Patent GB-579346-A (GB)
Title: An improved radio aid to navigation
IPC: G01S1/02
Filed: 1944-03-01
Abstract: 579,346. Directional wireless systems. MITCHELL, H. T., and KILVINGTON, T. March 1, 1944, No. 3865. [Classes 40 (iii) and 40 (v)] In a navigational system in which the aircraft or other receiving station determines the direction of a beacon station by measuring the difference in transit time between signals received from spaced beacons, the beacons emit continuous-wave signals of the same frequency but distinguished from one another by a characteristic such as timing, and the phase difference is measured with a phase-meter or the like. Indicating bearings ; sender. In an embodiment employing three beacons A, B, C, Fig. 14, half a wave-length apart, each beacon emits continuous waves of radio frequency f, from a crystal-controlled oscillator 27 through pre-set phase-adjusters 31, 32, 33. In order that the three emissions may be distinguished at the receiving station they are modulated with staggered rectangular envelopes by a commutator 43, which applies positive biasing potential in succession to the three transmitters 28, 29, 30. The phase comparison is made at a low frequency #2 such as 83<1>/3 c./s. after heterodyning at the receiver, and in order that the frequency #2 may be stable a heterodyne wave having a frequency (f1Œf2) is continuously emitted from a further beacon D at the beacon station, the transmitter 34 being locked to a stable low-frequency oscillator 38 so that the difference #2 or 83<1>/3 c./s. is stabilized. To this end the output of the stable oscillator 38 at a frequency 12f2 or 1000 c. /s. is applied to a frequency-divider 39 whence a stable frequency #2 of 83<1>/3 c./s. passes to a discriminator 37. The latter also receives for comparison a frequency which is to be stabilized at 83<1>/3 from a mixer 36 fed by the main transmitter 34 at (#1Œ83<1>/3) c./s. The commutator 43 is driven by synchronous motor 42 which is energized by an amplifier 41 controlled by current of stabilised frequency from the divider 39. For monitoring, the output of a monitoring receiver 47 is applied to the Y plates of a cathode-ray tube 46, whose X plates receive voltage of stabilized frequency derived from the divider 39. To facilitate phasemeasurements, each commutator segment corresponds to an. integral number of half wavelengths at the low frequency f Instead of the mechanical commutator an electronic switching- device may be used, Fig. 10 (not shown). Speech may be superimposed on the emission of the auxiliary beacon D. When the beacons A, B, C are half a wave-length apart, the locus of zero phase-difference for a given pair such as A, B is a single approximately straight line which can be oriented by setting the phaseadjusters 28, 29, but preferably is perpendicular to AB. If the distance between the beacons is increased, the bearing can be indicated with greater precision but provision must be made against ambiguity which is in that case introduced by the presence of more than one locus of zero phase-difference, Figs. 6, 11 (not shown). In one arrangement, Fig. 7 (not shown) the bearing is given grossly without ambiguity by a closely spaced pair of beacons, and at the same time precisely though with ambiguity by one of these in co-operation with a third widely-spaced collinear beacon. Indicating bearings ; receiver. At the mobile receiving station the bearing is given in units such that 600 of them go to 360 degrees, the tens and units being indicated on the scale 23, Fig. 13, of a phase-meter 64, Fig. 15, and the hundreds on the scale 26, Fig. 13, of a switch 63, Fig. 15, which determines which pair of beacons shall control each of the phasemeters 23, 24. This switch is turned until the pointer of the subsidiary meter 24 is in the third quadrant 25 and its contacts are so arranged that in that case the readings of the meter 23 are free from ambiguity. The received signals are demodulated at 48, Fig. 15, and the audio frequency of 83<1>/3 c./s. is applied through a receiving distributer 49 to band-pass filters 60, 61, 62 in succession. The low-frequency waves modulated with staggered rectangular envelopes are transformed by these filters into unmodulated low-frequency oscillations whose phase-differences can be compared in pairs, the switch 63 (with scale 26, Fig. 13) serving to apply the signals from one pair of beacons to the main phase-meter 64, Fig. 15 (with scale 23, Fig. 13) and of another pair to phase-meter 65 (with scale 24) the selection being such that the main pair of beacons is that, the perpendicular bisector of whose base passes nearest to the receiving station. Distance along the course near the beacon station may be indicated on a scale 68, Fig. 13, graduated for a standard height. Instead of phase-meters of the type suitable for the embodiment described above, a calibrated phase-shifter may be used, Fig. 4 (not shown) in conjunction with a phase-discriminator and a null indicator or with a centtezero dynamometer. Alternatively a stable oscillation may be applied to the Y plates of a cathode-ray tube, Fig. 5 (not shown), and shifted in phase until it yields a straight trace when one of the oscillations to be compared is applied to the Y plates. The other oscillation is applied to the Y plates through a calibrated phase-shifter which is adjusted until a second straight trace is obtained. Alternatively the rectangular wave-trains whose phases are to be compared may be converted to steady oscillations by means of filters as described above and applied to the X and Y plates respectively of a cathode-ray tube, the relative phases being adjusted by means of a calibrated phase-shifter so as to yield a standard pattern. Alternatively the wave-form may be rendered square and then converted by a differentiating circuit into pulses on a linear or circular time base. Blind landing ; aerials. Lateral guidance is given to landing aircraft by means of a pair of beacons A, B, Fig. 18, not more than 11/12 of a wave-length apart, so that the glide-path is the only course giving zero phase-difference. Vertical guidance is afforded by a pair of more widely-spaced beacons A, C, the phase-meter pointer co-operating with a lubber line 79, Fig. 17, which can be adjusted against a scale 81 graduated in glide-path angles. The aerials may be sunk in pits below ground-level and covered with material giving small dielectric loss. The circuits operating the indicators shown in Fig. 17 may be similar to those described with reference to Fig. 13. Ambiguity is avoided by observing the general behaviour of the pointer 80, Fig. 20 (not shown). Position along the landing track may be indicated by means of a lateral pair of beacons, E, F, Fig. 18, giving a low-frequency modulation different from that characterizing the beacons A, B, C. An additional phase-meter and filters are provided, Fig. 21 (not shown), and the modulation is furnished by an additional emission from the auxiliary beacon D. Aural course-indication. A desired course may be set, with equipment of the type described above, by means of a phase-shifter 87, Fig. 22, the pair of signals being applied to a discriminator 88 which is such that when the signals are in quadrature equal direct-current voltages are applied to the ends 89, 90 of a resistance earthed at its centre. In the contrary case the voltages at 89, 90 control the variable-gain amplifiers 93, 94 to unequal levels, so that distinctive signals such as dots or dashes from a source of tone 91 and ticking relay or commutator 92 are audible in the receiver 95. Electric synchronous movements. The receiving distributer 49, Fig. 15, is synchronised with the sending commutator 43, Fig. 14, by means of corrections applied through the differential gear 52 whenever signals reach one of the segments S1, S2, the wiper being dead for these segments so long as synchronism is maintained. When synchronism is lost a signal passes through a filter 69 or 70 and a rectifier 71 or 72 to a relay 73, 74 whereby positive or negative voltage from battery 75 is connected to a directcurrent motor 57 which applies a correction by way of the differential gear 52, through which the distributor 49 is driven by a synchronous motor 51 under the control of alternating current derived from the filter 60 and amplified at 53. Relay 74 is, however, cut out when errors of synchronism are so great that both segments S1, S2 receive signals.