Patent GB-1101326-A (GB)
Title: Radar system for controlling the flight position of a plurality of aircraft in formation flight
IPC: G01S13/76
Filed: 1965-09-08
Abstract: 1,101,326. Radio-navigation; aerials. GENERAL DYNAMICS CORPORATION. 8 Sept., 1965 [18 Sept., 1964; 30 Sept., 1964], No. 38448/65. Headings H4A and H4D. Radar system.-In a radar system for controlling the flight position of a plurality of aircraft 17, 18, Fig. 2 (not shown), relative to a control position 17, each aircraft has a transponder, and the transmitter of the control (master) aircrafts (17) transponder transmits via a rotating (10 r.p.s.) directional beam, an interrogation pulse signal to a different aircraft each revolution, said aircraft retransmitting said pulses omni-directionally to the control aircraft, which in turn retransmits it such that a pulse train is set up having a repetition rate representing the range of the aircraft and available on each aircraft for range use in PPI display, Fig. 3 (not shown). On one revolution, when not interrogating an aircraft the control transmitter transmits a variable frequency pulse train, representing by its repetition frequency the azimuth of the control beam, whereby a signal may be set up in each aircraft synchronous with the control beam's rotation so as to provide azimuth information for the PPI displays and to determine which aircraft should apply to the interrogation pulse in any particular revolution. In the control aircraft's transponder, Fig. 4, the interrogation pulses are produced by free running multivibrator 21 at rate of 1200 p.p.s. or one pulse per three degrees rotation. During the first revolution however and each subsequent tenth revolution the interrogation pulses cease to be transmitted. This is achieved by a 10 : 1 reduction gear 31 coupled to the antenna motor 25 and controlling a switch 30 which shuts off the multivibrator 21. The switch simultaneously lets through to OR gate 22 and transmitter 23 the output from a variable frequency pulse generator 29 coupled to the antenna motor, and producing pulses having a repetition rate changing with the antenna azimuth by rising from 5 Kpps. for 0 degree up to 10 Kpps. for 60 degrees, and repeating this sweep for ever subsequent 60 degrees. At the start of the next antenna revolution the multivibrator 21 restarts and the switch 30 closes to the output of generator 29. An interrogation pulse is received and re-transmitted by the slave aircraft allotted to the particular antenna revolution with an added delay of td equal to the maximum expected propogation time. The retransmitted pulse is received and fed to receiver 26 which is automatically blanked for 10Á sec. to exclude noise pulses. The pulse is fed from the receiver 26 to the OR gate 22 with the addition of another td delay for retransmission to the slave aircraft. The pulse train thus set up has a repetition frequency of where tr is the propogation period. It is seen that even at zero range with tr = 0, a pulse train free from overlapping of frequency 1/2td will be produced. The frequency sweep from zero range to maximum range is, in the described embodiment 50 Kpps. to 100 Kpps. It may be desired to produce artificial response 20a, 20b, Fig. 3, on the PPI displays and for this purpose during an unused antenna revolution, a pulse train having repetition frequency representing the desired range co-ordinates on the PPI is produced by generator 39 and transmitted in a burst having a timing representing the desired azimuth co-ordinate. The desired timing is determined by phase shifting at 34 a 10 c/s. signal synchronous with the antenna rotation and using the phase shifted signal to operate a pulse generator 37 which in turn operates switch 38 to pass the output of generator 39. The transponders on the slave aircraft are the same as described, except for the removal of generators 21 and 29 and the use of an omnidirectional antenna so that a response by a slave aircraft is radiated to and received by all the other aircraft for display. On each aircraft the receiver output is fed to the processing display circuitry of Fig. 6 (not shown), wherein the range pulse trains, having repetition frequencies of 50 Kpps. to 100 Kpps. are passed to line 43 and the azimuth synchronizing pulse trains, having repetition frequencies varying from 5 Kpps. to 10 Kpps. are passed to line 44. The range pulse trains are fed to a six stage 25 counting binary counter 55 such that the last stage of the counter is in one state for a count of sixteen to produce a pulse of duration inversely proportional to the repetition frequency of the pulse train. This pulse operates a pulse generator 60 at 3À2 Mc/s. to produce a number of cycles proportional to the range of the aircraft. This number is counted in counter 58 to produce an analogue output at 61 which is fed to one of the inputs of analogue multipliers 62 and 63. A digital output is available at 61a. The output of counter 55 is used to intensity modulate the C.R.T. beam at 75. To ensure that a random pulse e.g. noise does not start counter 55, it is present to - 3, such that the pulse train must comprise at least three pulses for a positive count to be possible. Both counters are automatically reset to zero at the end of their counts. The frequency varying azimuth synchronizing signals are separated out by a decoder, and a 10 c/s. oscillator is synchronized with the control antenna rotation by means of a control loop comprising a phase detector 46 determining the phase between the synchronizing signals as received directly and after delay in a variable delay pulse generator 47, a sawtooth generator 48 controlling the delay of generator 47 and a 10 countering counter 49, receiving the output of oscillator 50 and passing a pulse to generator 48 on every tenth cycle of the oscillator output to correspond to the one in ten azimuth signal transmitting control antenna revolutions, the frequency of the oscillator being controlled by the output of the phase detector. Each stage in counter 49 has its output brought out to a time block selector tap, whereby a signal derived from a selected tap determines in which revolution of the control antenna the particular slave station shall reply to the interrogation signals. The derived signal is applied to OR gate 22, Fig. 5 (not shown). Signals, representing the sine and cosine of the scanning angle of the control antenna are produced by oscillator 50 and are fed to analogue multipliers 62 and 63. Since the other inputs of the analogue multipliers are fed with range representative signals the outputs of the multipliers are fed via amplifiers to the horizontal and vertical C.R.T. deflection plates respectively to ensure that ranges greater than that required are not displayed, a pulse monitor 56 produces a signal wherever the space between range pulses is greater than 20 Ás, which signal resets the counters 55 and 58 and blanks out the C.R.T. by a signal on the grid thereof. Transistorized embodiments of the decoder 45 phase detector 46, generator 48 are given, Fig. 7 (not shown), the decoder and generator 47 using variable period Schmitt trigger circuits. Antenna.-An antenna suitable for mounting on the outside of an aircraft with little aerodynamic resistance and changeable from a directional to an omnidirectional mode is described, Fig. 8. It comprises three circular metal plates 210, 211 and 212, sandwiching two polyfoam dielectric layers 230 and 232. Plates 211 and 212, and layers 230 and 232 have central holes whereby a quarter wavelength antenna 216 formed from the bared inner conductor of a coaxial cable 215, may pass vertically from one dielectric layer to the other. The upper layer 232 has no reflecting media therein and so has an omnidrectional pattern, whilst the lower layer 230 has a parabolic reflector 217 embedded in the rein such that with the antenna 216 in the lower position, a directive lobe is produced. The antenna is moved vertically by means of handle 228. The rotating coaxial cable 215 is telescopic into a fixed coaxial cable 227. The antenna is rotated by means of a motor 222 engaging a toothed antenna base plate 220. Slots 219 are cut in the rims of plates 211 and 212 quarter wavelength deep, to cut down side lobes.