Patent GB-652709-A (GB)
Title: Improvements in or relating to navigational warning systems employing radio beacons
IPC: G01S3/68
Filed: 1948-04-08
Abstract: 652,709. Radio direction finding and navigation; pulse generating circuits. SPERRY CORPORATION. April 8, 1948, No. 9869. Convention date, April 10, 1947. [Classes 40 (vi) and 40 (vii)] A navigational aid comprises one or more radio beacon transmitters arranged to radiate, either successively or simultaneously, trains of electromagnetic wave energy having frequencies (some of which may be swept over a range) characteristic of the beacon and a receiving system on a mobile craft comprising a rotatable directional antenna coupled to a receiver whose reception frequency is periodically swept through the range of frequencies radiated by the beacon(s) so that outputs are given for the several successive wave trains at successive times. The craft receiver indicator may comprise a cathode-ray tube radially scanned according to the receiver frequency sweep at an angle corresponding to the direction of the antenna maximum sensitivity. The display may be combined with the P.P.I. display of a radar system carried by the craft. The beacon transmitters. The frequency modulation of the beacon transmitter is preferably arranged so that identification letters comprising dots and dashes are displayed on the receiver radial sweep. For example (Fig. 2, not shown) the transmitter may be modulated to radiate a frequency F1 for a time, then change to F2 which is gradually changed to F3, and finally F4. It is shown that if the frequency of receiver sweep is large compared with that of the beacon transmission cycle and the receiver has a suitable bandwidth, the radial display will consist of a dot-dash-dot indication. In an alternative arrangement, frequencies F5, F6, F7 and F8 may be radiated simultaneously, F6 and F7 being spaced so that the bandwidth of the receiver overlaps both and F5 and F8 having greater spacings. This also provides a dot-dash-dot display (Fig. 3, not shown). It is possible by rearrangement of the modulating voltages to produce any desired identification signal. Pulse generating circuits. The circuit arrangements for producing the modulating waveform for the first example above, this being the composite waveform A, B, C, Fig. 10, is shown in Fig. 9. A double triode 60, coupled as a free running multivibrator, produces the waveform 61<SP>1</SP> of Fig. 10. This waveform 61<SP>1</SP> after differentiation (75<SP>1</SP>, Fig. 10) is applied to trigger a flip-flop circuit including valve 70. The output waveform of this circuit (72<SP>1</SP>, Fig. 10) is, after differentiation (waveform 851, Fig. 10), applied to a second flip-flop circuit including valve 80. The output of this circuit (82<SP>1</SP>, Fig. 10) is differentiated and the negative " pip " (66<SP>1</SP>, Fig. 10) fed back to the grid 66<SP>1</SP> of the free running multivibrator valve 60 to initiate a further cycle. An output lead from the grid 72 of valve 70 is applied to a network including resistors 87 and 88 and capacitor 89 which creates a waveform such as shown at 96<SP>1</SP>, Fig. 10, at the grid 96 of mixer valve 90. This waveform comprises a substantially instantaneous rise followed by a slower increase and is mixed in mixer valve 90 with the waveform 82<SP>1</SP>, the average potential of the latter being fixed by the potential applied by switch 102. These waveforms together form parts B and C of the composite waveform and part A is determined by the potential applied to switch 103. The receiver. One form of receiving system comprises a directional antenna 7 rotated in synchronism with the radial trace of the cathode-ray tube display and feeding a mixer 35. Sweep generator 32, the amplitude of whose output is controlled at 33, causes the frequency of local oscillator 31 to sweep over the required frequency range. The mixer 35, to which the output of the local oscillator is applied, feeds I.F. amplifier 36 whose output is fed through amplitude limiter 37, band-pass filter 38, detector 39, to video amplifier 42, the output of which intensity-modulates the radial trace of the indicator. The sweep of the radial trace is synchronized with generator 32 by means not shown. Where the receiver is used in conjunction with a radar system showing on a P.P.I. the surrounding objects, the antenna and display systems may be common to both, Figs. 6 and 7 (not shown).