduced from the speed of the aircraft, gives the altitude. A.V.C. is applied to the audio stages 25, 27 from the rectifier circuit 28, the strength of the rectified current-being shown by the meter 30. The number of maxima in a given horizontal distance can also be shown automatically by the luminous indicator 60. When using this, switch 37 is closed and switch 42 is thrown down causing part of the audio output to pass through the rectifier 36, smoothing-circuit 38, and amplifier 39 to close relay 40, which completes the circuit through battery 80, brushes 77, slip-ring 74, filaments of lamps 72, slip-ring 73, and brush 78. The two lamps 72, the filaments of which are connected in parallel to the sliprings 73, 74, are rotated about the shaft 62 by the motor 67 behind a transparent and luminescent segment 75, so that each time the relay 40 closes as the receiver passes through a maximum a luminescent spot is produced on the segment. The speed of the motor 67 is adjusted in accordance with the speed of the aircraft by means of the rheostat 69, so that the lamp 72 moves across the entire segment 75 while the aircraft traverses a given horizontal distance. Alternatively the frequency of passing through the maxima could be measured by regulating the motor speed to cause a single light spot to appear stationary on the screen ; in combination with the known speed of the aircraft, this frequency would give the altitude. A direct indication of the altitude obtained by measuring the frequency of passing through the maxima may be obtained on the meter 56 in which case the switch 42 is thrown up so that each time the relay 40 closes the condenser bank 43, which has a small time constant, is charged by the batteries 52, 53. When relay 40 opens, the condenser bank 43 discharges into condenser 48 which has a large time-constant, so that the voltage across condenser 48 which is applied to the grid of valve 55 is proportional to the rate of operation of relay 40. Since this rate of operation depends upon both the spacing of the maxima and the speed of the aircraft, the timeconstant of the condenser bank 43 is varied with the speed by means of the switch 45, so that the plate current of valve 55 is dependent upon the spacing of the maxima only and the meter 56 gives a direct indication of the altitude.