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Towed Array

Several underwater sonar applications exist for steered directional acoustic beams. Towed multi-line acoustic arrays, also known as streamers, are known. Such acoustic arrays are used to detect ships, marine life, marine geology, etc. Streamers also have military applications. The node points of the acoustic array (also referred to as a lattice) are comprised, for example, of hydrophones that receive acoustic energy. The acoustic energy detected by the hydrophones may be generated by the feature detected itself, such as a marine animal. Alternatively, the acoustic energy may be a reflected signal, emitted by an acoustic source, such as a sparker or a boomer.

A towed array comprises a hose of an elastomeric material, for example rubber, having an outside diameter of about 2 to 4 inches, which may be several miles long, and may be towed by a submarine. Inside the hose, or cable, are many transducers, perhaps as many as 2,000. There are many wires because the transduced acoustic energy must transmit the corresponding electrical signals to the support vessel. Since one wire for each transducer is impractical, the towed array includes a coaxial cable and telemetering equipment, including amplifiers for amplifying the signals.

Typical sensors used in today's towed line arrays consist of solid piezoceramic spheres or cylinders. However, in some special cases they consist of fiber optic cables wound onto mandrels or miniature flextensional transducers. In any case, the element dimensions are small compared to the acoustic wavelength, and thus these sensors operate below their first mechanical resonance.

Submarines deploy fat-line towed-arrays using a process known as flushing, wherein water is pumped into the fat-line stowage tube to exert pressure upon and hence deploy the fat-line towed-array. Deployment success can be determined by measuring the flushing water force applied to the fat-line towed-array. Since effective deployments are critical to successful submarine missions, it is essential to maintain a method to evaluate the flushing mechanics and effectiveness. There is currently no reliable method to evaluate a submarine's flushing procedure.

Submarines deploy thin-line towed-arrays using mechanical handling systems. A thin-line array element includes an outer sheath or hose that contains hydrophones and supporting electronics. When the towed-array is deployed or retrieved, it is fed through a guide tube by a handling system. There is a great interest to quantify the handling system effects on the thin-line towed-array and its internal elements. Such quantitative information is useful for thin-line towed-array maintenance scheduling and design. By knowing the amount of stress applied to the thin-line towed-array during a typical deployment or retrieval process, faults may be predicted more accurately. Additionally, new sensor and material durability may be evaluated against existing designs.

Tactical towed-array sonar systems (TACTAS) are passive sonar systems that can provide much longer-range detection of submarines than is normally possible with active sonars. Towed-array sonar systems consist of a long linear array of hydrophones towed well behind a ship by a wire, together with sophisticated electronic equipment aboard the ship for analyzing the signal from the hydrophones. They offer the surface ship, for the first time, the possibility of achieving parity with the submarine in passive listening capability.

The long-range detections made possible by towed-array sonar systems will be of limited value, however, without a means of localizing and attacking enemy submarines - the function performed by helicopters and/or other ASW aircraft in the vicinity of the towed-array ship. Together, these can extend the surface combatant's ASW engagement range to something more commensurate with that of modern submarine weapons.

Conventional towed arrays are generally built of ceramic piezoelectric transducers that are distributed and mounted within a tubular sheath. The construction, size (including diameter) and spacing of these ceramic transducer elements define the frequency band of operation of the array in a water medium. Conversely, the required operating frequency places restrictions on the minimum dimensions achievable using ceramic type array elements. The sheath type array is generally filled with any of several types of acoustically transmitting materials which provide structural integrity together with some measure of isolation from noise-producing turbulent flow. Since array diameter has a direct correlation with turbulent flow, it is desirable for the purpose of further reducing the noise effects of this turbulent flow to have the diameter of the array be as small as possible. Most present thin-line sheath type arrays have minimum diameters of approximately one inch.

Furthermore in present towed arrays, in order to provide steerable beams, the amplitude and phase information from each array transducer element must be individually transmitted to beamformer electronics external to the array. This requires that along with the transducer elements themselves, at least one telemetry wire for each element must be packaged within the array sheath. In addition, other wires are required to deliver power to each piezoelectric ceramic array element. The need for such power supply and telemetry wires places further constraints on the minimum array diameter achievable and contributes to the very high production costs. Telemetry schemes that reduce the number of wires and hence wire bundle diameter require complicated circuitry to be contained in the array proper, thus again limiting the minimum cost and diameter achievable while potentially decreasing reliability. In summary, present towed arrays have larger than desired diameters and must contend with the concomitant high flow noise associated therewith, have high manufacturing costs and provide less than desired reliability.

Although the current towed systems provide spatial discrimination in the direction of tow, a left right ambiguity exists due to the omni-directional sensors employed. Given the orientation of the array in revolution, a directional sensor would resolve this ambiguity and provide essentially three-dimensional spatial discrimination.

The data detected by the hydrophones is centrally processed to provide a detection, mapping, etc. of the feature. The hydrophones are connected through an electronic and processing backbone, which serves to coordinate and process the data received by the array. For example, if a boomer is used, timing is coordinated between the chirp of the boomer and the listening of the hydrophones. The waveform and timing of the acoustic energy received by the hydrophones is analyzed by the processing backbone to determine if it represents the same feature. In addition, once a feature is identified, its position can be determined by virtue of the timing and the relative positions of the hydrophones in the array. As is known in the art, this can be readily accomplished via generation of a set of simultaneous equations based upon the distance determined via the time of receipt of the acoustic signal by each hydrophone.

Prior art towed arrays are plagued by various problems. One problem is boundary-layer noise, another one is internal noise caused by waves propagating inside the array from one transducer to another, caused by flow through the hose that the towed array is made of, or by "cable strumming" caused by towing the cable through the water. Towing the cable may cause the cable to vibrate, and the vibration is coupled into the array, and picked up by the transducers in the array.

A fundamental problem is that the positions of the hydrophones or other items comprising the nodal points of a towed acoustic array are inherently unstable. Because of currents, position in the array, speed of the boat, or any other of myriad influences, the relative positions of the nodal points change continuously over time. It is thus important to also continually monitor the relative positions of the nodal points of the array. Various systems are used in the art for this purpose. A common system uses a multiplicity of "birds" that clip on the tow lines. Each bird comprises a transducer used for determining the range between nodal points. Thus, the backbone causes one bird to ping and the hydrophones of the nodal points to listen. The measured time of flight (TOF) is used to calculate the distance between the pinging bird and receiving hydrophones. The distances are used along with known quantities (for example, the distance between nodal points on the same line) in a series of simultaneous equations to generate relative distances between nodal points and, thus, the shape of the array.

The accuracy of knowing the shape of the array is highly important to the detection, analysis, etc. by the array. Although there are known systems that perform the above-described determination of the shape of the array, such systems generally rely on match filtering to the shape of the envelope of a continuous wave (CW) tone burst or to a pseudo-random sequence of tone bursts to measure TOF. Matching the shape of the CW leads to a loss of detail, which is then compensated for by using higher frequencies. This, however, results in attenuation, reduction of the range that can be measured, and susceptibility to reflections off of array components, bulkheads, etc. Thus, the present systems can only measure the relative positions of nodal points on the order of 50 cm or greater. Errors of this magnitude result in a degradation in the performance of the array.

Presently used towed acoustic receiving arrays are very costly; normally use a complex telemetry system with a unique data acquisition telemetry module for each channel of data; are relatively large in diameter (2-3 inch) which causes storage, deployment and retrieval problems; typically use flammable fill fluid to achieve neutral buoyancy and to help dissipate internal heat; and usually require relatively high electrical power (1 watt or more) per array acoustic channel. Recovery of an array's many separate channels of acoustic data is necessary to improve array sensitivity and directivity in conjunction with modern array signal processing.

Multichannel telemetry data can be coded in many ways depending on the data bandwidth, the signal dynamic range, the number of channels required, and the telemetry cable limitations. Digital, analog and hybrid telemetry methods are presently used. A digital time division multiplexed format allows telemetry of many channels with wide dynamic range to be coupled as necessary over a distance of many thousands of meters using digital repeaters. However, the data cable must be larger in diameter to allow passage of the relatively wide telemetry signal bandwidth. In addition, power per channel tends to be high due to high speed clocking requirements. To maintain channel identity, a different address number is normally assigned to each channel. Unless a separate plug-in address module is used, this requirement limits interchangeability.

To convey acoustic information, analog multichannel carrier techniques use either frequency modulation (FM) or amplitude modulation (AM) of high frequency carriers. A single cable can pass up to approximately 20 FM channels before carrier drift and crosstalk become a problem. AM-FM techniques can carry more data by bandshifting groups of modulated carrier data, but circuit complexity and power increase and extremely linear repeaters are usually required to prevent harmonic distortion interference. Amplitude modulation allows more channels to exist in a given limited cable bandwidth, especially if one of several single sideband methods are used; however, the received signal level varies inversely with the cable length and low frequency response is usually poor due to the modulation limitations. Both FM and AM methods require that each data acquisition module must operate at a unique carrier frequency; therefore, particular module sparing and repairing problems arise.




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