Patent GB-1487257-A (GB)
Title: Air navigation and landing aid system
IPC: G01S13/91
Filed: 1974-12-13
Abstract: 1487257 Radio-navigation THOMSONCSF 13 Dec 1974 [14 Dec 1973 28 March 1974] 54104/74 Heading H4D A radio-navigation system comprises at least one ground transponder beacon 10, Fig. 1, which receives pulses having a frequency F 0  and retransmits them at a different frequency F 0  + #F and, airborne, an aerial 2, a mechanical drive device 5 for scanning the aerial, a weather transmitter/receiver 1 coupled to the aerial via a switching means 3, a special receiver 6 for receiving, via the switching means, beacon signals in response to interrogation pulses from the weather transmitter, and processing means 7 connected to the device 5 and the receiver 6. During cruising flight "sum" (#) signals received by the aerial correspond to returns from clouds or the ground. They have the same frequency F 0  as the transmitted pulses, and are fed directly to a weather indicator 4, which also receives aerial position signals from the device 5. The special receiver 6 also receives from the aerial 2 "difference" (#) signals indicative of angular displacement in one or in two planes, and from the switching means 3 synchronization pulses (S) corresponding to transmitted pulses. The #, #, and S signals are delivered by the receiver 6 to the processing means 7, which may also have an input from an accelerometric aid device 9. Output from the means 7 is fed to an indicator 8, which displays landing aid information to the pilot. As shown in Fig. 2, the switching means 3 comprises a circulator 31 and a coupling device 32. Transmitter pulses are fed directly to the # input of the aerial, a small fraction being taken off by the coupling device for application to the receiver 6 as S signals. Received # signals are coupled by the circulator 31 to a filter 21. If of frequency F 0 , they are reflected, and are coupled by the circulator to the weather receiver. If of frequency F 0  + #, they are passed by the filter 21. The aerial provides two difference (#) signals, designated #S and #G, and these are handled in time multiplexed manner by means of two synchronously operated changeover switches C1, C2. The path for the # signals contains a filter 11, similar to the filter 21 in the # signals path. In each path there then follows a pair of coupled switches 12, 15 or 22, 25 whereby either an attenuator 13 or 23, or a direct connection 14 or 24, may be inserted by a control circuit 41. The switches 15, 25 provide inputs to respective mixers 16, 26, which also receive inputs from a local oscillator 46. There follow wide band amplifiers 17, 27 and narrow band filters 18, 28, the outputs from which are combined in a circuit 19 to provide # signals for application via the switch C2 to the processing means 7. A switch C3, which may be automatically controlled by a distance measuring circuit, allows either wide or narrow band # signals to be applied via a variable gain amplifier 20 to a detector 29, which delivers a video signal to the processing means 7. The wide band # signals are used for precision measurements at short ranges, by determining the interval of time between their rise edges and those of the S signals, thereby overcoming the limitation imposed by the long durations of the pulses from the weather transmitter. The signals from the coupling device 32 at the frequency F 0  are changed in frequency in a device 42 whose output is connected to a mixer 43, which also receives an input from the local oscillator 46. An automatic frequency control loop for said local oscillator is completed by a narrow band filter 44 fed from the mixer 43 and coupled to a frequency discriminator 45, which provides a control signal. Detected output from the filter 44 at the output of a detector 40 provides S signals for the processing means 7 and control signals for the circuit 41. In an alternative form of switching means, Fig. 3 (not shown), a changeover switch provides for direct connection of the "sum" terminal of the aerial to the weather transmitter during cruising flight. In the second position of the changeover switch connections as in Fig. 2 are established (a duplexer may be used instead of a circulator). As above described, precision angular information is obtained by means of a special aerial, e.g., of monopulse type. In other embodiments, an ordinary radar aerial is used and in one (Fig. 4, not shown) no bearing information other than that from the mechanical drive device is fed to the processing means 7. Elevation data is obtained by combining the radar determined distance with altitude measurements furnished by the navigation equipment on board the aircraft, and is fed to the processing means. In a variation (Fig. 5, not shown) elevation data from ILS equipment is utilized. In further embodiments each ground beacon is arranged to re-transmit received pulses on at least two different frequencies, a respective radiation pattern being provided for each frequency, and said patterns being arranged to overlap so as to provide equi-signal planes. An ordinary radar aerial is used on an aircraft, and the received beacon response signals are separated according to frequency, and are compared in amplitude. A beacon providing a single equisignal or reference plane P, Fig. 6, comprises a parabolic reflector 101, six dipoles 102 to 107 symmetrically disposed in two groups G1, G2 in the focal plane of the reflector about the plane P, respective frequency changers 112 to 117, an oscillator 110 for the group G1, and an oscillator 120 for the group G2, said oscillators having different frequencies. The group G1 produces the radiation pattern D1, and the group G2 produces the radiation pattern D2. In a second form of beacon three fanwise disposed reference planes are produced by four groups of sources arranged in line in the focal plane of a reflector (Fig. 7, not shown). In a thrid form of beacon two reference planes which intersect at right angles are produced by four groups of sources symmetrically disposed about the axis of the reflector in the focal plane (Fig. 8, not shown). Two beacons may be provided for an airport runway, one being located on the axis thereof and beyond its end, and the other to one side of the runway. With beacons of the first form (Fig. 6) both may provide reference planes which are parallel to the glide slope plane, or the one at the end of the runway may have its reference plane vertical. With beacons of the third form (Fig. 8), in each, one reference plane may be parallel to the glide slope plane, and the other vertical. In the airborne special receiver 6, Fig. 13, the received beacon signals are separated in circuitry 200 according to their frequencies, to provide outputs S1, S2. These are added in an adder 500 to provide a # signal, and are compared in a comparator 400 to provide a # signal. Alternatively, one of the signals S1, S2 is delayed by a delay line with respect to the other and the two are combined on a single output of the circuitry 200. Comparison of the amplitudes of the signals is effected, for example, directly in the processing means 7, after digital coding, and the comparator 400 and the adder 500 are not required. In the circuitry 200 the two signals S1, S2 are separated by a diplexer either at radio frequency (Figs. 14, 15, not shown) or at an intermediate frequency (Figs. 16, 17, 18, not shown). A logarithmic amplifier is provided in each or the output channel before final detection, except in the case of the Fig. 18 arrangement, where, instead, a limiter amplifier is provided before the diplexer.