Patent GB-700602-A (GB)
Title: Radio navigation system
IPC: G01S13/91
Filed: 1949-01-14
Abstract: 700,602. Radio - navigation. STANDARD TELEPHONES & CABLES, Ltd. Jan. 14, 1949 [April 17, 1948], No. 1085/49. Class 40 (7) Relates to overlapping-beam equi-signal beacons for defining an azimuthal course line of the type in which a carrier modulated with a first frequency F1 is applied in parallel to one of two horizontal spaced non-directive aerials a directly and b through a 90 degree phase delay and is applied in parallel to the other aerial through 90 degrees and 180 degrees phase delays so that the resultant signals applied to the two aerials are in phase quadrature thereby producing a radiation pattern modulated at F1 of substantially cardioid shape, a reverse cardioid radiation pattern modulated at F2 being produced simultaneously by modulating the carrier with a second frequency F2 and applying it in a similar manner to the same aerials with the direct and 180 degree phase delay connections interchanged. A development of the above type of beacon is disclosed in Specification 700,601 in which the carrier of the above system is regarded as a sub-carrier and the modulated sub-carrier signals applied to the aerials are modulated on to higher-frequency carriers before being transmitted so that the effective directive patterns produced are defined by the sub-carrier frequency. The present invention represents a further development in which the sub-carrier comprises a pulse train instead of a C.W. oscillation, the corresponding C.W. subcarrier being produced in the craft receiver by applying the detected video pulses to a circuit tuned to the pulse recurrence frequency ; alternatively the selective circuit may be tuned to a harmonic of the pulse recurrence frequency (P.R.F.) which then becomes the effective subcarrier frequency and the delays produced by the pulse delay circuits in the beacon must correspond to the phase delays of 90 degrees and 180 degrees at the effective sub-carrier frequency. A pulsed overlapping-beam glide-path beacon is also described and an aircraft receiver is described for determining the horizontal and vertical deviations of the aircraft from the landing path defined by the pulse signals radiated by the two beacons. The radiated pulse signals may also be received at a ground receiver after reflection or retransmission by the aircraft to give an indication of the range of the aircraft and its deviation from the landing path for purposes of monitoring, traffic control and for emergency " talk-down " procedure. Azimuth course beacon.-As shown in Fig. 1 the beacon comprises a pair of aerials 1 and 2 spaced symmetrically on either side of the approach path 3 the spacing between the aerials being a fraction of the effective sub-carrier wavelength corresponding to a harmonic of the P.R.F. of a pulse train generated at 16 under the control of trigger circuit 17 the pulse width being so chosen that the pulse train contains a relatively large component at said harmonic frequency. The output from pulse generator 16 is applied in parallel to three modulators 13, 14 and 15 and the modulation frequencies F1 and F2 are produced by heterodyning the outputs from oscillators 4 and 5 in mixer 6 and selecting the sum and difference components F1 and F2 by filters 7 and 8, the frequencies F1 and F2 being combined in a tone-frequency bridge 9 which is so coupled to the modulators that frequency F1 is applied to modulators 13 and 14 and frequency F2 is applied to modulators 13 and 15. Modulator 13 is coupled via a 90 degrees delay 18 to the lower terminal of a bridge 21 which includes a 180 degrees delay 23 in one arm and modulators 14 and 15 are coupled directly and via 180 degrees delay 23a to the upper terminal 22, the outputs from the left and right hand bridge terminals being applied to modulate separate high-frequency carrier sources 19 and 20 coupled to aerials 1 and 2. The delays 18, 23 and 23a correspond to the effective sub-carrier frequency so that the F1 and F2 modulated pulses are applied to the aerials with the correct phasing to produce reversed overlapping sub-carrier frequency patterns modulated respectively by F1 and F2. If the outputs from the modulators are of unequal amplitude the shape of the sub-carrier frequency patterns and hence the sharpness of the course line will be changed, but the direction of the course will be unchanged. The glide-path beacon, Fig. 2 (not shown), is similar to that disclosed in Specification 590,491 except that the carrier source is pulsed instead of being continuous, the pulsed carrier being separately modulated by frequencies F3 and F4, the F3 modulated signal being applied in phase to two vertically spaced aerials and the F4 modulated signals being applied in anti-phase to the two aerials. In order to avoid interference between the two sets of beacon signals, the P.R.F.s of the two beacons must not be harmonically related. At the ground receiver, Fig. 3, the beacon signals reflected or retransmitted by the aircraft and received by aerial 35 are applied through frequency changer 3b and I.F. amplifier 37 to a detector 38, the output video pulses being applied to modulate the vertical time base of a range indicator cathode-ray tube 39 synchronized with the glide-path beacon P.R.F. control so that only the glide-path echoes will be stationary. In order to indicate the deviation of any particular aircraft from the landing path, a cursor 42 which is aligned with the corresponding echo is ganged with variable delay gating pulse generators 44 and 43 synchronized respectively with the azimuth and glide-path beacon P.R.F. controls, the generated gating pulses being applied to gating amplifiers 4b and 55 to which the video pulses from detector 38 are applied and the output from amplifier 46 is applied through a filter 47 tuned to the effective sub-carrier frequency to demodulator 48 the modulation signals Fl and F2 being selected by filters 49 and 50 and applied through rectifiers 51 and 52 to control the horizontal deviation pointer 53 of the cross-pointer indicator 54 ; the vertical deviation pointer 61 is controlled in a similar manner by modulation frequencies F3 and F4 derived from the gated glide-path beacon pulse signal output from 55 but in this case no sub-carrier frequency filter is required. Alternatively the deviations may be displayed on a back-to-back type A cathode-ray tube range display. In the airborne receiver (not shown) the gate pulse generators 43 and 44, range scope 39 and video amplifier 55 are omitted the input to demodulator 56 being taken from amplifier 46. Specification 590,491 also is referred to. Figs. 2 and 3 (Cancelled) (not shown) of the Specification as open to inspection under Sect. 91 illustrate respectively the time relations between the component pulses in the azimuth beacon and a vectorial analysis of these pulses. This subjectmatter does not appear in the Specification as accepted.