Patent GB-977129-A (GB)
Title: Air navigation information system
IPC: G01S5/00
Filed: 1961-04-28
Abstract: 977,129. Radar; radio signalling. STANDARD TELEPHONES & CABLES Ltd. April 28, 1961 [May 5, 1960], No. 15468/61. Headings H4D and H4L. A radio navigation system comprises a ground station radiating a repeating space scanning signal pattern and in succession a number of indicating signals progressing in phase relative to the scanning signal so that a mobile craft transmits a reply signal in response to one of the indicating signals received within a predetermined time from the incidence on the craft of the scanning signal, the receipt of such a reply signal at the ground station then occasioning the transmission of a response signal for a predetermined period at a constant phase relation to the indicating signals to initiate a communication cycle to the craft, the ground station having means to respond to, and establish communication cycles with a plurality of craft in sequence dependent upon the times of receipt of their reply signals. A first embodiment provides cyclically interrogation of, alternately, the regions defined by an azimuthal sector of predetermined range and angular width whose azimuth is progressively varied, and the regions defined by this range and a slice of predetermined thickness whose altitude is progressively varied. Response by aircraft within a sector or slice results in the successive establishment of an air-to-ground and a ground-to-air communication link which is extended from the ground station to a control centre via a narrow pass-band land line. Connection to aircraft responding in a sector or slice is established in turn according to their range. The system provides aircraft determination of range and azimuth together with ground station determination of range and sector or slice location of responding craft, and communication of this information to the control centre. In a second embodiment each cycle of interrogation involves the simultaneous interrogation of an altitude slice and all azimuth sectors and an aircraft responds to set up the communication link if within both a slice and sector interrogated. At the ground station there is then deduced the position of the craft in azimuth, altitude, and range, from the one response. Ground station equipment for the first embodiment comprises a known VOR (Very high frequency OmniRange) beacon I, Fig. 1, transmitting azimuthal information in the form of a carrier wave modulated at 30 c/s. by a reference signal of phase invariant with azimuth and by a signal of phase relative to that of the reference signal proportional to azimuth, this being obtained by rotation at 1800 r.p.m. of a cardioid radiation pattern. In a so-called " indicated sector interrogation" mode of operation an interrogation signal is radiated once during each revolution of the VOR cardioid from the aerial 20 of a known DME/T (Distance Measuring Equipment for Tacan) responder II. This is in the form of a pulsed signal (designated S#) whose phase relative to the positive-going zero crossing of the cycle of the VOR reference signal in which it occurs is interpreted by an aircraft transponder as defining the azimuth of the sector interrogated, the phase angle being caused to progress by 7¢ degrees with each VOR cycle until the whole 360 degrees of azimuth has been explored. Operation is then changed to a socalled " indicated level interrogation " regime in which a similar pulsed interrogation signal (designated Sh) is radiated once per VOR cardioid revolution, the phase progression per revolution being 2“ degrees in this case to represent discrete increments of 75 metres until the maximum altitude of 12,000 metres has been interrogated. In either mode, however, receipt of a response from an aircraft within 2 ms. of an interrogation (corresponding to a range of 300 km.) results in the suppression of interrogation signals together with the arrest of their phase progression during three VOR cardioid revolutions for the establishment of bothway communication at 750 bauds between aircraft and control centre. If there is more than one response during the 2 ms. interval a repetition interrogation signal is sent out (designated either S#r or Shr) after communication with the first responding aircraft has ceased. When all aircraft replying to an interrogation signal have been linked in this fashion to the control centre the progressing interrogation is resumed. The DME/T interrogation signals (S#, Sh &c.) are encoded by a five channel coder 127 to which pulses are supplied by a distributer 200. This is fed with pulses corresponding to aircraft response signals by the DME/T receiver 21, and with a pulse during each VOR cycle from one of two impulse generators 115 (for sector interrogation) and 116 (for level interrogation). A land line 301 connects the control and ground stations for the transmission of signals from the aircraft direct from the receiver 21 after passage through a decoder 122, and for the transmission of beacon information to the control station, the timing of which is controlled by a narrow pass-band transmission device 300. The 30 c/s. VOR reference signal frequencymodulates a sub-carrier wave which is used to amplitude-modulate the radiated carrier wave. This f.-m. sub-carrier wave is generated by a phonic wheel 16 for supply to the VOR transmitter's modulator and is also supplied to a discriminator 101 whose output forms the 30 c/s. VOR reference signal for timing the radiation of interrogation signals. It is fed to two resolvers 103 and 104 whose rotors are, respectively, coupled to step-by-step motors 105 (7¢ degrees per impulse) and 106 (2“ degrees per impulse). The impulse generators 115 and 116 are supplied, respectively, by the resolvers so that the phase relative to the beginning of a cycle of the VOR reference signal of their pulse outputs (designated I# 1 ) is dictated by the angular position of the step-by-step motors. The routing of these I# 1  pulses both to the motors and to, and within, the distributer 200 is dependent upon the state of a bi-stable circuit 114. Coupled to the step-by-step motors 105 and 106, respectively, are binary coded discs 107 and 108 each bearing an index co-operating with, respectively, reading stations 109 and 110, so that the passage of an index past a reading station results in switching of the bistable circuit 114. Thus, at the completion of sector interrogation throughout 360 degrees of azimuth, during which motor 105 stops, the bi-stable circuit 114 is switched by reading station 109. The appropriate output of 114 (the 1.h. one in Fig. 1) then opens level interrogation AND gates 118 and 120 to route I# 1  pulses from generator 116 to the distributer 200 and the motor 106, respectively, and bars the corresponding sector interrogation AND gates 117 and 121. On the next VOR cycle, therefore, the I# 1  pulse from generator 116 is routed to the cooler 127 to key the DME/T transmitter 22 for the radiation of an Sh interrogation signal. The same I# 1  pulse is routed to the input of a 2 ms. delay line 227 whose output is connected to a further AND gate 226. This gate is blocked only upon the receipt of one or more aircraft response signals during the 2 ms. interval (response signals are fed to the distributer at an input 203 from the DME/T receiver 21). In the absence of such a response the delayed I# 1  pulse is allowed to pass through gates 226 and 120 to step motor 106 through 2“ degrees for progression of the phase of the I# 1  pulse and Sh interrogation signal during the next VOR cycle. If a response is received, however, output from the delay line 227 through the gate 226 is barred and a counter 215 continues the barring of the next two 1f 1  pulses. The response also results in a " proceed to code " signal K being radiated by the DME/T transmitter so initiating air-to-ground transmission which is succeeded by ground-to-air transmission, the two occupying the three VOR cycles in which these three 1f 1  pulses occur. More than one response within the 2 ms. results in the counter 215 counting capacity being increased, for the emission of a repetition interrogation signal Shr after communication with the first responding craft has ceased, and for the barring of gate 226 to suppress phase progression for an appropriate number of VOR cycles. A further counter 312, operating at 200 kc/s., is started upon the transmission of an interrogation signal and is stopped by the first aircraft response signal thereafter to represent the range of the responding craft or is reset after 2 ms. Upon the reception by an aircraft of a proceed-to-code signal K in answer to its own response, the aircraft transponder transmits a message at 750 bauds followed by an "end of air-ground " signal, those signals being sent direct to the land-line 301 from a decoder 122 supplied by the DME/T receiver 21. The end of air-ground signal is then decoded by a decoder 302 which allows the transmission device 300 to initiate transmission on the telephone line 301 of the indicated sector or level from readers associated with the appropriate binary code disc 107 or 108, and then the range reading of the counter 312, these signals being clocked at 750 bauds by a pulse generator 304. The airborne equipment for the first embodiment comprises a conventional VOR receiver III and a DME/T transponder IV. The former supplies to a pulse generator triggering monostable circuit 411 a signal representative of the azimuth and via a phase shifter controlled in accordance with the altitude of the aircraft (in a manner described in Specification 968,097) the received VOR reference signal is supplied through another pulse generator to trigger a second monostable circuit 414. Thus, when the VOR cardioid reaches the azimuth of the craft the monostable circuit 411 is triggered to open a gate 412 for a predetermined period equal to that required for the VOR cardioid to rotate through a further 15 degrees. Similarly at a phase