GB2516292A - Navigation sonar - Google Patents

Abstract

A navigation sonar is described which incorporates an outboard part 20 and inboard part 22. An outboard part 20, which in use may be installed on an outboard side of a hull, comprises a transducer array (40) of sonar elements (and see figure 3), the sonar elements including a portion thereof operable both in receive and emit modes (A, figure 3), the remainder of the elements being operable only in a receive mode. The outboard part also includes signal processing (44) and conditioning elements (42) so that the driving of a sonar signal, and the detection of a resultant echo, is contained therein. The outboard part sends a digitised echo signal to an inboard part 22 of the sonar, where it can be processed and rendered into a graphical display, or used to trigger an alarm system.

Description

Navigation SONAR

FIELD

The field of the present disclosure is that of sonar technology and relates particularly, but not exclusively, to navigation sonar.

BACKGROUND

Sonar ("Sound navigation and ranging") is a widely deployed technology using propagation of sound for navigation and object detection, particularly in a marine environment.

Shipping remains a very widespread mode of transportation around the world.

Uninterrupted and efficient use of ships in transporting goods and people is vita! to world trade. One risk which presents itself to shipping is the presence of underwater obstacles, natural or otherwise, which can give rise to the possibility of a ship running aground. Such events frequently occur around the world, causing significant damage to ships, and rendering them immobile for extended periods. This can cause significant cost to ship owners (and/or insurers) in repair and consequential loss of income.

Human error, instrumentation failure, and chart inaccuracy all contribute to the risk of running aground.

There are few known systems available for use, which address this specific problem.

Suppliers of Obstacle Avoidance systems often claim to provide some capability but these fall short of the real operational environment. This is because Obstacle Avoidance sonars are optimised for detection of point objects anywhere in the water column, using techniques which help reduce reverberation from the sea surface and seabed, in other words, short FM pulses.

Passive sonar operates on the basis of detection of ambient sound and deducing therefrom the existence of sound emitters, such as other marine vessels. Active sonar, on the other hand, operates by way of the emission of a controlled burst of sound waves, and then detection of reflections of that sound wave from nearby objects. The interaction of sound waves with various fluid/solid (or fluid/fluid) boundaries is well understood, and prior knowledge of the manner in which a sonar emission will be affected by reflection at such a boundary can impart information to the receiver as to the nature of the boundary, and the distance of the boundary from the emitter/receiver.

It is also known to provide an array of individual transmitter elements, to provide a facility for steering and/or beamforming a sonar emission, and to beaniform reception of sonar reflection soundings. For instance, International patent application W02008/0621 56A1 describes a forward facing sonar system with a transmitter comprising an elongate array of ultrasound transmitter elements, intended to be mounted substantially vertically on the bow of a ship, and a receiver comprising an elongate array of ultrasound receiver elements intended to be mounted substantially horizontally on the bow. By steering both the transmitted ultrasound signal and the directional sensitivity profile of the composite receiver defined by the receiver elements, a 3D visualisation can be built up of the observed space covered by the sonar system.

See Echo", an imaging sonar product offered by Marine Electronics Limited, Barras Lane Industrial Estate, Vale, Guernsey, GY6E 8EQ, is another example of a forward looking active underwater acoustic device. This emp[oys the Mills Cross" arrangement of separate line arrays (aligned orthogonally) as detailed in the above referenced international patent application, to provide a 3D real time display of underwater terrain ahead of a vessel to which it is mounted. By this, improved information can be offered to a captain of a marine vessel in order to improve the prospect of ground avoidance and detection of submerged objects.

The Marine Electronics product is a wideband CW system, producing a series of sequential transmit beams of relatively short duration (generally in the order of 2ms) at different frequencies, and vertical steer from a vertical line array around a centre frequency of approximately 75kHz.

The directional sensitivity of the receiver is provided by use of a horizontal line array.

Considering the directional sensitivity of the receiver as a "receive beam", this beam has a large vertical beamwidth and narrow azimuthal beamwidth. This enables a wide range of detection in the relevant axis, with relatively fine granularity, by virtue of the fact that each element can distinguish horizontally effectively.

The Marine Electronics system is claimed to have a range of up to bOOm for depth determination and detection of small objects. However, it is evident that the Marine Electronics system, and that proposed in the earlier referenced international patent app}ication, have certain drawbacks.

Due to the Mills Cross arrangement of separate line arrays, the footprint of the outboard elements of the sonar system is relatively large. The two orthogonally arranged elements cannot be miniaturised without sacrificing beamwidth or accuracy.

Further, the sequential steered transmission limits the amount of energy transmitted in any particular direction. This will limit the detection range in high noise conditions such as those experienced at speed. As marine vessels are being developed which are capable of relatively high speeds, this presents a significant technical problem to the designer of a sonar system.

Embodiments described herein may present a solution to the problem of long range detection of sea bed and surface at high self noise conditions prevalent when ships are travelling at speed.

An aspect of the invention provides a sonar comprising a transducer assembly, the transducer assembly comprising an array of sonar transducers, the transducers being arranged such that, in use, they form a plurality of columns, and wherein a portion of the transducers are configured for both reception and transmission of sonar signals.

An aspect of the invention provides a sonar comprising a transducer assembly, the transducer assembly comprising a plurality of transducer elements, wherein some of the elements are solely operable to emit a sonar signal while others of the elements are operable to emit and detect sonar signals.

An aspect of the invention provides a sonar transducer assembly, for mounting on the hull of a marine vessel, the assembly comprising a rectangular array of sonar transducer elements, the elements being arranged in rows, said elements being operable to detect sonar signals and wherein, in a subset of said rows, the subset not encompassing the whole array, said elements are further operable to emit sonar signals.

An embodiment of the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a marine vessel with a sonar system in accordance with a described embodiment in situ; Figure 2 is a schematic diagram of an outboard unit of the sonar system illustrated in figure 1; Figure 3 is a schematic diagram of a transducer array of the outboard unit illustrated in figure 2; Figure 4 is a schematic diagram of a signa] conditioning unit of the outboard unit 1 5 illustrated in figure 2; Figure 5 is a schematic diagram of a signal processing unit of the outboard unit illustrated in figure 2; Figure 6 is a schematic diagram of an inboard unit of the sonar system illustrated in figure 1; Figure 7 is a schematic diagram of system architecture established by execution of an inboard unit program of the inboard unit illustrated in figure 6; Figure 8 is a flow diagram illustrating signal processing by the inboard unit illustrated in figures 6 and 7; and Figure 9 is a flow diagram illustrating data processing by the inboard unit illustrated in figures 6 and 7.

Figure 1 illustrates a sonar system in situ on a marine vessel 10. The sonar system in accordance with the disclosed embodiment is characterised as a forward looking sonar comprising an outboard unit 20, an inboard unit 22 and an interface unit 24. The outboard unit 20 is mounted rigidly at the front of the vessel 10 with a fixed depression and ideally at a minimum depth of one metre below the vessel's water line. The system has a wide horizonta] and narrow vertical field of view. The interface unit 24 provides power isolation and is mounted dryside' in close proximity to the outbDard unit 20. The inboard unit 22 is mounted on a bridge 12 of the vessel 10.

A fibre optic cable link 26 provides connection between the outboard unit 20 and the interface unit 24, and a further fibre optic cable link 28 provides communications connection between the interface unit 24 and the inboard unit 22.

An electrical power supply 30 which1 in practice, will be driven by the ship's source of motive power, provides power to the interface unit 24 and the inboard unit 22. The interface unit 24 provides power conversion and isolation for provision of an isolated power supply to the outboard unit 20.

While in this embodiment the provision of signal communication between the interface 24 and the inboard unit 22 is by fibre optic means, it will be appreciated that a copper wire electrical link could alternatively be provided. In such circumstances, means woutd be provided in the interface to convert between electrical and fibre optical based signal transmission, as it is desirable that signal transmission across the dryV'wet' boundary be by non-electrical means. However, the reader will note that fibre optic communication is usual for communication over relatively long distances, and will be less susceptible to interference than copper cable. Transmission over the dry/wet boundary would normally be by copper cable due to difficulties of fibre optic connection in this environment.

Unlike mine warfare products where there is a requirement to reduce reverberation in order to enhance the detection of smalF objects, the requirement for a grounding avoidance sonar is to promote sea bed and surface reverberation. In order to achieve this, and to maximise the sonar range, the system employs a narrowband, long continuous wave (CW) purse. The fad that the pulse is narrowband will serve to reduce the resultant noise level; its characteristic as a long CW pulse will impart high levels of sound energy into the water to increase the bottom and surface reverberation levels.

The outboard unit 20 is illustrated schematically in figure 2. In terms of practical detail, the outboard unit 20 comprises a metal enclosure, which can be designed for watertight integrity. Within the enclosure, the outboard unit 20 comprises a transducer unit 40, a signal conditioning unit 42 and a signal processing unit 44. The construction of the transducer unit 40 will be described in due course but, for the benefit of the reader, it comprises a rectangular array of transducers. A subset of the transducers can switch between transmitting and receiving sonar signals, while the remainder are functionally dedicated to receiving sonar signals. That is, all of the transducers can be used for receiving sonar signals, while a subset can also be used for emitting. As such1 and as indicated in figure 2. the transducer unit 40 is shown with a direct signal feed to the signal conditioning unit 42, but also with a bidirectional feed with a transmit/receive switching unit 46. This is indicative of the dual function of a portion of the transducers of the transducer array.

The signal conditioning unit 42 performs amplification, filtering and digitising of signals fed thereto from the transducer unit 40 (either directly or via the switching unit 46).

Resultant signals are fed to the signal processing unit 44, which performs scaling, heterodyning and digital filtering on the signals. The processed signals are passed to the interface 24 previously described.

A system controller 48 provides general system control activities including but not limited to an operating system, communication, programme control/timing and data storage.

The system controller 48 controls the signal conditioning unit 42 which, on receiving trim information from an attitude sensor 50, controls beamforming of receive beams developed at the transducer unit 40 in receive mode.

In a transmit mode, a signal is generated by a power amplifier 56, powered by an energy store 54, and thirty-two acoustic signals are passed to a matching unit 58. The matching unit 58 provides a matching between the power amplifier 56 and the array 40.

The matched acoustic generation signals are passed by the matching unit 58 through the previously mentioned switching unit 46 which then switches the lower third of the transducer array (i.e. 32 elements thereof) to a transmit mode, driven by the acoustic signal generation signals.

The attitude sensor 50 provides vessel positional (attitude) information into the Sonar System. In particular, it feeds attitude information into a pulse storage and generation unit 52, which performs a waveform storage and transmit pulse generation function.

This function provides a waveform storage capability and provides the capability to produce the digital waveforms that are suitable for driving the transducer array to provide the sonar system required transmit pulse.

The energy store 54 provides a constant high voltage power source for the transmitters of the transmitter array 40. It draws energy from the ship's power supply. via the interface 24. Using the energy store 54 as a source of energy, the power amplifier 56 amplifies the digital waveforms provided by the pulse storage and generation unit 52 to provide a high voltage digital waveform for use in generating a sonar emission, This high voltage signal is passed to the signal matching unit 58, which performs a signal matching function between the power amplifier 55 and the transducer unit 40 (via the switching unit 46). under control of the pulse storage and generation unit 52. In essence, it looks to ensure that the amplified signal matches the originally stored and generated digital waveform. This is essential]y to reduce losses but it also performs some filtering of the signal.

The transducer unit 40 will now be described in further detail with reference to figure 3.

As shown in figure 3, the transducer unit 40 comprises a transducer array 60 composed of 96 sonar transducer elements 64, arranged in sx rows of sixteen. Each row constitutes a transducer module 62. In operation, the modules 62 are substantially horizontal, such that reference can be made to a top row and a bottom row hereinafter.

All of the transducer elements 64 are operable to detect acoustic signals, such as sonar emissions, and to convert the same into electrical signals. The lower two modules 62 (indicated by Fetter A in figure 3) are configured also to selectively emit acoustic energy, on being driven by driver signals.

Thus, as referred to previously, the two lowermost transducer modules 62 are dual purpose. By this, the array 60 is more compact than one which employs dedicated and separate transmit and receive elements, It is possible to use a smaller array than with previous deployments, with consequently reduction in the size of outboard equipment.

This has a self evident impact on shipbuilding and fitting. Further, it is possible that the a detection range of such equipment will be higher than with previous implementations, through provision of a higher signal excess over noise.

To achieve a wide horizontal transmit beam, the beam is defocused to achieve wide coverage. The transmit vertical beam is fixed with a narrower vertical receive beam which is steered based on the position of the vessel. This is achieved through an internal attitude and heading reference system.

The use of a two dimensional array, to reduce outboard footprint! reduces the height of the array and consequently reduces the vertical directivity index (Dl), in comparison with a purely linear array. However, this is traded against a longer pulse. This longer pulse increases reverberation, and results in a narrower bandwidth. The larger area of the receiver array produces a higher Dl which results in lower noise. These all result in a greater signal excess over noise generated in conditions such as would arise if a vessel to which the array is fitted is travelling at high speed. It also provides the ability to detect seabed and surface returns in shallow water at greater range.

With reference to the enclosure of the outboard unit 20, the transducer array 60 is positioned at a front face of the enclosure, to enable emission of acoustic energy into water. The active face of the array elements are encapsulated in a waterproof material that maintains the watertight integrity of the unit. The rear of each transducer module 62 has a connector (not shown) which links to each of the 16 elements. In use, the enclosure is to be fitted to the hull of a vessel, beneath the water line, so that the aforementioned front face is presented outwards of the vessel. In some cases, the enclosure could be incorporated into a cooperative formation of the hull of the vessel, so that the front face of the enclosure is substantially in line with the envelope of the hull.

The frequency of operation is centred at 70kHz and has a minimum bandwidth of 25kHz centred on the centre frequency.

The horizontal 3dB beamwidth of each element is greater than 60 degrees at the centre frequency. With the output from all elements summed the array produces a beampattern with a nominal horizontal 3dB beamwidth of 3.9 degrees at the centre frequency.

The vertical 3dB beamwidth of each e]ement is less than 10 degrees at the centre frequency. With the output from all elements summed the array produces a beampattern with a nominal vertical 3dB beamwidth of 1.5 degrees at the centre frequency.

The signal conditioning unit 42 will now be described in further detail, with reference to figure 4. The function of the signal conditioning unit 42, as noted above, is to performing amplification, filtering and digitising on the 96 signals received from the transducer unit 40. Firstly, an array of pre-amplifiers 70 pre-amplify each receive channel and then the amp]ified signals are sent to a corresponding array of bandpass filters 72 to reject noise outside of the receiver bandwidth. The conditioned signals are then presented to buffers 74 which buffer the receive channels, and then these are passed to ADCs 76, to enable conversion thereof from analogue to digital.

For convenience, dual channel 24 bit ADO devices are employed, leading to 48 digitised streams. The ADO sample frequency is 192kHz and with a transmission frequency of 70kHz and a maximum echo freqLlency of about 71.4kHz with a ship's forward speed of 3okts the sample rate requirement is met (Nyquist).

A multiplexer 78 then acquires the digitised signals and converts these to a serial data stream. This serial data stream is then passed to the signal processing unit 44, which will now be described with reference to figure 5.

The signal processing unit 44 is configured to provide scaling, heterodyning and digital filtering functions. A scaling unit 80 scales (from 24 to 16 bit) each received channel, and the scaled data is basebanded in a heterodyne down-mixer 82 using complex heterodyning followed by low pass filtering in a low-pass filter 84.

The purpose of this process is to reduce the sample rate whilst retaining all of the information contained in received echoes. The complex heterodyne frequency is the mean of the transmission frequency and the maximum echo frequency received at maximum forward speed. The maximum echo frequency arising from the 70kHz transmission frequency at 3Okt ship's speed is 71.4kHz. Using 70.7kHz complex heterodyne frequency the basebanded echo bandwidth extends from -0.7kHz to +0.7kHz. The low pass filter 84 embodies decimation in order to minimise the output sample rate and the ratio is 80:1 so that the filter output sample rate is Z4kHz.

The decimated signal data is then passed to the system controller, which converts the data to a Gigabit Ethernet output data stream suitable for connection to the inboard unit 22 via the interface 24.

The inboard unit 22 will now be described in more detail with reference to figure 6. In this embodiment, the inboard unit 22 is implemented by way of a general purpose computer 100, with a visual display unit (VDU) 102, a keyboard 104 and a mouse 106.

This stand-alone solution will be understood by the reader to be but one possible implementation of the embodiment, and others, such as client/server based arrangements, would also be possible. Nowadays, tablet or smartphone implementations may also be possible.

The provision of suitable functionality to the Inboard unit 22 is described herein in the context of configuration of a general purpose computer configured by program executable instructions. Software products may provide these instructions, either as an integral program providing all facilities or by way of a program which calls for use of pre-existing software facilities on the computer in question, such as in the form of an operating system, libraries such as dynamic linked libraries, or master programs of which a further, introduced, software product is but an update or plug in. The skilled reader will appreciate that the invention is not limited to any one implementation.

Moreover, the skilled reader will appreciate that one or more functionalities associated with a disclosed or envisaged embodiment may involve provision of firmware, br hardware. Such implementations are entirely within the scope of the present

disclosure.

The computer 100 comprises a processor 120, with access to a working memory 124 and a mass storage device 122. The mass storage device 122 may be a hard drive and/or solid-state storage devices such as Flash memory.

The processor 120 also has access, via a bus 126, to a keyboard interface 130, connected to the keyboard 104, operable to receive and buffer keystroke signals indicative of user input action at the keyboard 104. A mouse interface 132 is provided an the same basis. Ihe reader will appreciate that these two interfaces may be combined in some implementations and that other forms of user input interface exist and may be equaFly appropriate.

A display driver 134 provides hardware/firmware support for the provision of VDU signals for establishing a display on the VDU 102.

An audio output driver might also be provided, such as to provide a user with warning alert sound effects, such as in case an obstacle has been detected in respect of which action is required.

A communications driver 136 enables establishment of Ethernet communications via the interface 24 with the outboard unit 20. The communications driver 136 may provJde other communications functionality also, such as communication with other computers on ship with other ships or with land, air or satellite based systems. These are not relevant to the present disclosure and so will not be described further.

All of the above components are under the command and control of the processor 120.

The behaviour of the processor 120 is itself governed by the execution thereof of computer program instructions. Computer program instructions are, for convenience, stored in the mass storage device 122 and, to enable execution thereof, may be loaded into the working memory 124 at the time of execution -it being understood that the working memory may be of lower storage capacity than the mass storage device but faster to access. To illustrate this, a computer program is illustrated as being stored in working memory, namely an inboard unit program 140.

The end effect of the running of the inboard unit program 140 is the ability of the computer 100 to implement the inboard unit 22. In terms of function and the passage of data through the computer 100. the computer develops a functional architecture as illustrated in figure 7. That is, a signal processor 150 receives a data stream from the outboard unit 20, and the data stream is processed thereby, involving 213 beam forming, spectrum analysis and peak picking.

The resultant signals then pass to a data processor 152 which is operable to process the received signals to form a map of a seabed, and certain depth estimations, for further use. The resultant mapping data is then passed to a display processor 154, which applies input threshold data, colour coding preferences and the like to present a display output to the VDU 102.

A system controller 160 interacts with the signal processor 150, the data processor 152 and the display processor 154, to provide appropriate presentation of an image at the VDU 102. Moreover, information fed back from the various processors 150, 152 and 154 to the system controller 160 can give rise to output of an indication of a hazard, such as the relative range and bearing thereof.

[he signal processor 150 performs beamforming, spectral analysis and peak picking.

Figure 8 illustrates the operation of the signa] processor 150 on received data, Two dimensional basebanded beams spanning the sonar receive angular coverage are formed using the basebanded receive channel data (step S 1-2). Since the receiver is narrowband (the echo bandwidth is about 2% of the transmission frequency) each beam sample is formed as the appropriately phase and amplitude weighted sum of the corresponding samples from the basebanded receive channels. The errors arising from such an implementation compared with a full time or frequency domain implementation are insignificant. The phase and amplitude weights are selected to steer each beam in the appropriate direction whilst maintaining appropriate balances between mainlobe widths and sidelobe levels. In this instance 4096 samples per beam, corresponding to a range interval of 1280 metres are generated.

The 4096 beam samples are organised into 32 50% overlapped blocks each containing 256 samples. The block interval is 256/2 = 128 samples or 40 metres in range when the speed of sound is 1500m5t. Therefore the final output is 32 samples per beam at metres range resolution. Each block is input to an FF1 (step S1-4). The magnitudes of the FFT outputs are calculated and the peak magnitude of each FF1 is recorded (step S1-6). The recorded peaks in each beam represent the estimated reverberation level in the beam as a function of range.

[he data processor 152, as mentioned above, performs depth estimation and sea bed mapping. Each of the 32 horizontal beams has an associated set of 32 vertical beams and the following process is carried out on each of these sets to estimate the position of the sea surface and the sea bed. Figure 9 iltustrates the steps performed by the data processor 152 on the data.

For each range step there is an array of comprex data with one number for each beam.

The form of data for a single point target alone is known and this can form a template in a model-based algorithm. In this implementation, there are two known targets and these are the sea surface and bottom and any additional noise. The location of the surface is known, however the location cf the bottom is unknown and is to be found.

The strength and relative phase of the two targets are not known.

Each separation (in beam steps) is trialled using the template. A least squares fit with the data is tound (step S2-2) which estimates the target amplitudes and most importantly, a goodness of fit measure for each separation (beam) is calculated and provides as output complex amplitudes. Using the Fowest value from the net square error resulting from each trial fit, the lowest number indicates the beam position of the sea surface and sea bed (step 82-4).

The display processor 154 performs thresholding, colour coding and contour mapping.

The input data for each beam will then be colour coded depending upon depth thresholds defined by the user. This range-versus-bearing data will then be displayed to the user. The data would be presented as either a plan position indicator (PPI) or B-scan at the display.

A system as described above makes use of a long CW pulse at low frequency, thus maximising reverberation, and hence detection of sea surface and seabed. This can achieve a range of up to 1 000m in 4Dm water depth.

In a large commercial sea-going vessel, such as a cruise ship or a passenger ferry, equipped with a sonar in accordance with an embodiment described herein, with Fie]d of View of 600, it is expected that the system will be operational up to at least a vessel speed of 30 knots (approximately 15.4 mIs) to provide information to an operator, to enable that operator to take evasive action, with regard to operator reaction time. This provides an adequate operational limit which will not, in most circumstances, impose a constraint on operation of the vessel.

Such a system can be installed simply in a retro-fit manner to an existing vessel. By enabling assembly of the system from three operational units, each connectable to another by a single connection, eases installation further.

The reader will understand that the foregoing description is of a navigation sonar system provided as an aid to navigation. As such, it is not expected that an operator will need to continually man the navigation sonar system. The system as described above can be continually functional, raising an alarm (such as an audible alarm output) whenever an area of seabed be detected which could cause a heightened risk of grounding or collision. This would then cause an operator to examine a display of the sonar system, determine the cause of alarm, and take any evasive action deemed necessary in the circumstances. The system could, in such circumstances, provide the operator with a facility to cancel such an alarm. Cancellation of an alarm might be accompanied by an authorisation check, and the system might record information such as identifying the person making the cancellation request, in case such information might be required by a third party monitoring operation of the vessel at a later time.

The system as described above does not rely on input from another system to provide position sensing, or ship speed, to enable cancellation of Doppler shift which would otherwise distort sonar readings.

The outboard unit as described includes a miniature attitude and heading device. This a]lows the sonar system to compensate for varying pitch and trim.

In general terms, the described embodiment comprises a forward Idoking sonar capable of assessing water depth in a wide azimuthal view, out to a long range ahead, to enable a crew of a ship travelling at speeds up to maximum operational speeds (30 knots being above operational maximum for many sea vessels), to which the sonar has been fitted, to be warned of hazardous areas of shallow water (or other grounding risks) ahead.

Certain advantages of embodiments such as disclosed herein include the fact that a simple signal processing technique can be employed, thus reducing the need for sophisticated processing hardware. Embodiments provide easy Doppler determination for use in such processing. Embodiments provide reduced need for sizable outboard equipment on a ship: outboard equipment in line with an embodiment described herein may be made more compact than previous examples in the field of the art.

It may be appreciated by the user that, for a transmit array surface in accordance with an embodiment as described herein, in comparison with transmit arrays as previously disclosed in the art, performance is substantially six times worse with respect to vertical Dl, though the performance with regard to energy transmitted into water is substantially fifty times better. This is due to the increased pulse length employed. The resultant overall gain is thus substantially eight times (i.e. 9dB) for the transmit performance.

In terms of the performance of the receive array, the area concerned is substantially five times larger than for comparable prior art examples, and hence the noise performance is substantially 7dB better.

The pulse bandwidth for embodiments disclosed herein is fifty times smaller than for the comparable example from the prior art, and hence the noise performance is substantially 1 7 dB better.

Overall, the performance improvement for both the transmit and receive operations, in comparison with previous devices, is 33dB.

Operational exampj To demonstrate the potential capability of embodiments such as those described herein, an operational analysis will now be described. The purpose of this analysis is to show the skilled reader how requirements can be determined, for a specific implementation, for acceptable performance of a sonar used for the stated purpose.

This is achieved using mathematical modelling. The vessels to be modelled are commercial and include cruise ships passenger ferries including RORO (roll on, roll off) ferries, and so on.

Simple modelling can be used to demonstrate the ability of a ship to avoid obstacles detected by a sonar. The model simulates movement of the ship, as the sonar system detects an object, allowing for operator decision time, time for an operator to initiate evasive action (such as a turn) and execution of the turn. During movement of the ship, the position of the ship's centre of gravity, position of the sonar array, and the sonar coverage are modelled.

By the model, Ft is possible to identify the range and Field of View which are suitable for the Navigation Sonar arid to estimate the operational performance of the Navigation Sonar when used on the targeted ship types.

The mode] assumes that the speed of the vessel is constant throughout the manoeuvre, and equal to the approach speed. That is, it is assumed that no deceleration takes place through the turn. It also assumes that the movement of the vessel is a straight line followed by a circular motion.

The conclusion from the modelling shows that for most vessel types with speeds up to 30 knots (15.4 mIs), a sonar Field of View of 60° and range of 75Cm, is adequate to enabe a vessel lo take avoidance action to manoeuvre around shallow areas or obstacles, Sonar modelling can also be carried out using a proprietary performance prediction tool, details of which are not essential to an understanding of the present disclosure.

This tool uses sonar, environmental, target and ship parameters to predict sonar performance against range for the specified parameters. Key features of this modelling tool provide the ability to realistically predict performance and allow for example, hydrophone efficiency, receiver noise etc, calculate reverberation and target multipaths, calculate spreading loss in non iso-velocity environments and model environmental boundary conditions (for example, sea-surface boundaries). This allows the optimisation of the system characteristics with respect to the expected environment.

The reader will observe that a Navigation Sonar in accordance with the described embodiment uses a long CW pulse to maximise reverberation of the sea surface and seabed thus allowing better detection of these boundaries.

During any high speed transit of a vessel it is assumed that the sonar system is turned on and the ship is travelling at maximum speed. The operator selects a depth threshold based on knowledge of the area (from charts, echo soundings and measurements etc) and water draft and any likely squat. The system is intended to alert the operator when there is an area ahead of the ship that is estimated to have a water depth shallower than the depth threshold. The operator then monitors the sonar dispray and can change the depth threshold as necessary as well as obtain estimates of the water depth and some measure of the accuracy (resolution) at manually marked points on the screen (note accuracy will improve at shorter ranges). The estimated time to close (possibly to within a defined range) can also be obtained from the dieptay.

Based on the information gathered from the sonar display, the operator can decide if precautionary/avoidance action (slowing turning etc) is required and can act accordingly.

WhiFe certain embodiments have been described, these embodiments have been presented by way of exampte only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms: furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the sprit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims (29)

1. CLAIMS: 1. A sonar transducer device comprising a plurality of transducer elements arranged in a rectangular array comprising juxtaposed rows! each row comprising the same number of transducer elements, each transducer element being operable, in a first mode, to generate an acoustic detection signal on the basis of acoustic energy incident thereon, and wherein, in a subset of the rows, the transducer elements are further operable in a second mode to emit acoustic energy in response to a generation signal applied thereto, whereas, in the rows not in the subset, the transducer elements are not operable in the second mode.
2. 2. A sonar transducer device in accordance with claim 1 and further comprising a mode switch operable to switch operation of said transducer elements of said subset of rows between the first mode and the second mode.
3. 3. A sonar transducer device in accordance with claim 1 or claim 2 wherein the rows of said subset are contiguous.
4. 4. A sonar transducer device in accordance with any preceding claim wherein the rows not in said subset are contiguous.
5. 5. A sonar transducer device in accordance with any preceding claim wherein the number of rows not in said subset s substantially double the number of rows in said subset.
6. 6. A sonar transducer device in accordance with any preceding claim wherein the number of rows in the device is between 2 and 3 times the number of transducer elements in any one row.
7. 7. A sonar transducer device in accordance with any preceding claim wherein the number of rows in the devrce is 2.7 times the number of transducer elements in any one row.
8. 8. A sonar transducer assembly, for installation outwardly of a maritime vessel, comprising a sonar transducer device in accordance with any one of the l9 preceding claims! the assembly being configured such that the rows of the sonar transducer device are oriented so as to be substantially horizontal with respect to the vessel, in use.
9. 9. An assembly in accordance with claim 8 and further comprising a driver for driving the transducer elements within the subset to generate an acoustic signal, and a detector operable to detect, at any of the transducer elements-signals responsive to acoustic energy incident on said transducer elements.
10. 10. An assembly in accordance with claim 9 and further comprising a switch operable to switch operation of said transducer elements within said subset between said first mode and said second mode.
11. 11. An assembly in accordance with c]aim 10 wherein the detector comprises a signal conditioning unit operable to receive a plurality of input signals from The plurality of transducer elements, each input signal being associated with acoustic energy detection by a respective transducer element! the signal conditioning unit being operable to process received input signals into a stream of serial signal data bearing information corresponding to the acoustic energy detection at the transducer elements.
12. 12. An assembly in accordance with claim 11 wherein the detector further comprises a signal processing unit operable to receive a serial data stream from the signal conditioning unit] and to generate a processed output data stream on the basis thereof
13. 13. An assembly in accordance with claim 12 wherein the signal processing unit is configured to receive binary serial data from the signal conditioning unit, and further comprises a scaling unit operable to scale received binary serial data from a first word length to a second word length, the first word length being larger than the second.
14. 14. An assembly in accordance with claim 13 wherein the first word length is 24 bits.
15. 15. An assembly in accordance with claim 13 or claim 14 wherein the second word length is 16.
16. 16. An assembly in accordance with any one of claims 12 to 15, wherein the signal processing unit comprises a down-mixer, operable to process the binary serial data so as to demodulate a signal of which the binary señal data is representative from an acoustic carrier frequency to a baseband frequency, thereby resulting in down-mixed binary serial data.
17. 17. An assembly in accordance with claim 16 wherein the down-mixer is a heterodyne down-mixer.
18. 18. An assembly in accordance with claim 16 or claim 17, wherein the signal processing unit further comprises a low pass filter, operable to filter the down-mixed binary serial data, to produce a processed output data stream.
19. 19. An assembly in accordance with any one of claims 8 to 18 and further comprising an emission driving unit operable to drive emission of acoustic signals by the subset of transducers operable to do so.
20. 20. An assembly in accordance with claim 19 wherein the emission driving unit comprises an energy store operable to store energy for use in generating an acoustic emission, a pulse generator operable to generate a pulse waveform for application to an acoustic frequency signal, and a power amplifier driven by energy stored in the energy store and operable to generate. for each emitting transducer, a driving signal comprising said acoustic frequency signal with said generated pulse waveform imposed thereover.
21. 21. An assembly in accordance with claim 20 wherein the emissron driving unit further comprises a matching unit operable to match signals generated by the power amplifier to the subset of transducers.
22. 22. An assembly in accordance with any one of claims 8 to 21, and further comprising an attitude sensor operable to provide vessel attitude information into the assembly.
23. 23. An assembly in accordance with claim 22 when dependent on craim 20 wherein the attitude sensor is operable to feed attitude information into the pulse generator.
24. 24. An assembly in accordance with claim 23 wherein the pulse generator comprises a pulse waveform store, and wherein the pulse generator is operable to select a pulse waveform from the pulse waveform store on the basis of received attitude information.
25. 25. An assembly in accordance with any one of claims 8 to 24, and further comprising an interface, the interface offering a facility for interconnection of the assemb]y with in-board facilities of a vessel to which the assembly is installed in use.
26. 26. An assembly in accordance with claim 25 when dependent on claim 20 wherein the energy store is connected to the interface, to enable the energy store to source electrical energy from an electrical energy store of a vessel to which the assembLy is installed in use.
27. 27. An assembly in accordance with claim 25 or claim 26, when dependent on claim 12, wherein the interface offers an interconnection facility with a vessel to which the assembly is instaLled in use, for the output from the assembly of the processed serial data stream.
28. 28. A navigation sonar system comprising an assembly in accordance with any one of claims 8 to 27, and an in-board control unit, the in-board control unit being capable of receiving a serial data stream from the assembly and comprisFng a data processor operable to process the serial data stream into a seabed contour.
29. 29. A system in accordance with claim 28 wherein the data processor of the in-board control unit comprises a beamforming unit operable to process the serial data stream by extracting, for each transducer element, a plurality of samples over time and to convert these into a plurality of beam signals for each time sample.3D. A system in accordance with claim 29 and further comprising transform means, the transform means being operable to transform the time sampled beam signals into a plurality of beam signals in the frequency domain.31. A system in accordance with claim 30 and comprising a peak picker, the peak picker being operable to identify value ranges, within each frequency domain beam signal, and thereby to develop a seabed contour.