| 摘要 |
A distance measuring system is described which may be used as an altimeter for a helicopter or for a load carried by its winch. The system comprises: a transmitting transducer for converting a first modulated electrical signal into a corresponding acoustic signal to be directed towards and reflected by a surface; a receiving transducer for converting the received reflected acoustic signal into a corresponding second electrical signal; and a correlation processor for receiving the second electrical signal, or the first and second electrical signals, and correlating the modulations thereof to produce an indication of a time delay thereof from which the distance travelled by the acoustic signal can be determined. The frequency of the acoustic signal is preferably chosen so that it is not at or near the fundamental resonant frequency of either transducer and so that it satisfies at least one of the following conditions: at or near a higher-order resonant frequency of one or both of the transducers; at a frequency at which a frequency-dependent directivity index of the transducers is relatively high; at a frequency at which the ambient acoustic noise level is relatively low; and at a frequency at which the acoustic reflection loss at the surface is relatively low. The transducers are preferably mounted adjacent each other, with an acoustically absorbent material between them, and by respective mounting systems to a common member, with each mounting system providing acoustic damping and isolation between its transducer and the common member. The correlation processor is preferably arranged so that it does not attempt to correlate the electrical signals in respect of a time delay which is less than or generally equal to the time taken for the acoustic signal to travel directly between the transducers. The first electrical signal is preferably modulated by an M-sequence. An altitude-related aspect of the helicopter may be controlled in dependence upon the indicated time delay.
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