Patent GB-661394-A (GB)
Title: Landing information radio receiving system
IPC: G01S1/02
Filed: 1949-01-07
Abstract: 661,394. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. Jan. 7, 1949 [April 17, 1948], No. 481/49. Class 40 (vii). In a pulse direction-finding system comprising two spaced aerials, signal energies obtained from the two aerials are combined to produce sum and difference components and these components or two further components derived therefrom are modulated by two different frequencies and combined, the amplitude ratio of the resultant modulation components being indicative ot the incoming direction of the received pulses. The invention is described as applied to a ground pulse-radar system for determining the range and horizontal and vertical deviations of an aircraft from a predetermined landing path such information being then relayed to the aircraft; the aircraft may reflect or re-transmit the incident pulses. Determining horizontal deviation, Figs. 2-7. As shown in Fig. 4, the two aerials A and B are spaced apart symmetrically on either side of the runway and the received signals are demodulated at 9 and 9A, the resultant video pulses being applied through amplifiers 10 and 10A to tuned amplifiers 11 and 12, which produce CW signals of " sub-carrier frequency " equal to the pulse recurrence frequency or a multiple thereof which are combined in phase and anti-phase in bridge 42 to give sum and difference components at terminals 44 and 45 respectively. The sum component is phase delayed by 90 degrees at 18 and combined with the difference component in phase and antiphase in the bridge 43 giving two resultant signals at terminals 46 and 47, whose amplitude ratio depends on the horizontal angular deviation of the aircraft from the perpendicular bisector of the aerials, being unity for zero deviation, and this ratio is measured by modulating the two signals separately at 48 and 49 with frequencies F1 and F2 respectively, combining the modulation products in bridge 50, demodulating at 32, separating the modulation signals F1 and F2 by filters 33 and 34 and applying them after rectification to a ratiometer type meter 35. Although the resultant indication is substantially independent of the relative amplitude of the pulses received at A and B, the amplifiers 11 and 12 are preferably provided with A.G.C. operating from a common reference level to maintain equality of outputs. The pulses received by aerial A may be applied to produce an off-centre P.P.I. display 36 whose sweep generator 37 is triggered from the radar transmitter and in order to ensure that the deviation displayed on meter 35 corresponds to a particular aircraft, the video amplifiers 10 and 10A are gated by a gate generator 41 coupled to a cursor 39 which is manually aligned with the response 40 of the required aircraft. In order to deal with several aircraft a number of systems may be provided operating with a common pair of aerials. In a modification, Fig. 2, the bridge 42 is replaced by a hybrid coil 13 and the 90 degree-delayed sum component from 18 is modulated at 19 with frequencies F1 and F2. The difference component from terminal 17 is applied to a balanced modulator 27 together with frequencies F1 and F2, the F1 frequencies applied to modulators 27 and 19 being in phase and the F2 frequencies being in anti-phase, and the sideband components from balanced modulator 27 and the carrier and side-bands from modulator 19 are combined in hybrid coil 30 the combined modulation products being then treated as in the Fig. 4 embodiment. The two frequencies F1 and F2 are produced by applying the outputs from oscillators 21 and 22 to a mixer 23 to give sum and difference frequencies F1 and F2 which are separated by filters 24 and 25. A vectorial analysis of the operation of Figs. 2 and 4 is given respectively in Figs. 3 and 5 (not shown). Figs. 6 (not shown), and Fig. 7 (part shown), illustrate modifications of Figs. 4 and 2 respectively in which the video pulses are converted to a C.W. sub-carrier frequency after combination and modulation. The side-band generator of Fig. 7 corresponding to balanced modulator 27 of Fig. 2, comprises two separate pulse modulators 67 and 68 with the outputs combined in parallel, the " difference " pulse components being applied to modulators 67 and 68 directly and via a 180 degrees delay 66 and the modulating frequencies F1 and F2 being applied to the two modulators in phase with the corresponding frequencies F1 and F2 applied to the " sum " component modulator (not shown). In these two embodiments the electrical value of the pulse delays is the corresponding phase delay at the sub-carrier frequency and a 180 degree delay may be effected by a polarity reversal. In all the above embodiments the horizontal discrimination of the system depends upon the spacing of the two aerials in terms of the wavelength # of the sub-carrier frequency, a suitable value being slightly less than #/2. Determining vertical deviation, Figs. 8 and 9. As shown in Fig. 8, two aerials 6 and 8 located at one side of the runway are spaced vertically, the upper aerial having a vertical polar diagram with a null in the direction of the glide slope and the lower aerial having a vertical polar diagram with a maximum in this direction, Fig. 9 (not shown), and the received signals are combined in phase and anti-phase in a carrier frequency bridge 71 to give sum and difference components whose amplitude ratio depends upon the vertical angular deviation of the aircraft from the glide slope, being unity for zero deviation, and this ratio is measured by modulating the two components separately at 74 and 75 with frequencies F1 and F2 respectively, combining the modulation products in bridge 76, extracting the modulated video pulses in receiver 77, demodulating the pulses at 79, separating the modulation frequencies F1 and F2 by filters 80 and 81 and applying them after rectification to a ratiometer meter 82. In order to select received pulse signals from an aircraft at a particular range the receiver 77 is gated by a gate generator 83 as described above. In this embodiment the shape of the vertical polar diagrams of the two aerials and hence the vertical discrimination of the systems depends upon the vertical height of the aerials in terms of the carrier wavelength. The lower aerial 6 may constitute one of the aerials of the horizontal deviation system, Fig. 1 (not shown), and one of the aerials of the horizontal or vertical deviation systems may be used as the radar transmitting aerial if a P.P.I. display is not required. The Specification as open to inspection under Sect. 91 states that the horizontal discrimination may be increased by generating several harmonics of the sub-carrier in addition to the fundamental and that if it is desired to use a high sub-carrier frequency the radar pulses may be modulated with such a frequency, the pulse duration being long enough to include several cycles of the frequency. This subjectmatter does not appear in the Specification as accepted.