Patent GB-1397142-A (GB)
Title: Aircraft monitoring systems
IPC: G01S13/935
Filed: 1972-09-08
Abstract: 1397142 Radar LICENTIA PATENTVERWALTUNGS GmbH 8 Sept 1972 [30 Sept 1971] 41751/72 Heading H4D In a radio position-finding system for assisting a plurality N of aircraft, e.g. helicopters, to fly in close formation, each aircraft carries a pulse radar interrogator/transponder and a directionfinder for determining the range and (true) bearing of each of the other aircraft and said position data as determined at any one aircraft is also transmitted to each of the other aircraft so that if radio contact between two aircraft X and Z, Fig. 1, fails (e.g. due to the presence of an obstacle O) then each of said two aircraft X and Z can compute the position of the other aircraft from the position of a third aircraft Y (which is still in radio contact with X and Z) and the received data signal (from the third aircraft Y) giving the position of the other aircraft from said third aircraft Y. If the N aircraft in the formation are denoted Al, A2-A N  then during a first subcycle the aircraft Al simultaneously interrogates the other aircraft A2-A N  which transmit corresponding reply signals having different predetermined delays to allow time separation of said signals at Al; in addition the reply signal from any one aircraft An includes a coded data component giving the position of another aircraft, e.g. An + 1, from An. During the next N - 1 subcycles the aircraft A2 - A N  operate sequentially as interrogators, the position data component of each reply signal being as in the first subcycle but during the next full cycle (comprising N sub-cycles) the position data component of each reply signal from an aircraft An gives the position (from An) of a different other aircraft, e.g. An + 2, and this process is repeated so that at the end of N - 1 full cycles each aircraft has transmitted the positions (from said aircraft) of all the other aircraft. In Fig. 2 (which shows the equipment carried by each aircraft) the range R of another aircraft is determined by a ranging stage 28 from the reply signal received at 26 in response to an interrogation signal from the transmitter 44 and the relative bearing α<SP>1</SP> of the other aircraft is determined by a direction-finder comprising a plurality p (e.g. p = 8) of directional aerials with a uniform angular spacing of 360 degrees/p. The directional patterns of adjacent aerials partially overlap (or touch) and a pulse signal received by any one aerial enables a corresponding modulator, the p modulators being fed by a local oscillator of frequency f 0  such that the phase difference between the f 0  signals applied to adjacent modulators is 360 degrees/p and the outputs from all the modulators being combined to give a resultant whose phase relative to a reference signal f 0  is equal to the relative bearing α<SP>1</SP>. The bearing signal α<SP>1</SP> is then combined in an adder 20 with a heading signal from a unit 12 to give the true bearing α, the signals R and α being stored at 38 and then read out as required to an encoder 40 for use as the position data component of a reply signal to be transmitted by the transmitter 44, a corresponding received data signal being decoded at 34. Each transmitted reply signal also includes a further data component indicating the altitude of the aircraft as given by a barometric altimeter 10 and the timing of the equipment is controlled by a unit 30 which is synchronized in a known manner with the corresponding units in the other aircraft.