Patent GB-582085-A (GB)
Title: Improvements relating to arrangements for determining distance and bearing by electromagnetic waves
IPC: G01S1/02
Filed: 1941-11-14
Abstract: 582,085. Radio navigation. STANDARD TELEPHONES & CABLES Ltd., and JACOBSEN, B. B. Nov. 14, 1941, No. 14707. [Class 40 (v)] Means for determining the bearing or position of a mobile receiver comprises two or more spaced radio transmitters radiating different synchronised indicating waves, and means at the receiver for comparing the phases of the received waves, thereby determining solely by the indicating waves one or more hyperbolµ, each having two transmitting aerial as its focii, on which the receiver is situated. Thus, since the difference in the distances RT1 and RT2, Fig. 1, of any point R on either branch A or B of an hyperbola having T1 and T2 as foci is constant, measurement of the phase difference of the indicating waves received from transmitters at T1, T2 will locate the receiver R as being on a particular hyperbola. Ambiguity as to whether the receiver is on the A or B branch may be resolved by determining the sign of the phase difference. Since other confocal hyperbolµ, such as C, exist in which the phase difference differs from that on the hyperbola A, B by an integral multiple of 2#, the same readings will in general be obtained when the receiver is situated on any one of them. If the distance from the base line is small, or the frequency of the indicating waves is low, only one of such hyperbolµ will, however, be reasonable. In other cases, the ambiguity may be resolved by using a different indicating frequency for a second measurement, thereby specifying a second family of hyperbolµ of which, if the frequencies are suitably chosen, only one will be common to both families, or if there is more than one only one will be reasonable. Since a low indicating frequency leads to inaccuracy in measurement of distance, but usually to no significant ambiguity, whilst with a high frequency the reverse is the case, it is desirable to make two measurements at relatively high and low frequencies to obtain accuracy and at the same time resolve the ambiguity. The use of two transmitters, as shown in Fig. 1, enables the bearing of the receiver to the mid-point of the base-line to be found when it is at a substantial distance, since it will be substantially that of the asymptote of the hyperbola. To indicate position, another pair of transmitters is employed to define a second base line, from which a second hyperbola intersecting the first at the position of the receiver may be similarly determined. For this purpose three transmitters, one being common to both pairs, may be used. The transmitters may radiate carriers of the same or different frequencies, modulated by the same indicating frequency or by frequencies having a simple ratio. In one form, two transmitters at spaced stations radiate carriers of different frequencies modulated by indicating waves of the same phase generated by an oscillator at one station, the phase shift introduced by the communication line to the second station being compensated by a phasechanging network at the first. Each transmission is received on a separate receiver, the demodulated outputs of which are applied to a phase-measuring device the indicator of which may be calibrated to give the difference between the distances to the two transmitters, subject to the ambiguity above referred to. This ambiguity may be avoided by the use of high and low modulating frequencies, either in alternation or simultaneously. To prevent unauthorized use of the transmissions, phaseshifting networks are provided at one transmitter and at the receiver which are adjusted according to a pre-arranged schedule so that the variable phase-difference introduced between the two transmissions is compensated at the receiver. Fig. 3 shows a modification in which transmitters T1 and T2 at stations I and II supply carriers of the same frequency, that from T1 being modulated at frequency f from an oscillator O, and that from T2 at frequency nf being applied from the same oscillator through transmission line L, frequency multiplier FM, and filter BP2. The demodulated output of a single receiver at station IV is divided into. the frequencies f and nf by filters BP1, BP2 respectively, the latter being brought back to the frequency f by a frequency divider D before application to the phase meter PM. Obscuring phase-changing networks VN1 and VN2 are adjusted according to a time-schedule to prevent unauthorized use of the transmissions. A network CN compensates for the phase-change produced in the line L. The carrier wave from station II may be suppressed, that from station I being used at station IV to demodulate the sidebands. In this case, when the phase of the carrier as received from station I differs by an odd multiple of #/2 from that which would have been received from station II there is no demodulated output of the frequency nf. This fact is used to determine a family of hyperbolµ defined by phase differences of odd multiples of #/2 by comparison of the carrier waves, giving accurate location with a considerable ambiguity which is resolved by a rough measurement obtained by comparison of the waves f and nf. The phase-comparison measurement of the carrier waves is made in the manner described in Specification 581,568, by introducing a cyclically-variable phase changer or delay network in the path of the carrier at station II before modulation, which varies the phase of the carrier through 360 degrees. Once in each cycle phase opposition of the carriers occurs at station IV, and the time interval between its occurrence and that of a reference signal (originated at a fixed point in the cycle by the phase changer) is measured at the phase meter 'and determines the phase difference at carrier frequency between the paths to the two transmitters. One or more additional transmitters may be used to determine a second hyperbola so that the receiver may fix its position. A system is described (Fig. 4, not shown) for setting the course of an aeroplane or other vehicle, in which three different carriers modulated at the same frequency are radiated from three stations and demodulated by three separate receivers. The demodulated waves from two of the receivers are passed through the corresponding one of a pair of phase adjusters, after which they are compared by a null method in two meters with the waves from the third receiver, the phase of which is not shifted. The two phase shifters are coupled by gears so that the phase changes produced by them are in a constant ratio, which may, however, be adjusted. Fig. 5 shows portions of the two sets a, b of hyperbolµ defined by the three transmitters, having equal phase steps of, say, 10 degrees between them, and illustrates that when a course line such as PT is followed the changes of phase difference produced by passage across the two systems are substantially in constant ratio. If, therefore, the gear-box is set for this ratio, phase opposition will be indicated simultaneously by both meters as long as the course is held. Deviations from the course are indicated in magnitude and sign by one phase meter when the other indicates phase opposition. With this system any number of aeroplanes may set their own courses independently of each other. In a modification, Fig. 6, the three transmitters operating with different carrier frequencies are each modulated with two consecutive harmonies, mf and (m+1)f, of a frequency f, which is chosen as high as possible consistently with the avoidance of significant ambiguity, say 1000 per second. The frequency (m+1)f may be 20,000 per second. The demodulated frequencies (m+1)f and mf from the receiver R1 are separated by filters BP1, BP2 and passed respectively to phase-shifters PA1, PA2, the outputs from which are combined in a modulator M1 to give the frequency f. The phase-shifters PA1, PA2 are so geared together that phase-changes of (m+1)# in the frequency (m+1)f and of m# in the frequency mf occur simultaneously, so producing a phase-change # in the frequency f, which is compared in a meter PM2 with the same frequency obtained, without phase-shift, from the comparison receiver R2. Adjustment of the phase-shifters PA1, PA2 to give a null reading in the meter PM2 therefore defines one hyperbola giving a rough indication of location, free from significant ambiguity, using the frequency f. An accurate reading is then obtained at frequency (m+1)f by further slight adjustment, if necessary, of the phase-shifters until a meter PM1 indicates phase-opposition between the frequencies (m+1)f supplied to it from the phase-adjuster PA1 and the comparison receiver R2. Corresponding readings similarly obtained on meters PM4, PM3 from receivers R3, R2 define a second hyperbola intersecting the first. In the two systems last described the phasecomparison apparatus may be stationed on the ground, the signals received on the aeroplane being re-transmitted to it, and the corresponding course instructions being automatically or otherwise transmitted back to the aeroplane. The synchronising of two transmitters may be effected by a monitoring station receiving ground waves from both and emitting a signal which is characteristic of the phase difference between them and is used to adjust the phase or frequency, or both, of the carrier or modulating wave at one station.