| 摘要 |
1514422 Diversity receivers MEL EQUIPMENT CO Ltd 12 Jan 1976 992/76 Heading H4L A radio system for an aircraft having front and rear antennae AF, AR, wherein a receiver Rx is cyclically switched between the aerials until a signal of a first standard of acceptability is received whence switching ceases until the signal coupled to the receiver decreases to a second, lower standard of acceptability additionally comprises means operative whenever an acceptable signal is being received and sensitive to the magnitude and sign of the line-of-sight component of the aircraft velocity relative to the transmitter to ensure that, provided the magnitude exceeds a given threshold value, the front, resp. rear, aerial is coupled to the receiver when the component is directed towards, resp. away from, the transmitter regardless of any previous selection made. It is thus ensured that antenna changeover occurs shortly after a closest approach to the transmitter has been made. The acceptability of the signal is assessed by monitoring the r.f. amplitude, the receiver a.g.c. signal, or the rate at which cyclically repeated messages are validated, to provide a signal Vs, antenna cyclic switching occurring under control of flip flop FF and oscillator OSC until the signal Vs, smoothed by network R 1 C 1 , is great enough to trigger threshold circuit (Schmidt trigger) Th 1 exhibiting hysteresis, to disenable the oscillator so that one of the antennae is selected, and to enable velocity threshold Th2 to receive the velocity component signal Vv. Time delay circuit TC2 ensures that, should fading occur, Th2 will continue to receive an input to enable holding of the selected antenna for a time dependent on the signal Vv strength prior to fading. The signal Vv may be derived by time differentiation of range measurements. The receiver Rx is associated with a transmitter Tx, both being coupled to the antennae via circulator T-R. The system is part of a landing and guidance system, position data being transmitted automatically or in response to interrogation.
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