Patent GB-358123-A (GB)
Title: Improvements in and relating to electrical means for determining altitude from aircraft
IPC: G01S13/36
Filed: 1930-04-08
Abstract: 358,123. Indicating proximity of earth. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London. -(Assignees of Alexanderson, E. F. W.; 8, Adams Road, Schenectady, New York, U.S.A.) April 8, 1930, No. 11138. Convention date, April 8, 1929. Addition to 349,426. [Class 37.] In order to determine the altitude of an aircraft above the earth, high-frequency waves having a wave-length which is short compared with the altitude are radiated from the craft and means are provided for detecting the resulting high-frequency oscillations having cyclic frequency variations caused by the combination of the oscillations reflected from the earth with the radiated oscillations, energy is stored in a condenser in an amount dependent on the detected combined oscillations, and the voltage across this condenser is measured in order to give the required indication. Carried by the aircraft is the apparatus shown in the Figure, in which oscillation generators 1, 2 are arranged to oscillate respectively at a fixed frequency and at a frequency which varies cyclically with the height of the aircraft above the earth, as described in the parent Specification. The frequency of the oscillations generated by the generator 1 may be set to one of three predetermined values by means of a switch 3 which may connect one of two condensers 27, 28 in parallel with a coil 26 coupled to the grid coil 23 of the generator 1, the third position of the switch being on open circuit. Connected with the oscillatory circuit of the generator 2 is an antenna 4, suspended beneath the craft and connected through a plug-and-socket coupling 19, 20, cable 5, and condensers 6, 6<1>. The frequency of the oscillations generated by the generator 2 is dependent upon the phase relation existing between the radiated waves and the reflected waves which arrive back at the antenna from the earth. The frequency accordingly varies cyclically as the craft varies in altitude, the amplitude of the cyclic variations being dependent upon the altitude of the craft above the earth. The beat frequency produced between the oscillations of the generators 1, 2 is amplified by a valve 7 and applied through a transformer 8 to a detector valve 9, the anode circuit of which includes a relay 62 which closes the circuit of a lamp 13 when the current increases above a predetermined value. The secondary winding of the transformer 8, owing to its inherent capacity represented by the condenser 8<1>, has a natural period of oscillation slightly outside the range of cyclic frequency variations produced. Between the anode and cathode of the valve 9 are connected a resistance 40 and a condenser 42, across which are connected a condenser 41 and resistance 43. The potential across the resistance 43 is applied to the grid of a valve 44, in the anode circuit of which is a resistance 45 and a battery 46. An energy storage condenser 10 is connected between the anode and cathode of the valve 44 through a diode 47 and a battery 48 which is so arranged that the anode of the diode 47 is maintained at a potential which is equal to or slightly negative with respect to the potential of the cathode, and consequently no current flows. When the potential of the anode of the valve 44 increases due to frequency variation of the oscillator 2, current flows through the diode 47 and charges the condenser 10. The potential of this condenser then overcomes that of the grid biasing battery 49 of a valve 12 and permits current to flow in the anode circuit, which includes an ammeter 11. Thus during the cyclic variations in the beat frequency which occur when a maximum has occurred, a certain charge is impressed on the condenser 10 and a certain deflection is produced on the meter 11. During the remaining portion of the cycle this deflection is substantially retained by the charge upon the condenser, which charge can only leak slowly away through a shunting resistance 10<1>. If the amplitude of the next cyclic variation is greater than the last, the charge on the condenser is increased and the deflection correspondingly increased, this increase taking place in steps each of which occurs at a definite altitude as determined by the maximum in the cyclic variations. The meter 11 has three scales, corresponding to the three frequencies of the oscillation generator 1 as determined by the three positions of the switch 3. When the maximum scale deflection is reached, the circuit of the lamp 13 is closed by the relay 62, warning the pilot to move the switch on to the next contact, corresponding to the second scale on the meter 11. Coloured glass windows 51, 52, 53 are provided on the switch 3 and are brought in front of the lamp 13 in the corresponding positions of the switch. Keys 65, 66 enable the condensers 41, 42, 10 to be shortcircuited prior to taking an observation during an ascent. The antenna 4 may be hauled in through a conduit 18 by means of an insulating cord 16 wound on a reel 15.