Patent GB-601672-A (GB)
Title: Improvements in or relating to radio navigation systems
IPC: G01S13/91
Filed: 1944-12-22
Abstract: 601,672. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. Dec. 22, 1944, No. 25832. Convention date, Jan. 26. [Classes 40 (v) and 40 (vii)] A radio-navigation system for guiding an aircraft to one of a plurality of ground landing points comprises means on the aircraft for transmitting to a selected landing point a train of pulses characteristic (e.g. in carrier frequency or pulse-spacing) of that landing point, means at each of the landing points for responding to its own characteristic train and retransmitting a train of impulses also characteristic of itself, and further means on the aircraft for indicating in response to the retransmitted train the direction of and, if desired, also the distance to the selected point. The system described is particularly suitable for guiding a helicopter provided with a continuously rotating aerial system comprising a cylindrical parabolic reflector 22, Fig. 4, directed obliquely towards the ground and having a directivity pattern which in the vertical plane has a component vertically downwards so that a response will be obtained even when directly over the landing point. The landing points have fixed aerial systems with a directivity pattern omnidirectional in the horizontal field, but with vertical components in all directions. Fig. 5 shows equipment at a landing point designed to respond only to pulses having a particular repetition period. The received pulses after detection are applied directly and after a delay equal to the interval between two pulses to separate grids of a valve 65 which only responds when both grids receive pulses at the same time. Thus, only at the selected point will a train of pulses 66 be passed by valve 65 and these are retransmitted from aerial 60 after a delay sufficient for a blocking circuit 68 to render the detector 62 ineffective. On the aircraft, the beam of a cathode-ray tube 37 is rotated in synchronism with the rotation of the aerial system 22 by connections to plates 38, 39 and the received signals are applied to the grid 31. producing an illuminated arc 43, the centre of which indicates the bearing of the landing point. As the craft approaches the landing point, the length of are increases, becoming a complete circle when overhead. To 'indicate also the distance, the beam is deflected radially by coils 139 rotating in synchronism with the aerial and fed from a time-base circuit 40. (The elements 44-46, 144, 145, Fig. 4, and 160-162, Fig. 5, do not appear in the drawings as accepted and are referred to below). The Specification as open to inspection under Sect. 91 comprises also the following subjectmatter : (1) The equipment, Fig. 5, may serve only as a beacon transmitter and not as a repeater by throwing switch 160. Transmitter 161 then transmits a signal characteristically modulated in tone or pulse-spacing by a modulator 162. On the aircraft, demodulator and filter 44 is connected over switch 45, and indicator 46 identifies the beacon. A coupling circuit 144 controlled by a blocker 145 may be added so that only the pulses corresponding to the desired signal are applied to tube 37. (2) In a simplified modification of the aircraft equipment, Fig. 7 (Cancelled) (not shown), the outputs of a simple voltage generator driven by motor 23, Fig. 4, and of the receiver 27, provided with automatic volume control, are applied to separate coils of a zero-centre meter which gives indications for guiding the craft to the beacon. (3) In another system, the aircraft has a fixed aerial 100, Fig. 10 (Cancelled), and the landing point a rotating inclined aerial 87, Fig. 9 (Cancelled). In response to pulses from transmitter 101, which are detected at 90, a train of pairs of pulses is sent back in which the spacing between pulses of a pair is continuously varied by a delay circuit 92 coupled to the aerial so that the spacing is a measure of the direction of the aerial. On the aircraft, the first pulse of a pair shock-excites a sweep-circuit generator 111 to cause the beam of the cathode-ray tube to rotate once for each pair of pulses. The two pulses produce indications at 115, 116 and the angular position of 116 gives the direction. To indicate distance also, a time-pulse generator 118 is connected to plates 114 and the received pulses to plates 113 giving an indication 180, 181 of which the first peak gives the distance. An electronic switch 119 changes over the connections for the two types of indication at a high speed so that both are seen simultaneously. The ground equipment, Fig. 9 (Cancelled) , may also act as a continuously transmitting rotating beacon, transmitting pairs of pulses, the spacing of which indicates direction. In a modification, Fig. 12 (Cancelled) (not shown), the aircraft equipment responds only to signals from a particular beacon. This subject-matter does not appear in the Specification as accepted.