Patent GB-671037-A (GB)
Title: Improvements in or relating to distance-difference-measuring systems employing electro-magnetic waves
IPC: G01S1/30
Filed: 1948-10-21
Abstract: 671,037. Radio navigation. SPERRY CORPORATION. Oct. 21, 1948 [Oct. 28, 1947], No. 27385/50. Class 40 (vii). Relates to means for resolving ambiguities in a C.W. beat-frequency hyperbolic navigation system of the type in which frequencies a and b radiated respectively from spaced aerials A<SP>1</SP> and B<SP>1</SP>, Fig. 1, produce a beat frequency (a-b) at a mobile receiver S whose phase is compared with a reference frequency (a-b) which is transmitted as a modulation on frequency a from the A<SP>1</SP> station. In such a system although the phase comparison is effected at a low frequency (a-b), the measured phase difference #1 is dependent on the carrier frequency b which results in an accurate but ambiguous " fine " indication. According to the present invention a second pair of carrier frequencies d and e such that (a-b)=(d-e) and b>d are radiated from A<SP>1</SP> and B<SP>1</SP> respectively the reference frequency (d-e) being transmitted as a modulation on frequency e from station B<SP>1</SP> giving at the mobile receiver a second " fine " phase difference #2 which is dependent on carrier frequency d and a less ambiguous " medium " phase indication is obtained by taking the difference of #1 and #2 which is dependent on a lower frequency (b-d). A still less ambiguous " course " indication #3 is obtained by comparing the phases of the modulation frequencies (a-b) and (d-e) transmitted from A<SP>1</SP> and B<SP>1</SP> respectively; in this case it is necessary to provide phase-locking means between the difference frequencies (a-b) and (d-e) at one station and this may be effected by automatically controlling the frequency and phase of one carrier, e.g. the b frequency at station B. The frequencies a, b, d and e are preferably chosen so that the ratio of & : (b-d) : (a-b) is equal to 400 : 20 : 1 suitable values being 80 kc/s., 4 kc/s. and 200 c/s. In order to provide a navigational fix, a third station C, Fig. 2A (not shown), is provided radiating frequencies c and f which co-operate with the frequencies a and d radiated from A<SP>1</SP> to provide " fine," " medium " and " course " indications related to a second set of hyperbolic equi-phase contours intersecting those defined by stations A<SP>1</SP> and B<SP>1</SP>. In this case the carrier a radiated from station A<SP>1</SP> is additionally amplitude modulated by a beat frequency (a-c) produced by heterodyning the frequencies a and c. A suitable receiver for use with such a system is illustrated in Fig. 2B (not shown). In all the embodiments described the reference beat frequencies (a-b), (d-e), &c. are frequency divided by a factor K before being modulated on to their respective carriers and at the mobile receiver the modulation signals are frequency multiplied by a factor k before being applied to the phase comparators. As shown in Fig. 1, transmitters 61 and 66 at station A<SP>1</SP> radiate carrier frequencies a and d respectively and transmitters 72 and 67 at station B<SP>1</SP> radiate carrier frequencies band e respectively. At station A<SP>1</SP> the carriers a and b are received at 62 and heterodyned to give a beat frequency (a-b) which is frequency divided at 64 by a factor K and the resultant frequency (a-b)/K is applied to amplitude modulate the transmitter 61 which accordingly radiates side bands a(a-b)/K in addition to the carrier frequency a. Similarly at station B<SP>1</SP> the carriers d and e are received at 68 giving a beat frequency (d-e) which is frequency divided at 70 by the factor K and applied to amplitude modulate transmitter 67 which radiates a carrier e and side bands e(d-e)/K. At the mobile receiver S the signals a, band a(a-b)/K are received at 77, the modulation signal (a-b)/K is selected by filter 80 and frequency multiplied at 83 by a factor K giving a frequency (a-b) which is applied to a phase comparator 85 together with a signal of equal frequency (a-b) obtained by heterodyning the carriers a and b in the receiver 77 and selecting the beat frequency by a filter 79 and it is shown that the " fine " phase difference 61 indicated on meter 170 is equal to 2#b/C(M+Ra-Bb) where C is the velocity of propagation of radio waves, M is the separation of stations A<SP>1</SP> and B<SP>1</SP> and Ra and Rb are respectively the distances of the mobile receiver S from stations A<SP>1</SP> and B<SP>1</SP>. A similar group of components 78, 81, 82, 84 responsive to the received signals d, e and e(d-e)/K produces at 87 the second " fine " phase difference #2=2#d/C(M+Rb-Ra) and the difference between the phase differences #1 and #2 is displayed on meter 88 as [the " medium " indication, the difference (61 - 62) being dependent on a lower frequency (b-d). The course indication is obtained by measuring the relative phase #3 of the modulation reference frequencies radiated by transmitters 67 and 61 by applying the outputs from multipliers 83 and 84 to the " coarse " phase meter 86 so that #3. is dependent on a still lower frequency (a-b). The necessary phase locking of the two modulation frequencies is achieved by receiving at 73, station B<SP>1</SP>, the reference modulation (a-b)/K transmitted from station A<SP>1</SP> and applying it together with the modulating frequency (d-e)/K from 70 to a phase discriminator 75 whose output controls the. frequency and phase of transmitter 72. The servoactuated phase meters 85, 86 and 87 may be as illustrated in Figs. 3-5 (not shown). The Specification as open to inspection under Sect. 91 describes a C.W. beat-frequency secondary radar system providing " five," " medium " and " coarse " indications which is identical to that described in Specification 671,661, as open to inspection. This subjectmatter does not appear in the Specification as accepted.