Patent GB-1399769-A (GB)
Title: Radio guidance method
IPC: G01S1/14
Filed: 1973-10-18
Abstract: 1399769 Radio navigation TULL AVIATION CORP 18 Oct 1973 [20 Oct 1972 (2)] 48589/73 Addition to 1346473 Heading H4D The system described in Specification 1,346,473, is modified in that instead of transmitting only one standard frequency signal, e.g. 5000.2 MHz, irrespective of the microwave band A, B, C, D or E (Fig. 2) in which the position signals are transmitted and necessitating a heterodyning of the frequencies of the guiding signals, to the VHF bands normally employed in ILS systems, in two distinct stages i.e. the microwave receiver converter 24 and translator 28, Fig. 1, of said Specification 1,346,473, a different standard frequency 41A, 41B, 41C, 41D and 41E is transmitted for each band used. If each reference frequency is spaced exactly the same amount from its corresponding band the conversion to the VHF, ILS frequencies may be carried out in one stage only. Thus assuming the position signals are being transmitted in the frequency band A i.e. 5000.3 to 5004.1 MHz for the ILS localizer signals and 5221.5 to 5227.2 MHz for the glide slope signals, the standard frequency signal 41A will be 5005.2 MHz. At the microwave receiver and converter 24, Fig. 3, local oscillator 50A is selected by section switch 52 to generate a local signal at 4892.2 MHz which, when mixed at 46 with the position signals and the standard frequency signal, converts the standard frequency signal to a fixed frequency of 113 MHz, the converted position signals being passed to a gate 32. The converted standard signal is further converted in mixer 74 by a local oscillator 76 operating at 102.3 MHz to give an output normally at 10.7 MHz. After amplification at 80 this last signal is detected at 86 for any deviation from the desired frequency of 10.7 MHz and any difference due perhaps to Doppler shift is used on line 88 as a correction signal for the local oscillator 50A. The output of the discriminator 86 is also used to enable the threshold circuit 84 to detect when the standard frequency signal is being received with sufficient strength to allow the enablement of gate 72 and the passage of the position signals to the VHF localizer and glide slope apparatus. The microwave ILS localizer or glide slope transmitter may be of the form shown in Fig. 4. The modulated microwave radiation is switched between directive beams 138 to 152 produced by an array of aerials 122 to 136, beams 138 and 152 being clearance or cover beams. The switching sequence is such that the radiation alternates from one side to the other of a central navigation plane 154, but with consecutive beams never being adjacent, e.g. a sequence may be 138, 142, 146 and 150. The standard frequency signal is transmitted by an omni-directional aerial. The modulation of the beam is by pulse width modulation and is a mixture of standard modulation frequencies, 150 Hz and 190 Hz the 150 Hz modulation predominating for beams 138, 140, 142, 144 and the 90 Hz modulation predominating for beams 146, 148, 150, 152. An aircraft on the plane 154 will detect equal amplitudes of the 150 and 90 Hz modulations. Each scanning sequence is determined by scanner 166 which in turn controls primary gate 172 feeding the modulated signal to secondary gate 176 for beams 138-144 or secondary gate 174 for beams 146-152.