Patent GB-954496-A (GB)
Title: Radio navigation control systems
IPC: G01S13/78
Filed: 1960-07-29
Abstract: 954,496. Radio navigation systems. STANDARD TELEPHONES & CABLES Ltd. July 29, 1960 [July 31, 1959], No. 26433/60. Heading H4D. A radio navigation system includes a beacon producing a rotating directional pattern and radiating a coded signal when the said pattern is in a given position in azimuth, a distant craft co-operating with the beacon and provided with means to obtain a signal having a characteristic dependent upon a co-ordinate of the position of the craft, means to decode the coded signal received from the beacon and means to transmit a response signal at a time when energy received from the rotating directional pattern is in a given condition and the decoded signal has a particular relationship to the signal having a characteristic dependent upon the co-ordinate of the position of the craft, and means at the beacon obtaining the position of the craft from the received response signal. The invention is described as applied to a modified VORTAC system, the ground station of which is shown in Fig. 1. The V.O.R. beacon (inside rectangle I) comprises a V.H.F. transmitter 5, amplitudemodulated by modulator 6 with a 9960 c/s. signal, frequency modulated at 30 c/s. and obtained from a pick-up coil 17 and a co-operating phonic wheel 16. This modulated V.H.F. signal is fed directly to four symmetrically arranged antennµ 1, 2, 3, 4 to produce an omni-directional radiation pattern to provide a reference 30 c/s. signal at the remote aircraft. The modulated V.H.F. signal is also demodulated in demodulator 11 and the resulting pure carrier sine wave is fed to a capacitive goniometer 12 having two capacitive pick-ups 13 and 14 and rotated at 30 revs./sec. in synchronism with the phonic wheel 16. The result is to produce a rotating radiation pattern formed into a cardiod by phase-shifting means 9 and 10, producing, at the remote aircraft, a V.H.F. bearing signal amplitude modulated at 30 c/s. from whence the azimuth of the aircraft can be determined by comparison of the phases of the 30 c/s. frequency and amplitude modulations. The DME ground system comprises, within the rectangle II, a receiver 20 receiving the normal interrogation signal I 1 , at a frequency F 1 , and a transmitter 21 transmitting the normal responder signal I 2  at a frequency F 2 . In addition a discriminator 18 extracts the 30 c/s. component from the pick-up signal and feeds it to a pulse generator 19 which delivers a pulse 60 to the coder 28 every time the 30 c/s. signal passes through zero going positive, this corresponding to the maximum of the radiation pattern being due north. Each pulse received from generator 19 at the coder 28, causes the transmission, via modulator 22 and transmitter 21, of a signal Sp bearing coded information of a number p representative of a co-ordinate of the aircraft (in this case its distance p). Under normal operation (i.e. no response from an aircraft), the number p and therefore the radius of the annulus scanned, is increased by one unit for every rotation of the pattern. When p has increased to the maximum required it reverts to unity and the cycle is repeated. The 30 c/s. signal produced by the discriminator 18 is also fed directly to the coder 28 where it is delayed by an amount equal to the time of travel of a pulse over the distance represented by the number p and is converted into sine and cosine components which are multiplied by a factor Kp generated by the coder and applied to the orthogonal deflection system of the C.R.T. to produce a circular trace in synchronism with the rotating pattern and of a radius representative of the distance p. In the aircraft receiving equipment, Fig. 4, interrogation pulses I 1  are produced by transmitter 72 under the control of generator 74 and are radiated from antenna 71. Responder pulses I 2  are received from the ground beacon station by antenna 71 and are passed via a receiver 73 to a range gate 77 which lets through only those pulses that occur within a certain time of the transmission of the interrogation pulses I 1 , i.e. only those response pulses associated with the aircraft-not those associated with other craft. These filtered pulses are passed to a distance meter 76, together with pulses from generator 74 and the distance of the ground station found therefrom. The distance coded pulses Sp are received and a voltage proportional to the distance p is derived therefrom by a decoder 61 and generator 62. This voltage is then compared in comparator 63 with a voltage similarly proportional to the distance found by the distance meter 76. Equality of these two voltages (i.e. the aircraft to ground station distance is, to one unit of p, equal to the distance coded on signal Sp), results in a signal being sent to a bi-stable circuit 63<SP>1</SP> causing it to emit a constant voltage level which is fed to one input of an AND gate 64. The bearing 30 c/s. signal produced by passing the received amplitude modulated V.O.R. bearing signal through a low-pass (30 c/s.) filter 55 (it being compared at meter 56 with the reference 30 c/s. signal from F.M. discriminator 54, to give the aircraft's bearing), is passed to a pulse generator 60, to produce a series of pulses synchronized with the passages of the radiation pattern maximum through the aircraft. These pulses are fed to the second input of AND gate 64. The production of an output pulse signal i 3  from the AND gate therefore indicates that:- (a) The distance of the aircraft from the ground station equals the distance coded on the signal Sp. (b) The azimuth of the aircraft is the azimuth of the maximum of the radiation pattern at the time of the occurrence of the signal. The pulse i 3  is fed to a coder 66 via a sorting arrangement 65, wherein it is coded to render it distinguishable at the ground station, and with altitude information from altimeter 67. The thin coded pulse is fed to the modulator 75 to cause the transmission of a coded pulse signal i 3  to the ground station where it is received and decoded in decoder 24, Fig. 1, to give a pulse I 3  which is used to brighten up the trace on the C.R.T. 26. The C.R.T. thus shows the position of the aircraft relative to the ground station. The C.R.T. will similarly show the positions of a plurality of aircraft provided with the necessary equipment and at the same distance. Pulse i 3  is also fed to the coder 28 to keep the value of p constant and also causes the transmission of a second response signal I 4  which is received at the aircraft. Other aircraft co-operating with the ground station also cause the generation thereby of second response signals I 4  and these are also picked up by the aircraft being considered. All of the received I 4  signals are passed directly to one input of sorting arrangement 65 (Fig. 4), whilst the aircraft's own I 4  signal is separated by range gate 77 and is passed to a second input of the sorting arrangement. The sorting arrangement 65 triggers a cycle of coded information, firstly air-ground, then ground-air, between the ground station and each of the aircraft at a distance p successively in the order of their azimuth bearings from the North. Circuit details. Coder 28, Fig. 2.-This comprises a nine-stage binary counter 280, increasing its count by one digit each time a pulse i 0  is received by the first bi-stable stage 2801, from the pulse generator 19 via gate 285. The outputs of the nine stages are fed to a distributer 281 to provide a coded signal Sp representing the distance p being scanned. The coded signal Sp is fed to modulator 22, Fig. 1, and to a decoder 283 which produces a voltage proportional to the distance p which is fed to two sine and cosine resolvers in the sweep generator 27. The 30 c/s. reference signal # 0  from the discriminator 18 is applied to a variable delay network comprising a capacitor 2820 and five resistors 2821-2825, each having double the resistance of the previous one and short-circuited by contacts of relays 2831-2835. Each relay is controlled by one of six series-coupled bi-stable circuits 2826-2830, the first of which is controlled by pulses from the fourth bi-stable stage of counter 280. Thus each time the value of p increases sixteen times a signal is fed to circuit 2826 and one of the relays will be energized opening the corresponding contact across the resistance and introducing in the path of the # 0  signal a resistance producing a CR delay equivalent to the propogation time over the distance p. The thus phase-shifted 30 c/s. signal is applied to the two resolvers which produce orthogonal sine and cosine deflection signals for the display tube 26. The transmission by the aircraft of an I 3  signal produces an i 3  pulse from selector 24 and this closes the gate 285 to the next i 0  pulse such that the value of p remains constant. The following i 0  pulse is delayed in circuit 286 by the duration of the i 0  pulse and then reopens the gate 285 to let the next i 0  pulse through. If it is required to change the value of p quickly pulses may be fed to the counter 280 from an auxiliary generator 29. Sorting arrangement, Fig. 6.-This sorts out the various I 4  signals into those due to other craft and those due to the aircraft's own I 3  signal, and initiates and controls the various coded communications. Two cases will be considered:- (a) When the first i 4  pulse received is that due to the aircraft's own I 3  signal. (b) When the first i 4  pulse received is due to another craft. (a) The i 4  pulse due to the aircraft's own transmission (referred to hereinafter as the i 4 <SP>1</SP> pulse) is separated out from the set of i 4  pulses received in means 651, and is passed to one input of an inhibitor gate 653, the complete set of i 4  pulses, including i 4 <SP>1</SP>, being fed to the second input thereof, to give a pulse i 5  for each i 4  pulse except i 4 <SP>1</SP>. Pulse i 4 <SP>1</SP> is fed via open gates 654 and 655, to the coder 66 to initiate the