GB1469507A - Aircraft navigation systems - Google Patents

Abstract

1469507 Radio navigation PLESSEY CO Ltd 10 May 1974 [11 May 1973] 22671/73 Heading H4D An aircraft radio navigation system comprises means radiating a first signal in a narrow course beam radiation pattern and means radiating a second signal in a wider clearance beam radiation pattern, the phase of one of the signals being cyclically inverted. As described, an ILS (instrument landing system) localizer, for providing azimuthal guidance for aircraft, comprises means for generating a single pair of CSB (carrier plus sidebands) and SBO (sidebands only) signals. The sidebands are produced by amplitude modulation of the carrier signal by 90 Hz and 150 Hz signals. The CSB and SBO signals are applied to respective power splitters 3, 4, Fig. 2. A first pair of output signals CSB1, SBO1 are fed out to derive the narrow beam course field radiation pattern by conventional means. A second pair of output signals CSB2, SBO2 are fed out through respective phase reversal switches 5, 6 and 90 degree phase delay circuits 7, 8 to derive the clearance field radiation pattern by conventional means. The phase reversal switch circuits 5, 6 cause the signals applied thereto to be cyclically reversed in phase at a low frequency, e.g. 10 Hz. The effect of this is that if any reflection of the clearance signal on to the course line takes place, the DDM (difference in depth of modulation) thus imposed reverses in sense at the rate of the phase reversals, and is effectively self-cancelling. The phase delay circuits 7, 8 are provided to obviate the problems that can arise if the CSB and SBO signals are in phase where the course and clearance radiation patterns cross (Fig. 1, not shown) The phase reversal switches 5, 6 may each cnmprise a bridge arrangement of four arms 9, 10, 11, 12, Fig. 3, of which the first three have the same electrical lengths, whilst the arm 12 differs in electrical length from the others by a half-wavelength (180 degrees). The input signal is fed to the junction of the arms 9, 10 and the output signal is taken from the junction of the arms 11, 12. A diode D1 is coupled through a quarter wavelength (90 degrees) line 17 to the junction 15 of the arms 9, 12 and a diode D2 is directly coupled to the junction 16 of the arms 10, 11. When the diodes are made conductive (by application of a 10 Hz frequency rectangular wave bias signal) the junction 16 is short-circuited, whereas the junction 15 is opencircuited. Hence, the signal is fed via the arms 9, 12. Conversely, when the diodes are made non-conductive, the signal is fed via the arms 10, 11. The invention may also be applied to ILS glide-path systems.