Patent GB-574933-A (GB)
Title: Improvements in or relating to radio systems for landing aeroplanes
IPC: G01S13/34
Filed: 1940-04-02
Abstract: 574,933. Radiolocation; directive wireless systems. STANDARD TELEPHONES & CABLES, Ltd., EARP, C. W., and STRONG, C. E. April 2, 1940, No. 5912. [Class 40 (v)] [Also in Group XXXVI] In a system for guiding aircraft when landing, the height of the craft and its distance from a predetermined fixed target on the ground are ascertained in the aircraft by radio sounding arrangements and the ratio of height to distance is used for guiding the aircraft. Radiofrequency energy is radiated from a source A, Fig. 1, on the aircraft and its frequency is cyclically varied. Waves reflected from the ground and from a reflecting target C are received on a receiver B on the aircraft and are compared in frequency with the frequency of the waves emitted by the source A at the moment of reception by the receiver B. Currents of the two difference frequencies are passed to apparatus D by which these difference frequencies are converted into measurements of height above the ground and of distance from the target. The apparatus D may be arranged as shown in Fig. 2. The output from the receiver is applied to a first volume limiter V1 and comprises oscillations of two frequencies dependent on the difference between the maximum and minimum frequencies of the cyclic variation and the number of cycles of frequency variation per second of the source A and on the height of the aircraft and its distance from the target C. The output of the limiter V1 comprises a series of pulses of a frequency corresponding. to the frequency of the oscillations determined by distance from the target. Part of this pulsating energy is fed to a frequency counter F1 which feeds an indicating instrument 1D. The counter F1 may operate by charging and discharging a condenser, and the instrument 1D may be a moving coil milliammeter. The remainder of the output of the limiter V1 is passed to an amplifier E, the characteristics of which are such that currents of low frequencies are amplified to a greater extent than currents of high frequencies. The lowest frequency component, i.e. that representing height above the ground, thus forms the output of the amplifier E and is passed into a second volume limiter V2 from which energy pulses are fed to a frequency counter F2 and an indicating instrument H1. Indications are given on the instruments 1D, H1 of distance from the target and of height respectively. By maintaining a constant ratio between height and distance, a straight glide path towards the ground may be followed of any chosen gradient. The indicators H1, 1D may be combined in one instrument G1, indicating the glide path directly. One form of such an instrument is shown in Fig. 5, in which pointers h1, d1 indicate height and distance respectively. The points of intersection of the pointers are calibrated in a series of lines l indicating various constant ratios of height to distance. In a modification, the frequency counters are arranged to provide equal energy outputs for a predetermined frequency ratio and these outputs are fed in opposition to a single indicating meter, so that a zero reading corresponds to the predetermined frequency ratio. The target C, Fig. 1, may comprise a preselected reflecting object on the ground near the landing field or may comprise an aerial system for picking up energy from the aircraft, amplifying means, and an aerial system for radiating the amplified energy back to the aircraft. The aerial systems may be made directive in directions along which the aircraft is intended to land. Such a system is shown in Fig. 4. The incoming energy collected by the receiving antenna R is of frequency F1Œf, where F1 is the mean frequency of the source on the aircraft and f is the degree of frequency modulation prevailing at a given instant. Current of frequency F1Œf is passed into a mixer M1 together with a frequency F2 from an oscillator O. The output of the mixer M1 is of frequency F2-(F1Œf) and may be made of such a value, by suitably choosing F2, that it is efficiently amplified in an amplifier L. The output of this amplifier is passed to a second mixer M2 together with energy of frequency F2 from the oscillator O, restoring the original-frequency F1Œf, which is passed to the radiating aerial T. Automatic volume control may be provided to compensate for increasing signal strength received by the antenna R as the plane approaches. This control may operate so as 'actually to reduce the output of 'the repeater in addition to reducing its gain, so that as the plane approaches the re-radiated waves received at the plane remain of substantially constant intensity.