Patent GB-1403998-A (GB)
Title: Waveform communication sysem
IPC: H01Q3/42
Filed: 1972-10-27
Abstract: 1403998 Aerials; pulse radar; radio signalling INTERNATIONAL BUSINESS MACHINES CORP 27 Oct 1972 49542/72 Headings H4A H4D and H4L A communication system comprises first and second wave energy transmitting/receiving means which are spaced apart but which are within effective view of each other. One of said means is active, and the other may be either active or passive. At least one of said first and second means includes a retrodirective array of transducer elements and means whereby a communications loop over an optimal spatial path is automatically built between them out of noise in said loop. As described, a retrodirective oscillating loop comprises retrodirective aerial array terminals A, B, Fig. 1A, each being within the field of view 20A or 20B of the other. At the terminal A pairs of aerial elements 10A, 13A and 11A, 12A are connected together by respective lines of equal lengths, with amplifiers 19A, 18A interposed. An incident plane wavefront from a direction within the field of view results in the propagation from the array of a plane wavefront in the opposite direction. The terminal B is similar to the terminal A, and when the loop gain is sufficient, noise initiates the build-up of an oscillatory condition, when radiation from either terminal becomes automatically beamed upon the other (lobes 21A, 21B). A frequency change may be effected at each terminal, so that different frequencies are used in the two directions A to B, and B to A (Fig. 3B, not shown), and heterodyning may be accomplished, to allow amplification at an intermediate frequency (Fig. 3C, not shown). The oscillations around the loop may be modulated, for conveying information. In one embodiment, each amplifier at one terminal is modulated, and detection is accomplished at one amplifier at the other terminal. Filters are provided between the receiving aerial elements and the amplifiers at the first terminal, to prevent oscillation at undesired frequencies (Fig. 4A, not shown). In another embodiment, two terminals communicate via a satellite. Each terminal comprises a retrodirective aerial array and cooperates with the other terminal in an oscillating loop circuit, the satellite being a simple repeater (Fig. 4B, not shown). Modulation circuits are described both for arrangements in which each aerial element is exclusively for transmitting or receiving (Fig. 5A, not shown), and for arrangements in which each aerial element is used for transmitting and for receiving (Figs. 6, 7, not shown). The Fig. 7 arrangement includes means for compensating for local oscillator drifts and for Doppler shifts. In another embodiment, the height and attitude of an aircraft 350, Fig. 9A, are determined by means of a retrodirective aerial array 356, and circuitry 358 for providing an oscillatory loop in conjunction with ground reflections. The array comprises three receiving elements 360-1, 360-2, 360-3, Fig. 9B, and three transmitting elements 360-4, 360-5, 360-6 which are coupled together in pairs by lines of equal lengths, with detectors 368, amplifier/limiters 362, and modulators 364 interposed. When the oscillatory condition is established the retrodirective array directs a beam of radiation along the shortest path (351) to earth. The height can then be measured by modulating with pulses from a source 366, and determining their times of travel with a measuring device 367. The slope angle # is determined by measuring the carrier phase difference between the extreme aerial elements 360-1, 360-6, with a detector 370. Filters 369-1, 369-2 eliminate any sidebands. Similar arrangements to those described may also be applied to acoustic waves.