Patent GB-653504-A (GB)
Title: Improvements in or relating to airway traffic control systems
IPC: G01S13/91
Filed: 1947-05-22
Abstract: 653,504. Radio navigation; pulse circuits. GENERAL RAILWAY SIGNAL CO. May 22, 1947, Nos. 13775 and 13776. Convention dates, May 28, 1946, and April 22, 1947. [Classes 40 (v), 40 (vi) and 40 (vii)] In a system for supervising traffic in an air lane, the lane is divided into contiguous or partially-overlapping blocks each provided with a central ground station A, B, Fig. 1a, and each block is again divided into a number of altitude zones. Each ground station transmits synchronous sets of pulses characteristic of the different altitude zones and the receiver of an aircraft is controlled by its altimeter so as normally to respond only to the pulses characteristic of the zone in which it is flying. The normal range of a ground station is defined by a hemisphere of radius R, its receiver being conditioned only to respond to reply pulses received within a time-interval less than that corresponding to 2R. Special arrangements may, however, be provided to enable an aircraft to " look in," i.e. to receive and respond to pulses characteristic of an adjacent higher or lower zone to seek permission to ascend or descend, and also for enabling a ground station to respond to reply pulses from outside its block from an aircraft seeking to enter the block from the side. In the system described, each ground station transmits four sets of pulses I ... IV, Fig. 2, each set consisting of a synchronizing pulse 50, 56, followed by four pulses 51 ... 54 etc.; the first pulses 51, 57 &c. of each set corresponding to one altitude zone, the second pulses 52, 58, &c. corresponding to a second zone and so on, so that the code signal characteristic of a particular zone consists of four pulses such as 51, 57, 62, 63. These are normally of different widths, different also from the widths of the synchronizing pulses, and the widths may be further varied to signal special conditions such as occupancy or double occupancy of a zone. The pulses 55, 61, &c. represent reply pulses from an aircraft in the first altitude zone. The synchronizing pulses from adjacent block stations A, B are also of different widths to enable an aircraft to identify a block when near the boundaries of two blocks, and the reply signals sent by an aircraft are of a corresponding width. The transmission of pulses from all the ground stations is controlled by a master timer sending out a series of timing pulses which are relayed from block to block by radio link or land line, delay networks being provided at each station except the last so that the synchronizing pulses are transmitted from all the stations at the same time. Ground station, Figs. 4a, 4b. The waveforms appearing at various parts of the apparatus of Figs. 4a, 4b, as denoted by letters A ... BB in small circles, are shown in detail in Figs. 5a-5d. Master-timer signals A comprising single 2 Á sec. pulses separated by seventeen 1 Á sec. pulses (corresponding respectively to synchronizing pulses and seventeen theoretically -possible altitude zones) are passed over lead 73 to a discriminator 74 which selects only the 2 Á sec. pulses, producing output F. These and the pulses A are fed to a trigger circuit 76 which controls pulse-forming circuits 675 of transmitter 71 to pass on the mastertimer signals to the ground station of the next block. The pulses A are also passed through a station-delay circuit 75 to produce a train of pulses B, all of 1 Á sec. duration, delayed by a suitable amount so that the synchronizing pulses and altitude-zone pulses are sent out from all ground stations at the same time. The pulses F control the production of saw-tooth waves G and gates H, one for each of the signal intervals I ... IV, Fig. 2, and also, after 4: 1 division (C), a saw-tooth wave D and a gate E corresponding to the complete signal period I ... IV. Wave-forms D, E, H applied to circuit 82 produce four staggered rectangular gates J ... M, each being triggered by a pulse I obtained by differentiating wave H and each starting a short interval after the preceding one has ended. Circuits 85 fed with waves G, H produce gates N for the synchronizing pulses and O, P, &c. for the altitude-zone pulses. The four pulses of a complete code for one zone have different normal widths 1, 2, 3, 4, respectively, any of which may be changed to a further width 5 or 6 to convey particular information. Thus the code 1-2-3-1 will be sent to an unoccupied zone, 1-2-5-5 to an occupied zone in which the occupying aircraft has a clear passage and 1-2-6-6 when the reception of the 1-2-5-5 code by the aircraft has been checked. The pulses 1 ... 4 (R ... U) are generated by circuits 88 and the synchronizing pulses Q (of A- or B-width) and pulses 5, 6 (V, W) by circuits 90, all under the control of gates J ... M and circuit 86 which is in turn controlled by gates N, O P and the signal-relay circuits 87 and is fed with the delayed pulsetrain B. The signal-relay circuits 87 control the codes sent in accordance with information received over line 41 as to the conditions in the next block and with the occupancy of the various altitude zones as determined by occupancy-detection circuits 84. Line X shows the complete code signal for two altitude zones, comprising synchronizing signals A and codes 1-2-6-6 and 1-2-3-1. Trigger pulses from modulator 91 are fed to circuit 83 to produce gates Y, Z for occupancy signals relative to the different zones and the same trigger pulses produce receiver gate circuits AA to switch receiver 94 on after the end of the longest transmitted pulse and off after a period corresponding to the distance 2R, Fig. 1a. Pulses BB received from an aircraft in an occupied zone (only pulses 1 ... 4 and not pulses 5, 6 are replied to) pass to circuit 84 and if two pulses are received from the same zone during one gate period the code is changed by circuit 87 to warn both aircraft of double occupancy of the same zone in the same block. A pulse-width discriminator 95 may be provided to reduce interference by disturbances. Aircraft equipment, Fig. 7b. Demodulated received signals pass to timer 7041 which is controlled by an altitude switch 705<SP>1</SP> so as to respond normally only to pulses corresponding to the altitude zone in which the aircraft is. Such pulses pass to normal discriminating circuits 706 which separate the various pulses 1 ... 6 to control local relay circuits 7091. Timer 7041 also separates the synchronizing pulses, determines whether they are from an A or B ground station, and determines that reply pulses shall be of the same width. The local relay circuits control a lamp indicator 7081 and the transmission of reply pulses through modulator 707<SP>1</SP> and transmitter 711. The indicator contains push-button switches for controlling the relay circuits to send " ascent," " descent," and " entry " requests, and the relay circuit also contains switches to modify the altitude switch 705<SP>1</SP> so that pulses of a different altitude zone may be received, such pulses being routed to " look-in " discriminators 714. In order that an aircraft may " look in " on a block from outside, the reply pulses are normally transmitted with a delay, provided in modulator 707<SP>1</SP>, the gate 92, Fig. 4a, having a similar delay, and the delay is removed from the aircraft reply pulses when sent from outside a block so that they will arrive before the gate 92 is closed. DETAILED DESCRIPTION OF UNITS. Pulse-width discriminator 74, Fig. 6a. Negative synchronization pulses A applied to the grid of valve 101, which is normally conducting, cut off the valve and condenser 105 charges. The grid of valve 106 is so biassed by potentiometer 109 that the saw-tooth wave produced by the discharge of condenser 105 at the end of the pulse is only sufficient to overcome the bias if the pulse is 1.5 Á secs. or longer. Thus only the 2 Á sec. pulses produce an output (F) in lead 120. 4:1 Divider 77, Fig. 6a (part) (not shown). Each positive pulse F changes over a multivibrator for a time determined by a resistance. The negative gate output is differentiated and the negative pulse corresponding to the leading edge is inverted and amplified in a cathodefollower. Saw-tooth and gate generators 78-81, Fig. 6b (not shown). Each comprises a gas tube fired by positive pulses from lead 120, Fig. 6a, or from divider 77, such pulses discharging a condenser, the subsequent charging of which generates a saw-tooth wave. The condenser is connected across the grid and cathode of a cathode-follower and in circuits 79 and 80 a feed-back circuit is provided from this cathode to the anode of the gas tube to increase linearity. Gate circuits, 82, 85, Fig. 6c. To produce the gate J, wave E applied to lead 172 is differentiated at 182, 183 to produce a pulse on the control grid of the left-hand gas tube 185 which fires for the duration of gate H applied to the anode. The gate J is taken off from a cathodefollower 187. To produce the gate K, a gas tube 190 is fired when the value of the saw-tooth wave D applied to its control grid overcomes a predetermined bias at potentiometer 194. The output of tube 190 is differentiated and applied to the right-hand tube 185 which acts similarly to the left-hand tube 185 for gate J. Gates L, M are produced by similar circuits having different biasses on the cathodes of tubes 190. To produce the gates N, O, P, one circuit 85 is employed for the synchronizing pulses and one for each altitude-zone pulse. Tube 165 is fired at a time determined by the value of saw-tooth wave G which overcomes the bias 204 and the wave produced is differentiated and applied to the cathode of valve 207, the positive pulse cutting off the valve and producing a positive pulse at the anode which is applied through cathode-follower 212 to lead 215. Station delay circuit 75, Fig. 6d. Gas tubes 220, 231 fired by pulses A produce through cathode followers 223, 235 a saw-tooth wave and a voltage gate at the control grid and anode resp