Patent GB-1452509-A (GB)
Title: Two-carrier instrument landing system
IPC: G01S1/02
Filed: 1973-12-19
Abstract: 1452509 Radio navigation INTERNATIONAL STANDARD ELECTRIC CORP 19 Dec 1973 [19 Jan 1973] 58736/73 Heading H4D An instrument landing system (ILS) includes localizer and glide slope transmitting stations, each generating clearance and course radiation patterns, the clearance carrier frequencies lying respectively within the ranges 108 to 112 MHz and 328 to 336 MHz. At least one of the course carrier frequencies lies in the microwave region (over 1000 MHz) and at an airborne receiver is converted into the range of the carrier frequency for the associated clearance radiation pattern, the two signals then being applied to a further stage of the receiver. Deviations from course are determined by comparing in the receiver the depths of modulation by 90 Hz and 150 Hz signals. As described, the localizer transmitter station comprises a 110 MHz oscillator 1, Fig. 1, outputs from which are modulated in modulators 2, 3 by 90 Hz and 150 Hz signals respectively. A bridge 4 derives sum and difference outputs from said modulated signals and these are fed via a power divider 5 to an array of dipoles 5, which develops the clearance radiation pattern. A second oscillator 7 has a frequency of 110 MHz +9 kHz. The frequency difference from the oscillator 1 is necessary for the utilization of the capture effect in the airborne receiver as it comes within the narrow (Œ1 to 2 degrees) course radiation beam, and it is continuously monitored by a monitor 16 which is coupled to both oscillators. A third oscillator 9 has a frequency of 5000 MHz and output therefrom is mixed with output from the oscillator 7 to produce a signal of frequency 5110 MHz+9 kHz. This is amplified and is applied through modulators 11, 12, having respective modulation frequencies of 90 Hz and 150 Hz, and a bridge 13 which feeds an array of horns 15 producing the course radiation pattern. A 5000 MHz signal from the oscillator 9 is also radiated, either from an additional horn 18 or from the array 15, for reception aboard an aircraft and mixing with the course signals. The glide slope transmitting station is similar, except that the oscillators equivalent to nos. 1, 7 have frequencies of 330 MHz and 330 MHz+8 kHz, respectively. An airborne ILS receiver comprises conventional localizer and glide slope equipment comprising aerials 30, 33, Fig. 3, receivers 31, 34 and cross-pointer instrument portions 32a, 32b, respectively. Additional equipment comprises a microwave receiving horn 39, a mixer 40 containing a tunnel or point-contact diode and converting the course signal frequencies to 110 MHz+9 kHz and 330 MHz+8 kHz using the 5000 MHz reference signal from the transmitting station. The frequency-converted signals are separated by filters 36, 37 and are applied as inputs to respective bridges 35, 38, which also receive inputs from the aerials 30, 33. Outputs from the bridges 35, 38 are applied to the receivers 31, 34. The airborne ILS receiver described may also be used when landing at an airport equipped with conventional ILS facilities, for which purpose the additional equipment is ineffective. Conversely, an aircraft having conventional ILS equipment may utilize it when landing at an airport equipped as shown in Fig. 1 by making use of only the clearance radiation patterns. The use of microwaves for the course radiations pattern obviates multipath reflections which might cause distortions.