Distance measuring systems, altimeters and aircraft

Abstract

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.

What I claim is:  
     
         1 . A distance measuring system, comprising: 
 means for generating a first modulated electrical signal;    a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and    processing means for receiving at least the second electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined;    wherein the frequency of the acoustic signal is 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.  
   
     
     
         2 . A distance measuring system, comprising: 
 means for generating a first modulated electrical signal;    a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and    processing means for receiving at least the second electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined;    wherein the transmitting and receiving transducers are mounted adjacent each other, and an acoustically absorbent material is mounted between the transmitting and receiving transducers.    
     
     
         3 . A distance measuring system, comprising: 
 means for transmitting a modulated acoustic signal to be directed towards and reflected by a surface;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding modulated electrical signal; and    processing means for receiving the electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined;    wherein the processing means does not attempt to correlate the electrical signal in respect of a time delay which is less than or generally equal to the time taken for the acoustic signal to travel directly from the transmitting means to the receiving transducer.    
     
     
         4 . A distance measuring system, comprising: 
 means for generating a first modulated electrical signal;    a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and    processing means for receiving at least the second electrical signa and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined;    wherein the transmitting and receiving transducers are mounted adjacent each other, each by a respective mounting system to a common member, at least one of the mounting systems providing acoustic damping and isolation between the respective transducer and the common member.    
     
     
         5 . A distance measuring system, comprising: 
 means for generating a first modulated electrical signal;    a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and    processing means for receiving at least the second electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined;    wherein the first electrical signal is modulated by an M-sequence.    
     
     
         6 . A helicopter having a distance measuring system for use as an altimeter or for obstacle detection, the distance measuring system comprising: 
 means for generating a first modulated electrical signal;    a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and    processing means for receiving at least the second electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined;    wherein the frequency of the acoustic signal is 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; and  
   wherein the frequency of the acoustic signal is chosen so that it satisfies at least one of the following conditions: 
 at a frequency at which the acoustic noise level produced by the helicopter below the helicopter is relatively low; and  
 at a frequency at which the acoustic reflection loss at an interface between air and grassland, rock or water is relatively low.  
   
     
     
         7 . A helicopter as claimed in  claim 6 , wherein at least one of the transducers is stationarily mounted with respect to the body of the aircraft.  
     
     
         8 . A helicopter as claimed in  claim 6 , wherein at least one of the transducers is stationarily mounted with respect to a winch-hook, or the like, of the helicopter.  
     
     
         9 . An aircraft having a distance measuring system comprising: 
 means for generating a first modulated electrical signal;    a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface over which the aircraft is flying;    a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and    processing means 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 a distance travelled by the acoustic signal can be determined;    means for controlling an altitude-related aspect of the aircraft in dependence upon the indicated time delay.    
     
     
         10 . An aircraft as claimed in  claim 9 , wherein the lift produced by the aircraft is controlled in dependence upon the indicated time delay.  
     
     
         11 . An aircraft as claimed in  claim 9 , wherein the length of a winch of the aircraft is controlled in dependence upon the indicated time delay.  
     
     
         12 . An aircraft having a distance measuring system comprising: 
 receiving transducer means arranged to receive a modulated acoustic signal generated by the aircraft, both directly and indirectly after reflection by a surface, and to produce a corresponding modulated electrical signal; and    processing means for receiving the electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the reflected acoustic signal can be determined.    
     
     
         13 . An aircraft as claimed in  claim 12 , wherein the receiving transducer means comprises a directional transducer which is directed towards the surface.  
     
     
         14 . An aircraft as claimed in  claim 13 , wherein: 
 the receiving tranducer means further comprises a second directional transducer which is directed towards the source of the acoustic signal generated by the aircraft to produce a further electrical signal; and    the processing means is operable to correlate the modulations of both of the electrical signals.