GB1444046A - Method and system for determining aboard an airplane its position with respect to an airport landing strip - Google Patents

Abstract

1444046 Aircraft position determining system ELECTRONIQUE MARCEL DASSAULT 22 Feb 1974 [26 Feb 1973] 8111/74 Heading H4D The Specification describes a method and system for determining the position of an aircraft approaching an airport runway P, which comprises, a plurality of beacons 12, 14, 16, positioned at fixed known positions relative to the runway, a central airport transmitting station which generates measurement pulses produced by an atomic clock and enables individual beacons in a predetermined order to transmit a measurement pulse and information corresponding to its altitude. The aircraft has aboard a receiving and computing system linked to a less accurate clock, and a memory, storing the position of the beacons, which operate so that the aircraft's position is ascertained by processing a plurality of received measurement pulses, each representing the aircraft's distance from a particular beacon, so that the aircraft clock is brought progressively into synchronism with the airport clock thereby allowing the aircraft's space and time co-ordinates to be computed. Provision is made in the computing system to take account of the aircraft's speed and heading. The ground equipment includes the atomic clock which is linked to a master transmitter which first transmits a pulse code, e.g. a 3 in Fig. 4 which enables one particular beacon, and at a particular carrier frequency. The beacon then converts the master transmitter carrier frequency, e.g. from 1À5 GHz to 1À7 GHz, and retransmits a measurement pulse, e.g. c 3 received from the master transmitter after the pulse code enabling the beacon. The measurement pulse C 3 may be preceded, for each beacon by a pulse code b 3 corresponding to that beacon's altitude so that the aircraft computer may make the necessary adjustment. The master transmitter may have provision for monitoring the transmission from each beacon to check on the accuracy of the time measurement pulse transmitted. The aircraft receiver, tuned to the beacon frequency, e.g. 1À7 GHz, receives the information in a computer 52 (Fig. 5), which also receives from memory 54 the co-ordinates of the transmitting beacon, and from a system 56, the aircraft's estimated position Œ500 m. The computer estimates the position of the beacon, and includes the transmitted altitude information at input 51. The time of reception of the measurement pulse is compared with the time of occurrence of the aircraft clock pulse, and the time scale of the aircraft's internal clock is progressively brought, on successive receptions of measurement pulses, into synchronism with the ground based atomic clock, using recursive filtering techniques in computer 52. Provision is made, in the absence of a beacon in a particular position, for the measurement to be skipped until the next measurement pulse, or if a beacon is in a position other than that stored in the aircraft memory, for that beacon to transmit along with the information in pulse, e.g. b 3 , its actual co-ordinates.