Patent GB-642761-A (GB)
Title: Radio position indicating system
IPC: G01S13/78
Filed: 1947-12-04
Abstract: 642,761. Electric control systems. STANDARD TELEPHONES & CABLES, Ltd. Dec. 4, 1947, No. 32036. Convention date, Jan. 26, 1946. Addition to 623,188, [Group KL (c)] [Class 40 (i)] [Also in Group XL (c)] In a pulse radar system of the kind described in the parent Specification and comprising a ground transmitter and a plurality of receiving aircraft, means are provided enabling the ground transmitter to communicate exclusively with a selected one of the aircraft co-operating in the system. The communication signals comprise a narrow train of pulses transmitted at a time after the radar pulses corresponding to the range of the selected aircraft and the aircraft carry means enabling them to determine their ranges from the ground transmitter so that a gating circuit on each aircraft can be arranged to pass only the appropriate signal. The parent Specification describes a system comprising a fixed ground beacon transmitter and a plurality of spaced receiving and display apparatus carried by aircraft flying in the vicinity of the beacon. In addition to the apparatus described in the parent Specification, the beacon transmitter comprises means for radiating a command in the form of a pair of pulses of slightly different duration, the spacing between which is representative of the command. A pulse pair, which covers altogether a period comprising only a few micro-seconds, is radiated in each alternate radar-pulse cycle, i.e. in those cycles described in the parent Specification as 3PR cycles, at an interval after the radar pulse corresponding to the selected range from a directional aerial which is directed towards the selected aircraft. The aircraft apparatus comprises, as described in the parent Specification, a walking- strobe unit wherein in the 3PR cycles pulses are generated corresponding in time to the range of the aircraft. These' pulses are applied to a gate-circuit together with the communication signals which accordingly can only pass therethrough if the receiving aircraft is at the correct range. The output of the gate-circuit is applied to a plurality of selecting circuits arranged in parallel, each of these circuits being adapted to pass pulse-pairs having a predetermined respective spacing. The signals passed by the selectors actuate, respective indicators from which an operator in the aircraft can determine the command (e.g. to communicate with the ground station over a particular radio channel) transmitted from the ground. Responder equipment. The train of pulsepairs passed by the gate-circuit is also applied to a pulse-duration selector 181 which allows only the first pulse of each pair to pass to circuit 182 which together with the clipper circuit 183 delivers a pulse signal 184. This signal is applied over two channels, one containing a barometrically controlled delay circuit 187, to a circuit 190, the input to which consequently comprises a train of pulse-pairs, the spacing of which is a measure of altitude. The signal passing through the circuit 190 is modulated under the control of a circuit 180 with a telegraphic-code signal identifying the responding aircraft. This modulated signal is then transmitted back to the ground station. Means are provided, however, for preventing the aircraft from responding when the exploring beam of the radar system is directed at the aircraft. By this means interference is prevented with the radar signals received by the receiver on the ground. The aircraft responder may be adapted to re-radiate additional information, e.g. the airspeed and heading of the craft. Ground responsor equipment. The response signals transmitted from the aircraft are received at the ground station and passed through an appropriate width selector to an indicator arrangement. This comprises a flipflop circuit which generates pulse signals, the length of which corresponds to the spacing between the received pulse-pairs, these signals then being integrated and passed to a voltmeter to give a reading of altitude. A codetape printer is also provided which prints a group of dots or dots and dashes according to the modulation impressed on the signal at the aircraft, whereby the selected aircraft may be identified.