Patent GB-592748-A (GB)
Title: Improvements in or relating to radio navigational systems
IPC: G01S13/91
Filed: 1943-12-17
Abstract: 592,748. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. Dec. 17, 1943, No. 21251. Convention date, Aug. 28, 1942. Addition to 590,460. [Class 40 (v)] In a system for defining a course, pulses are transmitted from an aircraft to two spaced beacons on the ground which re-transmit pulses with differing delays ; observation of the relative timing of the received pulses on the aircraft enabling its position with respect to the desired course line to be determined. The equipment on the craft, Fig. 2, comprises a time modulated pulse transmitter 10 and receiver 11 associated with an aerial 12, which may also be employed for normal communication, a blocking circuit 13 being provided to render the receiver operative during the required interval. The output of the receiver is fed to a normal time modulation detector 14 for communication, to the course indicator 15 and a distance indicator 16, which may be of the type described in Specification 582,753, for measuring the time delay of signals from one beacon. The first beacon or repeater station A comprises a receiver detector unit 18 and transmitter 19 coupled to a common aerial, the output of the receiver controlling the pulsing of the transmitter with a delay of about 2 microseconds produced at 20 and 21. Beacon station B is similar to station A, but the delay device 21B is capable of introducing a delay greater by 10 microseconds in order to produce the desired spacing of pulses received by the aircraft when on the desired course. Stations A and B have also blocking devices 23, 23B, of the type described in Specification 591,353 which are controlled by received signals through delay devices 22, 22B to block the receivers following receipt of pulses, for a period of 75 microseconds. The recurrence period of the pulses of the aircraft transmitter is 80 microseconds, while the spacing between stations A and B is such that the traversal time of signals between them is about 30 microseconds, so that the interval during which the receivers 18, 18B are blocked is adequate to prevent oscillation between stations A and B occuring. Each receiver is thus also protected from the pulse emitted by its associated transmitter, but is operative in time to receive the next interrogating pulse and permit slight shifting of the pulse due to speech controlled time modulation. The stations A and B are preferably omni-directional, but directional aerials may be employed to prevent interaction and so eliminate the need for care in spacing and timing to prevent mutual iriterrogation. The course indicator 15, Fig. 3, operates on the same principal as that described in the parent Specification. The pulses from the output of receiver 11 are fed via limiter 30 and detector 31 to filter detector circuits 32, 33, 35, 37, 39 which produce output corresponding to the fourth, third, fifth, seventh and ninth harmonics respectively of the pulse recurrence frequency. For a spacing between received pulses of oneeighth of the recurrence period, the fourth harmonic is zero while the third and fifth are equal and the seventh and ninth are equal so that meters 40, 41 and 42 which indicate the fourth harmonic and the differentials of the third and fifth and seventh and ninth respectively, will all indicate zero when the aircraft is on the course defined by delay of one-eighth of the recurrence period between returned pulses. Specifications 511,222, 523,575 and 529,648 also are referred to.