GB2414800A

GB2414800A - Sonar receiver with low side lobes

Info

Publication number: GB2414800A
Application number: GB0001907A
Authority: GB
Inventors: Jack Marcus Keen
Original assignee: Thales Underwater Systems Ltd
Current assignee: Thales Underwater Systems Ltd
Priority date: 2000-01-27
Filing date: 2000-01-27
Publication date: 2005-12-07
Anticipated expiration: 2020-01-27
Also published as: GB0001907D0; AU784529B1; FR2902893B1; GB2414800B; FR2902893A1

Abstract

A beamformer and processing arrangement where the individual outputs of an array of sensor elements or transmitter elements are processed such that pairs of the elements can be regarded as a beamformer with a good front-to-back ratio. Means for applying weights and delays are provided to give the full array a desired directional characteristic. The arrangement is particularly suited to sonar receivers, as the array has very good side lobe rejection characteristics compared to prior art devices. However, the invention may be applied to arrays for sensing or transmitting other types of wave, for example sound in any other type of fluid or electromagnetic waves.

Description

24 1 4800

SONAR RECEIVER WITH LOW SIDE LOBES

This invention relates to a sensor. It arose in relation to a sonar system but could be applicable to other systems, such as radar receivers, for receiving radiated energy. The invention could also be of use in relation to transmitters.

A typical sonar receiver, of a type know as a "dunking sonar" is shown at 1 in Fig. 1. It is shown suspended from a helicopter 2 and immersed in water so as to listen for underwater sound. This sonar receiver 1 is shown in greater detail in Fig. 2 and comprises a central, vertically arranged housing 3 to which radially extending arms 4 are hinged, at the top and bottom of the receiver. Vertical support members 5 connect the radially extending arms, which form "vanes". These vanes can be opened from a folded condition, where they are folded close to the housing for ease of transport to a deployed condition shown in Fig. 2. Acoustic sensing elements 6 are disposed along the vertical support members 5. Fig. 2 shows these elements on only one vane for simplicity of illustration. A five-vane array is the most common configuration. A three-vane array is possible, as are a greater number of vanes, although arrays with a greater number of vanes are usually too expensive and complex to be commonly used. An array having only one vane could in principle be used, but such an array would have strong ambiguities in certain directions.

Fig. 3 is a view from the top of the sonar receiver of Fig. 1. It can be seen that each arm carries a number of acoustic sensing elements 6. Only three of these are shown on the drawing for simplicity of illustration, but in practice there are more. The distance d between the sensing elements on each arm is usually selected to be just a little less than half the wavelength of the radiation which the equipment is designed to receive. Sonar design engineers have used this spacing because it offers the largest apertures without causing grating lobes. Smaller spacing would not achieve any significant advantage, as more sensing elements would be required to establish the same aperture size.

Outputs from individual sensing elements are subject to weighting and time delays to produce a final output having the required directional characteristics. The time delays dictate the steer direction whilst the weights determine the side lobe and beam shaping.

The delays are usually chosen for hill reinforcement e.g. as described in the book "Principles of Underwater Sound" by Urick. The weighting can be calculated, for example, by using principles described in the paper "Amplitude Shaping of Sonar Transducers" by H.S.C. Wang JASA Vol 57 No 5 May 1975.

Conventional sonars of this type have had horizontal side lobes generally no better than 12 dB rejection. The invention arose when considering ways of improving this relatively poor performance. One possible technique considered was to infill the spaces between the arms with additional sensing elements, however this would have added unacceptably to expense and complexity.

When considering the problem, the inventor formed a theory that the poor side lobe structure was attributed to each arm having an ambiguous direction associated with it, ie.

equal sensitivities in two opposite directions mirrored on opposite sides of the arm.

According to a first aspect of the invention, there is provided a directional sensor comprising an array of multi-element beamformers and means for applying weights and delays to outputs of the beamformers to give a desired directional characteristic.

Although the invention arose in relation to creating a better sensor, how the beam pattern is achieved is reciprocally applicable to transmitters, as the principle is entirely similar.

Therefore, according to a second aspect of the invention, there is provided a directional transmitter comprising an array of multi-element beamformers and means for applying weights and delays to inputs of the beamformers to give a desired directional characteristic.

Each multi-element beamformer can be a discrete physical unit, for example two or more sensor elements held in spaced relationship by a suitable support; and the individual beamformers may be mounted on supports such as arms extending from a central support housing similar to the arrangement of Figs. 2 and 3. However, other arrangements are possible. The beamformers need not have their own individual support members; in fact, no supporting structure is theoretically required in order to implement the invention. It is sufficient that there be a suitable number of sensors whose spatial relationship is such that the outputs of groups of them can be processed in such a way as to give them a directional characteristic.

Thus, according to a third aspect of the invention, there is provided a directional sensor comprising sensor elements which can be considered to be arranged in groups, means for processing outputs of the elements of each group in one processing stage so that each group behaves as a beamformer and for combining outputs of the beamformers in another processing stage to produce an output of the sensor.

Correspondingly, for transmission, a fourth aspect of the invention provides a directional transmitter comprising transmitter elements which can be considered to be arranged in groups, means for processing inputs for the elements of each group in a one processing stage so that each group behaves as a beamformer and for splitting inputs for the elements in another processing stage from the input of the transmitter.

A sensor or transmitter array in accordance with the invention may be suited to sensing or transmitting a variety of waves, including those in a gaseous medium and those in a liquid medium and additionally in a vacuum. The types of wave include: sound in air, sound in water (sonar) and electromagnetic radiation of any frequency.

The invention is, however, particularly suited to underwater sonar receivers and transmitters, in which the sensor has very good side lobe rejection as compared to current devices.

The word "beamformer" is defined, for the purposes of this specification, as a group of at least two sensing or transmitting elements with their outputs or inputs being processed in association with one another so as to give the group a directional characteristic.

One way in which the invention can be performed will now be described with reference to Figs. 4A, 4B and S of the accompanying drawings, in which: Figure 1 illustrates a known "dunking sonar" in water suspended from a helicopter; Figure 2 is a schematic perspective view of the dunking sonar of Figure 1 shown in more detail; Figure 3 is a plan view of the known assembly of Figure 2; Figure 4A is a plan view, similar to that of Figure 3 but showing a sonar assembly in accordance with the invention; Figure 4B is an enlarged view of one of many beamformers as shown in Figure 4A; and Figure 5 is a block diagram showing how outputs of sensors of the Figure 3 arrangements are processed.

The overall physical structure of the dunking array shown in Figs. 4A, 4B and S is similar IS to that of Fig. 2, and corresponding parts are denoted by identical reference numerals.

However, the arrangement of Figs. 4A, 4B and S has three arms as opposed to five. Each arm 4 has several circumferentially extending supports 7 spaced along its length. The ends of each support 7 carry sensing elements 6A, 6B which, in combination, constitute a beamformer 6. The beamformers are spaced by a distance d, a little less than /. The sensing elements 6A, 6B are positioned at a little more than '/4i spacing.

Fig. 4B is an enlarged view of a single beamformer 6. Its directional characteristic is illustrated by the curve 8.

Fig. S shows a processor 9 which receives outputs of each of the sensing elements 6A, 6B and gives an output for the array. The processing is managed in two stages, as shown in the diagram. Stage 1 provides delays lOA, lOB for both sensors in the beamformer 6, the output of both delays being summed by an adder 11. The purpose of the delays lOA, lOB is to provide a relative delay between the two signals and, in practice, one or the other may be set to zero for any particular beamformer. The time delays chosen sets the beamformer in an "end-fire" configuration. This configuration gives the beamformer a beampattern with the directional characteristic as shown in Fig. 4B.

In stage 2, the outputs of the adders 11 of the respective beamformers are passed to variable delays 12. The delay imposed at 12 is controlled by a signal on line 12A so that the outputs from all beamformers derived from a sonar signal in chosen direction D are in phase. Thus, referring to Fig. 4A the delay 12 for any beamformer 6 is equal to the time taken for sound to travel the distance ea. lit or 12 between that beamformer 6 and a line 15 orthogonal to the direction D passing through the most downstream beamformer.

The output of each delay is then amplified by differing amounts by amplifiers 13, so as to give a characteristic "weight" to each beamformer. This technique suppresses side lobes of the array's beampattern. This is a known technique applied to current sonar arrays. The output of the amplifier 13 of each beamformer 6 is then summed by an adder 14, which gives an output for the whole array.

Mathematical predictions have suggested that a sonar array constructed generally as shown in Figs. 4A and 5 can have the most prominent side lobes with -20dB sensitivity relative to the main lobe for any given horizontal beam. Prior art dunking sonars generally have side lobe levels of up to -12dB.

Claims

1. A directional sensor comprising an array of multi-element beamformers and means for applying weights and delays to outputs of the beamformers to give a desired directional characteristic.

2. A directional sensor comprising sensor elements which can be considered to be arranged in groups, means for processing outputs of the elements of each group in one processing stage so that each group behaves as a beamformer and for combining outputs of the beamformers in another processing stage to produce an output of the sensor.

3. A sensor as described in claim 1 or 2 in which each beamformer has a significant front-to-back sensitivity ratio.

4. A sensor as described in any preceding claim in which each beamformer comprises two omni-directional sensing elements.

5. A sensor as claimed in any preceding claim in which the beamformers are mounted on arms extending radially from a central point and extend laterally with respect to the arms.

6. A sensor as claimed in any preceding claim in which the beamformers are mounted on three radially extending arms.

7. A sensor as claimed in any preceding claim where the sensor is a sonar detector.

8. A directional transmitter comprising an array of multi-element beamformers and means for applying weights and delays to inputs of the beamformers to give a desired directional characteristic.

9. A directional transmitter comprising transmitter elements which can be considered to be arranged in groups, means for processing inputs for the elements of each group in a one processing stage so that each group behaves as a beamformer and for splitting inputs for the elements in another processing stage from the input of the transmitter.

10. A transmitter as described in claim 8 or 9 in which each beamformer has a significant front-to-back gain ratio.

A transmitter as described in any preceding claim in which each beamformer comprises two omni-directional transmitting elements.

12. A transmitter as claimed in any preceding claim in which the beamformers are mounted on arms extending radially from a central point and extend laterally with respect to the arms.

13. A transmitter as claimed in any preceding claim in which the beamformers are mounted on three radially extending arms.

14. A transmitter as claimed in any preceding claim where the transmitter is a sonar source.

Amendments to the claims have been filed as follows 1. A directional sensor comprising an array of multi- element beamformers each beamformer comprising a group of at least two independent sensing elements with their outputs being processed in association with one another and means for applying weights and delays to outputs of the beamformers to give a desired directional characteristic, wherein each beamformer is mounted on an arm extending radially from a central point and each beamformer extends laterally with respect to the arm upon which it is mounted.

2. A directional sensor comprising sensor elements which can be considered to be arranged in groups, means for processing outputs of the elements of each group in one processing stage so that each group behaves as a beamformer each beamformer comprising a group of at least two independent sensing elements with their outputs being processed in association with one another and for combining outputs of the beamformers in another processing stage to produce an output of the sensor, wherein each beamformer is mounted on an arm extending radially from a central point and each beamformer extends laterally with respect to the arm upon which it is mounted.

3. A sensor as described in any preceding claim in which each beamformer comprises a circumferentially extending support upon which the sensing elements of the beamformer are carried. It

4. A sensor as described in claim 1 or 2 or 3 in which each beamformer has a significant front-to-back sensitivity ratio.

5. A sensor as described in any preceding claim in which each beamformer comprises two omni-directional sensing elements.

8. A directional transmitter comprising an array of multi-element beamformers each beamformer comprising a group of at least two independent transmitting elements with their inputs being processed in association with one another and means for applying weights and delays to inputs of the beamformers to give a desired directional characteristic, wherein each beamformer is mounted on an arm extending radially from a central point and each beamformer extends laterally with respect to the arm upon which it is mounted.

9. A directional transmitter comprising transmitter elements which can be considered to be arranged in groups, means for processing inputs for the elements of each group in one processing stage so that each group behaves as a beamformer each beamformer comprising a group of at least two independent transmitting elements with their inputs being processed in association with one another and for splitting inputs for the elements in another processing stage from the input of the transmitter, wherein each beamformer is mounted on an arm extending radially from a cer.tra] point and each beamformer exler,ds,arera]Ly Haiti-, respect to the arm upon which it 1 S noun ed.

10. A transmitter as described in any of claims 8 or 9 in which each beamformer comprises a circumferentially extending support upon which the transmitting elements of the beamformer are carried.

ll. A transmitter as described In claim 8 or 9 or 10 in which each beamformer has a significant front-back gain ratio.

12. A transmitter as described in any of preceding claims 8 to 11 in which each beamformer comprises two omni-directional transmitting elements.

13. A transmitter as claimed in any of preceding claims 8 to 12 in which the beamformers are mounted on three radially extending arms.

14. A transmitter as claimed in any of preceding claims 8 to 13 where the transmitter is a sonar source.

15. A transmitter substantially as described in any embodiment hereinbefore with reference to any of Figures 4A, 4B and 5 of the accompanying drawings.

16. A sensor substantially as described in any embodiment hereinbefore with reference to any of Figures 4A, 4B and 5 of the accompanying drawings. ll