Patent GB-1464761-A (GB)
Title: Vor receiver
IPC: G01S1/46
Filed: 1974-04-05
Abstract: 1464761 Radio navigation INTERNATIONAL STANDARD ELECTRIC CORP 5 April 1974 [25 April 1973] 15258/74 Heading H4D  A VOR receiver is adapted for receiving signals at carrier and sideband frequencies from a DVOR radio beacon and for determining azimuth by comparing the directionally-dependent phase of one 30 Hz signal with that of another, which serves as a reference. When the receiver receives signals radiated by the alternating- sideband (ASB) method from a DVOR beacon aerial array comprising n radiators arranged on a circle (there being also a central aerial for radiating a reference signal), a sequence of pulses of 9960 Hz of repetition frequency 30 Hz for the whole sequence is obtained, by demodulation of one only of two sidebands. A phase meter measures the phases of all the 9960Hz signals for each equivalent aerial rotation in order, beginning the signal received from the first radiator of the array, and using the phase of a locally generated signal as a reference. A computer determines the azimuth from the measured phase values by Fourier integration. As described, the beacon array (Fig. 2, not shown), has 39 omnidirectional aerials, nos. 0 to 38, arranged equidistantly around the circle of radius 8#/#, and an additional omnidirectional aerial at the centre. The radius to radiator no. 0 is directed to true North. A transmitter generates a reference signal of carrier frequency f Hz amplitude modulated at 30 Hz, which is radiated from the central aerial. The transmitter also generates two sequences of pulses with half-sine-wave shaped envelopes, one of which has the frequency (f+9960)Hz and the other the frequency (f-9960)Hz, the RF oscillations in both being coherent. The pulses are switched successively to the 39 omnidirectional aerials, by means of commutators. The pulses for the lower sideband are exactly time staggered with respect to the pulses for the upper sideband, Fig. 1 showing the radiation sequence. The duration of each pulse is 1/(30 x 39) second, and energization of all radiators in turn takes 1/30 second, equivalent to the rotational frequency of 30 Hz. It is arranged that the zero crossing of the 30 Hz reference signal from positive to negative values occurs exactly in the middle of the upper sideband pulse as it is radiated by the aerial no. O. Assuming that the carrier signal frequency is 110 MHz, said signal with sideband frequency signals are fed from a receiving aerial 50, Fig. 4, to a first mixer 51, where mixing with a 120 MHz signal from an oscillator 52, gives a first I.F. signal with carrier frequency 10 MHz. After amplification, this signal is fed to a second mixer 54, whose output of 100 kHz carrier signal and both sideband signals is applied to a second IF amplifier 55. A bandpass filter 56 for the 100 kHz signal provides one input to a phase-angle bridge 57, which also receives input from a 100 kHz crystal oscillator 58. Output from the bridge 57 is delivered to a 9À9 MHz voltage controlled oscillator 59, which provides the second input for the mixer 54. The signals from the amplifier 55 are applied both to conventional processing apparatus and to apparatus according to the invention. The former comprises an AM detector 60 which feeds a 9960 Hz bandpass filter 61 and a 30 Hz low-pass filter 64. The filter 61 is followed by an FM detector 62 and a selector 75, which provides a 30 Hz signal with directiondependent phase. This signal is compared in a phase meter 63 with the output signal from the filter 64, the meter provides an output which may be fed via a change over switch 73 to an azimuthal indicator 74. The signals from the amplifier 55 are also applied to a high-pass filter 67, which passes only those portions having carrier and upper sideband frequencies. An AM detector 68 follows, and a 9960Hz band-pass filter 69. At the output of this filter pulses with half-sine-wave shaped envelopes are developed, the frequency within these pulses being 9960Hz. A counter 66 with 39 steps is fed by the filter 69 through a rectifier 76. It is set to zero at the negative-going zero crossing of the 30Hz reference signal by a pulse derived from the output signal of the filter 64 by a differentiator 65. From zero, the counter 66 advances by one step for each pulse appearing at the output of the filter 69. In addition, a phase meter 70 measures the phase of the 9960Hz signal of each pulse with respect to the phase of a signal from a reference oscillator 72, which is locked to the phase of the 9960Hz signal of the pulse from the radiator no. O (Fig. 2, not shown). Under the control of the counter 66, the measured phase values, together with the numbers of the radiators, are fed to a computer 71, which performs Fourier integrations to determine the azimuth value. Output from the computer 71 is fed via the changeover switch 73 to the indicator 74. Controlled by the counter 66, the changeover switch 73 moves to the position shown in Fig. 4 only if said counter steadily advances step by step, which is the case if signals from an ASB beacon are being received. Otherwise, the switch 73 connects the indicator 74 to the phase meter 63.