Patent GB-911629-A (GB)
Title: Collision avoidance system
IPC: G01S13/933
Filed: 1959-01-09
Abstract: 911,629. Radio navigation. STANDARD TELEPHONES & CABLES Ltd. Jan. 9, 1959 [Jan. 10, 1958], No. 902/59. Class 40 (7). [Also in Group XXXVIII] In an aircraft collision warning system in which a warning is produced on an aircraft equipped with scanning pulse radar of possible collisions with intruding aircraft, the air space in the sector around the equipped aircraft in which collision is most likely to occur, e.g. the forward 90 degrees sector symmetrical with respect to the aircraft heading, is divided into a plurality of cells bounded by lines of constant range and bearing and the range and bearing of each object detected during each radar scan is determined in the form of a binary code word denoting the cell occupied by the object, the code words produced during a first scan being stored and then compared with the code words produced during the next scan in a logic circuit which automatically determines if any positional change in any of the objects is such as to constitute a collision threat and, if so, produce an aural and visual warning indication. Transponders may be installed on the intruding aircraft to assist detection by the radar equipped aircraft. The zone of possible collision with an object 4, Fig. 8, of less than a predetermined range, e.g. 40,000 ft., is taken to be the shaded area bounded by the lines 25a, 26a tangential to a clearance circle 22a centred at the equipped aircraft 1a and as shown in Fig. 6, the preferred size of each cell is such that the extensions of its diagonals are tangential to the clearance circle 22 of radius r 0 , e.g. 1000 ft., such that # R/R ## = R/r 0 . As shown in Fig. 4 this condition is approximated by taking the angular widths of the cells in the range zones 7000-12,000 ft., 12,000-25,000 ft., and 25,000-40,000 ft., as 12.5, 6.25 and 2.1 MR. (milli-radians), respectively, the corresponding range increments #R being as indicated; Fig. 4 shows only the central cell in each range increment. Fig. 7 shows the cells adjacent the cell 24 occupied by an object during the first scan, i.e., the reference cell R, the collision zone boundaries being indicated by the lines 25, 26, and the degree of hazard produced by movement of the object to any one of the other cells is indicated by the appropriate legend S (safe), P (possible threat) and T (threat), the bottom row of T cells corresponding to a " fast threat " and the upper set of three T cells corresponding to a " slow threat." As described, the equipped aircraft uses a weather radar having an azimuth scanning beam 2 degrees wide in azimuth, Fig. 2 (not shown), a sector display, Fig. 9, being controlled by the logic circuit so that objects constituting (1) a possible collision threat, (2) a slow threat and (3) a fast threat are displayed respectively as (1) a defocused blur 29, (2) a relatively short line 31 and (3) a relatively long line 30, the inclination of the lines 30, 31 indicating the relative course of the intruder and hence the direction, i.e., left or right, in which the equipped aircraft should turn to avoid collision. The bearing # is determined by a pair of spaced receiving aerials (spacing D) producing from each echo pulse a pair of pulses of amplitude K sin # and K cos #, where # = 2 # D /# sin #, the   signals from one pair of aerials 5a, 6a, Figs. 10 and 11, with a spacing D = 10# being used for objects within a range of 25,000 ft., and the signals from another pair of aerials 35, 36 with a spacing D = 30# being used for objects of range 25,000-40,000 ft., the changeover being effected automatically during each range scan by gates 72, 73, Fig. 11. Ambiguity due to the lobe structure of the spaced aerials is resolved by storing the angle data in a store comprising a separate section for each 6 degrees sub-sector of the 90 degrees sector, the input to the store sections being controlled by a commutator synchronized with the aerial scan (see data handling below). As shown in Fig. 11 the signals from the pair of aerials 5a, 6a are applied through switches 74c, 74b to mixers 58, 59 where they are heterodyned with a local oscillation applied through the gate 72 to give a pair of I.F. signals which are applied (1) directly to a synchronous detector 64 and (2) in phase quadrature to another synchronous detector 66 to give corresponding D.C. pulse outputs of amplitude X=K cos # and Y=K sin #. A variable phaseshifter 56 controlled by a servo 57 from a gyro is inserted in one RF channel to compensate for changes in the effective value of D when the aircraft banks. The signals from the other pair of aerials 35, 36 are applied to the detectors 64, 66 in a corresponding manner, the switches 74, 74a ... 74c being controlled by the radar aerial to select echoes within the forward 90 degrees sector. Data handling and display, Figs. 9, 10, 21, 22, 23, 24.-Each of the output pulses Y=K sin #, X = K cos # from the receiver 37, Fig. 10, is quantised into a parallel three-bit word in a corresponding digitizer 38, 39 (Fig. 12, not shown, see Group XXXVIII) whose pulse output is filtered and converted in a corresponding pulse filter 40, 41, Fig. 13 (not shown), into a serial four-bit word synchronized with 500 kc/s. clock pulses from a clock control 33, Fig. 12 (not shown), from which the radar sync. pulse is derived by frequency division. Each pair of simultaneous X and Y serial digital signals is then combined in an encoder 42, Figs. 14 and 15 (not shown), to give in an angle storage register 44, Fig. 14 (not shown), a single four-bit serial digital word specifying the magnitude of the angle #, the relationship between the angle code and the signs and relative magnitudes of X and Y being shown in Figs. 21 and 22. The timing of the angle signals relative to the radar sync. pulse, and hence the target range, is determined by applying an output from the encoder 42 to a range register 43, Fig. 15 (not shown), comprising a plurality of range gates opened sequentially by the time displaced outputs from a range counter, Fig. 12 (not shown), fed with clock pulses. The four-bit angle and range words in the registers 44 and 43 are then combined serially in a buffer storage 45, Fig. 16 (not shown), and applied by means of a commutator 48 synchronized with the aerial rotation to the appropriate section of a main storage 46, Fig. 18 (not shown), comprising 15 sections, each section corresponding to a different 6 degrees sub-sector of the 90 degrees collision sector and comprising a circulating store capable of holding three serial 13-bit target words, Fig. 23. The digits T6-T9 and T10-T13 in each target word specify the angle and range respectively, digits T1-T3 specify the nature of the collision threat, i.e., possible, slow or fast and digits T4 and T5 specify the direction to turn, i.e., left or right. After the first aerial scan the range and angle data corresponding to each new received echo is compared in a logic circuit 47, Fig. 16 (not shown), with the corresponding data of any old echoes stored in the appropriate section of the main store 46. Assuming the position of a stored echo is represented by the reference cell 24 labelled R in Fig. 7, then if the position of a new echo corresponds with any of the cells in Fig. 7 labelled P or T the resultant action initiated by the logic circuit 47 is denoted by the letters A, B, C and D in the corresponding arrangement of cells in Fig. 24, i.e., if a new echo is in a position A, the old information is held in the store 46, for positions B the old information is held and labelled as a possible threat, for positions C the new information is stored and labelled as a slow threat and for positions D the new information is stored and labelled as a fast threat. For positions B, C and D, the appropriate steering instruction left or right is inserted in positions T4 or T5 of the stored target word. Any echo in the main store 46 having a threat label actuates an aural alarm and is displayed on a sector display, Fig. 9, on a C.R.T. 53 by means of a scope decoder 52, Fig. 19 (not shown), which in response to the corresponding range and angle digital information from the store 46 and analogue angle information from the resolver 51, driven by the aerial servo 49, generates corresponding x and y D.C. deflection voltages, the beam focus and a " spike " generator producing the traces 30, 31, Fig. 9, being also controlled by the decoder 52.