GB2309301A - Vehicle mounted acoustics sensor systems - Google Patents

Description

VEHICLE MOUNTED ACOUSTICS SENSOR SYSTEMS The present invention relates to vehicle mounted acoustic sensor systems and in particular a vehicle such as an aeroplane or helicopter.

The invention may be adapted to perform the following functions: picking up speech or other audible communication, detecting surface vehicles such as automobiles, tanks, ships, hovercraft, for example, detecting other aircraft, detecting weapons such as artillery weapons, rocket launchers, for example, and, detecting projectiles, such as supersonic shells and rockets.

Acoustic sensing systems are known in the art, and an example of which is described in GB Patent No 9526420.6. This systems is used to locate artillery and mortar weapons, and provides useful information in situations when all other weapon location systems remain ineffective.

The acoustic sensing system described in the above mentioned patent, has a number of acoustic sensors distributed for example about a battle field, and each sense the acoustic wave produced when a heavy weapon is fired. The positions of the acoustic sensors are known and therefore processing equipment is used to calculate the position of the weapon being fired.

An aim of the present invention is to utilise the above known system in a highly mobile environment, such as an aircraft.

According to the present invention there is provided an airborne or ground based vehicle mounted acoustic sensor system comprising a plurality of acoustic sensors distributed about the surface of said vehicle and a processor arranged to receive signals from said acoustic sensors and perform a calculation upon the received signals to determine the location of the sound source detected by the sensors.

The vehicle may be an aircraft.

An embodiment of the present invention will be now be described with reference to the accompanying drawings, in which, FIGURE 1 shows an example of an acoustic sensor systems mounted on an aircraft, FIGURE 2 shows a block diagram of the sensing and processing system in accordance with the present invention, and, FIGURE 3 shows a diagram of local and absolute coordinate systems.

Referring to Figure 1, there is shown an airborne vehicle such as an aeroplane, having an antenna 2 and a plurality of acoustic sensors 4, 6, 8, 10, 12, distributed about the aerodynamic surfaces of the aircraft and in the nose of the aircraft or any other suitable surface of the aircraft. The acoustic sensors are devices for sensing the acoustic component of pressure variations. They may be pressure sensitive microphones with appropriate windshield assemblies, or devices used in combination with noise cancellation means. The aircraft also includes navigation, motion, attitude and rotation sensors, generally designated 14 and a processor 16. A sensor 20 may be provided to remove interference generated by the aircraft propulsion means.

The aircraft may be a remotely controlled aircraft or a manned aircraft and would generally fly at a speed of about 40 to 60 knots, for example, but other speeds are not excluded. It will be appreciated that at this low speed the effect of wind noise at the sensors does not create a major problem.

The navigation sensors are used to determine the position of the aircraft in space. The attitude sensors are used to determine the orientation of the aircraft. The motion sensors are used to determine the motion of the aircraft in three dimensions. The rotation sensors are used to determine the rate of rotation of the aircraft about its three principal axes. The processor 16 is used to combine the outputs of the acoustic sensors, navigation, attitude and motion sensors and is programmed to reference the detection of sound by the microphones to absolute co-ordinates generated by the navigation, attitude and motion sensors. A rotation sensor may be used to compensate for rapid changes in the orientation of the aircraft.

The acoustic sensors may be microphones and if an aircraft were only equipped with microphones the processor 16 would be able to determine an approximate relative bearing from the aircraft to the detector sound source, and in cases where this is sufficient, the other sensing systems need not be present.

However, in order to compute an accurate absolute bearing, and to be able to do so when the aircraft is manoeuvring, the sensors 14 are required.

The output of the processor 16 may be used to guide the flight of the aircraft or it may be communicated to a central command post where the information can be displayed and analysed further. The command post may receive detection data from a number of different airborne sources or ground based systems, which can be collated at the command post and used to calculate the locations in space of the detected acoustic sources and hence to build up a comprehensive picture over an extended area.

In the situation where the acoustic sensors are microphones and the aircraft is travelling at a higher speeds than those mentioned, the microphones will be subject to a high level of turbulence-induced noise. In this situation, a more sophisticated acoustic sensor would be required and such a sensor is described in co-pending patent application number 9526420.6, entitled 'Noise Cancellation Apparatus', which describes an acoustic sensor system that might be used with advantage in the present invention.

Referring to Figure 2, there is shown a block diagram of the sensing and processing systems in accordance with the present invention In Figure 2, there is shown an aerial 2 of the aircraft, which is connected to a telemetry system 18, having an output connected to an input of the detection processor 16. The acoustic sensors of which four are shown, 4, 6, 8, 10, are also connected to the detection processor 16 together with the navigation, motion, attitude and rotation sensors 14.

The operation of the detection processor 16 will now be described.

A plane acoustic disturbance propagating across an array of acoustic sensors will arrive at each sensor at a different time. The precise time differences will depend upon the layout of the acoustic sensors, and on the direction and speed of the acoustic disturbance. There are several possible methods for determining these arrival time differences. One of the simplest would be to set a threshold, and when the signal in each channel exceeds this threshold, the signal is deemed to have arrived at the corresponding microphone. The arrival time differences can then be computed directly. A more sophisticated technique would involve computing the cross-correlograms of pairs of microphone signals, and identifying peaks in these cross-correlograms, the temporal positions of these peaks giving the required arrival time differences. This process is described in GB Patent 2181239A.

A plane acoustic disturbance can be characterised by a propagation vector K, which is a vector normal to the wavefront, pointing in the direction of propagation, and with a magnitude equal to the inverse of the acoustic propagation speed. Using a suitable origin, the time of arrival ti at any point Mi can be computed as follows: ti = K M.

1 1 Thus, if the acoustic sensors are arranged on the vehicle at locations Mi, then the times at which the sensors will respond to the disturbance will be ti. In the present application, the positions of the sensors are known, the arrival times of the signals at each sensor can be measured and the only unknown is the propagation vector. This is resolved as follows: for each possible pair of microphones i and j, compute the following time difference: ti - t. = 1 = K This now provides a set of linear equations, which can be solved for K using a variety of well known techniques (singular value decomposition being especially favoured). If there are at least four microphones, and they are not coplanar, then all three components K can be computed directly. However, if all the microphones lie in a plane, only the component of K that lies in the plane can be computed. The component of K perpendicular to the microphone plane can then be resolved using a knowledge of the speed of sound in the medium, and a knowledge of the motion of the medium relative to the microphone array.

Generally, it will be convenient to determine the Mi in a coordinate system fixed with respect to the aircraft. Hence, the components of K will be relative to the aircraft. In some cases, this will be sufficient, because it will provide an indication of the relative bearing between the aircraft and the source of the sound.

However, if the information is to be used for anything other than local manoeuvring decisions, it would be useful to compute K relative to absolute coordinates.

The orientation of the aircraft can be described by three angles: its heading, h; its pitch p, and its roll angle, r. These are measured by means of the orientation sensors fitted to the craft.

In the following discussion, angles will be treated in the standard mathematical fashion, increasing anti-clockwise from the x axis.

The local coordinate system of the aircraft will be assumed to have the x' axis arranged along the normal direction of flight, the y' axis across the span of the wings, and the z' axis mutually perpendicular to x' and y'. This is summarised with respect to Figure 3.

A coordinate transform can be defined from aircraft coordinates to absolute coordinates as follows: = AK Kw 1 - K V w 1 K V Where Kw is the propagation vector relative to absolute coordinates, V is the vector describing the true motion of the aircraft relative to the air mass, and is measured by means of the motion sensors fitted to the aircraft. The vector V is expressed in coordinates local to the aircraft, but this does not mean that only its x' component is non-zero. When the aircraft is turning, side slipping, or flying at a high or low angle of attack, the aircraft's motion will not be directed entirely along its principal longitudinal axis.

The propagation of the sound can be described in another way, as a vector Uw pointing from the apparent source of the sound to the aircraft, with a magnitude equal to the speed of propagation of the wave front:

A is a change of basis matrix. The value A can most conveniently be described as a rotation around the roll axis (the x' axis) of the aircraft, followed by a rotation about the pitch axis (the y' axis) followed by a rotation around the direction axis (the z' axis). It can be expressed as follows:

cost) - sin(h) 0 cos(p) 0 - sin(p) f 1 0 0 sins ) cos(h) 0 0 1 0 0 cos(r) sin(r) 0 0 1 sin(p) 0 cos(p) 0 - sin(r) cos(r) The foregoing analysis has been performed with the implicit assumption that the propagation medium, the air, is stationary with respect to the absolute reference frame, the ground, in other words, there is no wind. If there is wind, this has the effect of moving the apparent position of a source of sound. The previous results remain valid, but they actually provide answers relative to the moving air mass rather than relative to the ground (hence the use of the subscript W).

In the simplest case, the wind can be described by means of a wind vector, W, the direction and magnitude of which are the same everywhere. The value W could be determined by comparing the outputs of the aircraft navigation and motion sensors, or it could be supplied by weather forecasting services.

What is required is the vector g which describes the motion of the wavefront from the source of the sound to the point of detection relative to the ground coordinate system. This is simply calculated as follows: ug =W+U g w Finally, by negating the components of U9 ug a vector is obtained directed from the point of detection to the true source of the sound, relative to absolute ground based coordinate. This information, coupled with the position of the aircraft recorded at the time of detection by the navigation sensors, is exactly what is required by the sound locating system described in GB Patent 2181238A, and hence, a vehicle or vehicles mounted acoustic detection system could, using the processing scheme outlined here, form part of such a system.

The above description assumes that the motion and orientation of the aircraft are changing only slowly when acoustic detections are being made. However, significant errors could result if a detection is made while the aircraft is undergoing rapid manoeuvres. In such cases, corrections would have to be made using information provided by the rotation sensors and they are achieved by computing the alterations in the positions of the acoustic sensors as the acoustic disturbance propagates across the aircraft.

The invention as described above provides the following benefits. The occupants (or controllers, if unmanned) of an aircraft are provided with information about potential targets or threats. The acoustic propagation is unaffected by clouds, haze, smoke, chaff, or other causes of poor visibility, and could continue to provide valuable information even when other sensing technologies are ineffective. The system would continue to work while the aircraft is on the ground or at a very low altitude, because sound can propagate around hills and the aircraft does not need to have a line of sight contact with the source of the sound to detect it. When used as part of a distributed acoustic locating system, such as that described in the above mentioned prior art patent, the present invention enables rapid and flexible re-deployment of acoustic sensors in response to changing circumstances.

Claims (11)

1. An airborne or ground based vehicle mounted acoustic sensor system comprising a plurality of acoustic sensors distributed about the surface of said vehicle and a processor arranged to receive signals from said acoustic sensors and perform a calculation upon the received signals to determine the location of the sound source detected by the sensors.

2. A system as claimed in Claim 1, wherein the vehicle is an aircraft.

3. A system as claimed in Claim 1 or Claim 2, wherein the vehicle includes a plurality of further sensing means each of which generates signals which are passed to said detection processing means and used in the calculation process to determine the location of the sound source detected.

4. A system as claimed in Claim 3, wherein the plurality of further sensing means comprise navigation sensing means, motion sensing means and attitude sensing means.

5. A system as claimed in Claim 4, wherein a rotation sensing means is used to compensate for rapid changes in the orientation of the aircraft.

6. A system as claimed in Claim 3, Claim 4 or Claim 5, wherein the vehicle includes an antenna and the telemetry system, which is used to transmit the data pertaining to the detected sound source to a command post location.

7. A system as claimed in any preceding Claims 2 to 6, wherein the acoustic sensors are microphones positioned on the aerodynamic surfaces of the aircraft, and in the nose of said aircraft.

8. A system as claimed in Claim 7, wherein the aircraft is an unmanned aircraft.

9. A system as claimed in Claim 8, wherein the detected signals are used to direct the aircraft.

10. A system as claimed in any of the Claims 4 to 8, wherein a further sensor is provided to compensate for interference generated by the aircraft propulsion means.

11. A system substantially as hereinbefore described with reference to the accompanying drawings.