Patent GB-978215-A (GB)
Title: Aircraft collision warning systems
IPC: G01S13/933
Filed: 1961-05-01
Abstract: 978,215. Radio navigation; transponder apparatus. SPERRY RAND CORPORATION. May 1, 1961 [May 31, 1960], No. 15624/61. Headings H4D and H4L. Collision warning apparatus in a craft comprises means deriving a signal (signal R) representing the range R between the craft and another craft and two signals representing respectively the component relative velocity of the two craft along the line of sight therebetween (signal L) and the component relative velocity perpendicular to the line of sight (signal P), means responsive to signals R and L to produce an output signal when the time taken, at the existing speeds and courses, to reach a predetermined minimum nearest approach distance along the line of sight is smaller than a predetermined time, and means responsive to the output signal and signal P to produce an alerting signal when signal P falls below a predetermined value. All aircraft carry the apparatus comprised in Figs. 1a, 1b, 1c. Considering two aircraft, " A " and "B," with " A " interrogating " B," " A " includes the apparatus referenced 1 and " B " includes responding apparatus 2. Within each repetition interval defined by oscillator 3 and generator 4, " A " transmits on a frequency f1 a pair of pulses, Fig. 2A, with a time separation therebetween controlled by a barometric altimeter 8 so as to represent the altitude h a  of " A." The responding apparatus on " B " is identical with that on " A " and for explanatory purposes the responding apparatus on "A" will be described: the received pulse pair Fig. 2B after detection is applied to a normally conducting gate 15 which together with a circuit 16 eliminates response during a time interval b following each received pulse so as to remove ground-reflected signals. Circuit 17 introduces a time delay proportional to the barometric altitude h b  of " B " minus a tolerance-factor t plus the delay b. The first pulse of the delayed pulse pair is represented by Fig. 2C and actuates gate generators 18, 19 which produce pulses of respective widths 2t, t, Figs. 2E, 2D. If " A " is within the altitude band 2t of " B " there is an output from " and " gate 20 and this output, together with that from generator 19 is applied to " and " gate 22. A generator 21 prevents the transponder of " B " from replying to other than the first of interrogations received from a plurality of interrogating craft. The output pulse from gate 20 is simultaneously applied via OR gate 28 for transmission on a frequency f 2 ; this pulse when received at " A " is utilized for range determination. The remainder of the reply signal of " B " consists of three pulses Fig. 2G the pulse spacings representing respectively the sense of the altitude difference between " A " and " B " and the northerly and easterly components of the velocity of " B "; a separation of h 0  + b Fig. 2F indicates that " B " is lower than " A " and a separation of b indicates the contrary. The signal received at " A " Fig. 2H is applied to receive 37 the sensitivity of which is controlled by the second interrogating pulse such that the sensitivity thereafter increases with time until stopped by the first pulse of the reply signal from "B"; thus side-lobe responses are suppressed. Circuit 39 prevents response to ground-reflected pulses for a time b following each received pulse, and a generator 43, actuated by the second interrogating pulse, provides for the rejection of pulses at the output of an " and " gate 40 which arrive later than the known maximum time for " B " to respond. The output pulses of gate 42 are applied to a distributer 41 which is reset at the beginning of every interrogation cycle by the timing pulses on line 72: if a fifth input pulse is received by the distributer it indicates that one or more of the input pulses are not valid replied by " B " and the fifth pulse is utilized to terminate an invalid collision-risk computation. Assuming that a normal four-pulse reply is received from " B," range R is determined by applying the range reference pulse Fig. 2A on line 45 to trigger a sawtooth generator 46, the sawtooth waveform is terminated by the first reply pulse, on line I, and the terminal value of the sawtooth is maintained at the output of generator 46 until reset by the next following pulse on line 72. Voltages related to the northerly and easterly components of the velocity of " B," V N  and V B , are produced in a similar manner by generators 47, 48. An appropriate " go up " or " go down " signal is produced on line 55 or 56. For the collisionrisk computation the additional quantities required are V A , the airspeed of "A," α, the heading of "A," and #, the true bearing of " B " with respect to " A." A voltage proportional to V A  is obtained from sensor 59 and resolved into components V NA  and V EA  by means 33; the train angle of the aerial is obtained from the rotating aerial drive and is combined with heading α from compass 61 in mechanical summator 62, along with a correction term # c , to form true bearing #: correction term # c  compensates for aircraft pitch and roll as measured by gyroscope 73. The collision - risk computation means includes gauged resolvers 66; a pair of outputs representing V NR  cos # and V ER  sin # are applied to summator 69 and the other pair, representing V NR  sin # and V ER  cos 0 are applied to summator 70. Summator 69 output represents V R , the relative velocity along the line of sight, and that of summator 70 represents V#, the relative velocity along the perpendicular thereto. A risk of collision is deemed to exist if V R  is less than a predetermined value V T , provided by a source 85, the computed time remaining to closest approach between the aircraft is less than a predetermined value t W , as explained in the Specification, and the aircraft are in the same altitude band. The Specification includes details concerning the second criterion and the corresponding computation utilizes an averaging process utilizing capacitors 75, 76, 77. The pilot's indicator is shown in unit 113 and vertical manourvre only is envisaged for collision avoidance. Pulse lengtheners 121, 122 provide for the deactivation of the transponder receiver on a given aircraft during the transmission of interrogation signals.