Patent GB-623188-A (GB)
Title: Radio position indicating system
IPC: G01S13/78
Filed: 1946-04-18
Abstract: 623,188. Radiolocation. STANDARD TELEPHONES & CABLES, Ltd. April 18, 1946, No. 12050. Convention date, May 14, 1945. [Class 40 (vii)] In a pulse radar system comprising a fixed beacon transmitter and a plurality of spaced receiving and display apparatuses carried by aircraft flying in the vicinity of the beacon, two alternating superimposed displays are produced, in one of which only objects carrying responding apparatus are displayed and in the other of which all reflecting objects within range (excepting those within a narrow zone between the beacon and the receiver) are displayed. The beacon transmits a narrow beam of microwave pulses rotating 50 times per minute, transmitting 1500 pulses during one complete rotation, alternate pulses initiating the two alternate displays respectively. During the first type of operation, termed " Three Path Radar " (3PR), the radio beacon transmits in addition to the pulse of width W1 on the microwave frequency f1, an omnidirectional synchronizing pulse of width W2 on a lower ultra-high frequency f2 to initiate a linear time-base sweep in the cathode-ray display apparatus of all receiving aircraft in the vicinity. This synchronizing pulse also conditions the receivers on the aircraft to 3PR working during the next 800 microseconds. The W1 pulse is re-radiated by receiving aircraft, lying in the path of the microwave beam, on the frequency f2. The re-radiated signals are received by the beacon where they may produce indications upon a P.P.I, display. The beacon re-radiates these signals omnidirectionally on the frequency f1 and with a changed width W3. These signals are received by the receiving aircraft where, on account of their changed width, they do not excite the responding apparatus but are passed to the display apparatus. To produce a P.P.I. centred about the beacon the rotation of the cathode-ray sweep is synchronized with the rotation of the directive beam. During the 3PR cycle when the beam is passing through North an omnidirectional pulse of width W5 is transmitted in place of pulse W2. This pulse serves to initiate the time-base sweep as before, but also restores to synchronism a shaft in each receiver rotating at slightly greater than 50 times per minute and driving the rotatable deflecting coil of the cathode-ray tube. During the next pulse cycle, the alternate system, here termed the Radio Lighthouse System (R.L.S.) comes into operation. In addition to the exploring pulse W1, f1, the beacon transmits an omnidirectional synchronizing pulse of frequency f2 and width W4. This pulse W4 generally conditions each receiver for R.L.S. working and initiates its sweep circuit. Considering the conditions shown in the Figure, where the microwave beam is directed approximately S.S.W., to produce reflections from the objects O, O<1>, these reflections of frequency f1, which are insufficient in amplitude to trigger the aircraft responders, are received by the aircraft A and passed to the display apparatus. Any object carrying a responder and lying along the path of the beam will also re-radiate a response on frequency f2, which will also be received at A and applied to the display apparatus. In order to provide a P.P.I. display centred about the beacon on the aircraft A, the value of M must be determined from the signals received. A shaft, similar to that driving the deflecting coil of the cathode-ray tube, is synchronized with the rotating beam, but, in this case, by means of the maximum amplitude W1, f1 pulse which occurs when the beam sweeps past the aircraft. This shaft then measures the angle p at any instant. The distance C of the craft from the beacon is measured by a shaft which is controlled by a walking double-strobe pulse which during one of the 3PR cycles locks itself on to are-radiated pulse of width W3. The strobe is only controlled during the 3PR cycles when the microwave beam is being received so that generally the W3, f1 pulse being followed is that corresponding to the aircraft, though if more than one responder is located at the same azimuth it is possible for the strobe to lock on to the wrong pulse. This, however, will produce distortion in the display, which will be easily recognisable by the observer who is provided with means for altering the timing of the strobe until it locks on to the correct pulse. The length (M+P - C), which is proportional to the delay between the direct and reflected pulses, is measured by the linearly increasing time-base voltage at the instant of reception of an echo. These factors p, C and the time-base voltage are inserted into a computing circuit which delivers a suitably distorted time-base voltage which is applied to the cathode-ray display unit to produce the radial deflection of the beam. In order that the heading of the craft carrying the receiving apparatus may be presented upon the display, a transparent disc, rotated by a compass carried in the craft and having several parallel arrows engraved thereon, may be mounted in front of the cathode-ray tube screen. Alternatively means, controlled by the compass, may be provided for rotating the display so that the top of the screen always corresponds to the heading of the craft. A rotatable transparent disc may then be provided to indicate North. Specification 617,511 is referred to.