GB683688A - Improvements in or relating to navigation aiding radio systems - Google Patents

Abstract

683,688. Radio navigation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 25, 1950 [Jan. 27, 1949], No. 2310/49. Class 40 (vii). This invention is a development of that of Specification 683,687, wherein is described a continuous-wave hyperbolic navigation system comprising two synchronized spaced stations each radiating two signals of frequencies symmetrically disposed with relation to a common mean frequency and spaced therefrom an amount small in relation thereto, the signals from each station being different in frequency. In such a system the mean frequency is preferably a common carrier frequency, the upper and lower frequencies being side-band frequencies derived by modulation, normally with carrier supression. The present invention contemplates the phase comparison of the sum frequencies of the signals from each station to provide a " fine " positional indication as in the above-mentioned Specification and of the difference frequencies of the same signals to provide a " coarse " positional, or track, indication. It is stated that, as the frequencies radiated are actually or in effect modulation sidebands, it is possible to separate the upper and lower frequencies at the receiver, demodulate to recover the " modulation" " frequencies, and change these for example by multiplication, to a common frequency for phase comparison to obtain the track indication. In a preferred embodiment however the actual or effective modulating frequencies are not recovered but the individual signals from each station combined subtractively to obtain signals of twice the " modulation " frequency which are phase compared at a common multiple frequency as before. In an example of such a system using three stations A1, A2 and A3 to define the receiver position in terms of intersecting contours, station A1 radiates sidebands 14,875 c/s. and 15,125 c/s. of the mean frequency 15,000 c/s. and A2 and A3 sidebands of 14,812.5 c/s. and 15,187.5 c/s., and 14,750 c/s. and 15,250 c/s. respectively having the same mean frequency. Station A2 also radiates the 15,000 c/s. carrier. Not only the mean frequency carrier of the stations but also the modulating signals of 125, 187.5 and 250 c/s. require phase synchronization and for this reason it is preferred to make the modulation frequencies sub-harmonics of the common carrier. All the signals are received at the position-determining receiver and may be mixed with a signal of 10,000 c/s. from a local oscillator L.O. to reduce the mean frequency to 5000 c/s. the difference signals being amplified and passed by broadly-tuned amplifier I.F.A. Frequency discriminator FD is provided to stabilize the output frequency of the oscillator LO. The output of I.F.A. is separated by sharply-tuned filters A1S1, A1S2, A2S1, A2S2, A3S1, and A3S2 into its component signals. The reduced signals from each station are mixed in mixers MA1, MA2 and MA3, the sum signals of 10,000 c/s. being separated and passed by filters SMA1, SMA2, and SMA3 whilst the difference signals of 250 c/s., 375 c/s. and 500 c/s. respectively are separated and passed by filters DMA1, DMA2, DMA3. For fine position indication the signals of 10,000 c/s., further filtered at F, are compared in phase in pairs and the phase difference values indicated by pointers moving over the outer scales of indicators RP, GP, BP. For coarse or track indication the outputs of DMA1, DMA2 and DMA3 are respectively multiplied in frequency by 6, by 4, and by 3 at X6, X4 and X3, and the resulting 1500 c/s. signals, filtered at CF, compared in phase in pairs, the phase differences being displayed on the inner scales of indicators RP, GP and BP. In the above embodiment the doubled carrier frequency 30,000 c/s. is an exact multiple of the " track " comparison frequency so that a complete revolution of the " track " indicator represents an integral number of revolutions of the fine indicator. However, these frequencies may have a simple rational fractional relationship for example 30,000 c/s. and 1800 c/s. give a relationship of 50 : 3; in such a case the coarse scale may be in the form of a three-turn spiral, the " coarse " indicator making 3 revolutions for 50 revolutions of the fine indicator. Fig. 3 (not shown), shows a composite indicator in which the three fine indication pointers move over an outer scale and three coarse indication pointers move over a spiral inner scale.