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Seeking the sounds of silence

Multistatic sonobuoy networks have been developed to pinpoint quietsubmarines

Anti-submarine warfare (ASW) forces that have honed their talents in the open oceans are facing tough new challenges posed by small, quiet diesel-electric boats operating close to shore. As the US Navy (USN) puts it: "ASW must now commence at the threat pier". Littoral waters have high levels of ambient noise generated by shipping, surf action and industrial facilities such as oil rigs. Furthermore, near river estuaries, water temperature and salinity can vary substantially. As a result, acoustic propagation is poor and difficult to predict. These subsurface challenges are accompanied by competition for radio-frequency bandwidth when operating near land. Although 99 VHF channels are available for communications between sonobuoys and maritime-patrol aircraft or other processing platforms, many of these frequencies are already in widespread use by civilian operators such as taxi despatchers.

Since no single sensor is completely effective across the wide range of ASW threats, environments and tactical phases, a family of complementary approaches is necessary. Non-acoustic types focus principally on surface and near-surface operations; active acoustic sensors counter threats that are submerged or lying on the sea bottom; and passive acoustic sensors provide valuable clues to assist in classifying submarines, while forcing them to limit their speed and operational maneuvers.

Many ASW operations rely on passive initial detection and fairly crude localization, followed by redetection using a combination of passive sensors and monostatic active systems. However, navies are increasingly adopting bistatic and multistatic implementations to detect quiet contacts. To provide tactical surveillance (also known as wide-area search), several are also turning to the use of multiple air-deployed sources and receivers incorporated in low- frequency sonobuoys. The latter can be relatively inexpensive and their use complicates the threat's problem of locating the ASW platform. However, this approach also imposes new demands, such as the need for accurate knowledge of the sensors' location - which may be provided by Global Positioning System (GPS) receivers - and a significant increase in data exchange. It also requires careful planning and management of the deployment pattern, to prevent direct blast from the source masking the presence of a contact.

Early implementations of multistatic systems have used relatively crude sources - essentially an explosive package generating a pulse of incoherent noise - such as the US-developed AN/SSQ-110 family. On command from an aircraft, these deploy noise-generating payloads to the desired depth. This approach provides a high source level, but is limited to one or two pings and the carriage of explosives inside the delivery aircraft causes safety concerns. As a result, developments under way in several countries focus on providing sources that are powered by alternative means and can generate many hundreds or even thousands of coherent pings, the characteristics of which can be matched to operating conditions.

Countries that are pursuing an airborne multistatic approach include Australia, Canada, Japan, the UK and the USA. Australia's Defence Science and Technology Organisation (DSTO), in collaboration with local industry, has been conducting research into airborne multistatic sonar techniques over the past five years. Current work includes a capability and technology demonstrator (CTD) program supporting Joint Project (JP) 1441, which focuses on providing an integrated family of subsystems to equip the Royal Australian Air Force's AP-3C maritime-patrol aircraft and the Royal Australian Navy's S-70B-2 Seahawk shipboard helicopters.

JP 1441 includes a bistatic version of the Barra sonobuoy; RASSPUTIN (Rapid Area Search Sonar Projector Used Tactically In Narrowband), an optimized version of the Low-Frequency Active Sonobuoy (LFAS) under development by Thomson Marconi Sonar; a digital version of the DIFAR sonobuoy, developed by Australian company Sonacom, in both C- and D-sizes, that provides greater range and bearing accuracy than its analog counterparts; and the associated Raptor real-time processor, developed by Australian company Acoustic Technologies.

Raptor, which largely employs commercial off-the-shelf (COTS) technology, has an integration time of approximately 60ms to permit the observation of transient sounds such as target echoes. It can operate with Bistatic Barra or DIFAR buoys and is planned to be integrated with the Computing Devices Canada AN/UYS-503 sonics processor selected for the AP-3C.

During bistatic operation of Barra, the sonobuoy acts as a receiver for signals emitted by a disposable, incoherent broadband source (which is useful in providing classification information) or the RASSPUTIN/LFAS coherent electro-acoustic source. Shallow-water operations typically provide excellent long-range acoustic transmission properties at frequencies from 500Hz to 2.5kHz, which coincides with the optimum passive detection performance of Barra. At these frequencies, according to DSTO, the buoy's narrow horizontal directivity provides excellent target bearing discrimination and rejection of unwanted off-beam reverberations.

Complementary areas

Several laboratories within Canada's Defence Research and Development Branch are working in complementary areas of multistatic systems and related sensors. Experience gained with the IMPACT (Integrated Multistatic Passive/Active Concept Testbed) has spawned the Wide Area Subsurface Surveillance (WASS) system. This system, also referred to as the Wideband Active Sonar System, is under development for the Canadian Forces' CP-140 Aurora maritime-patrol aircraft, together with the IVASP (Interim VME Acoustic Signal Processor) intended to equip that platform.

The Defence Research Establishment Atlantic has conducted several series of tests with prototype equipment to assess the applicability of GPS to conventional sonobuoys. In parallel, the Defence Research Establishment Ottawa has worked with the University of Calgary a GPS-based heading-determination system for use in sonobuoys deployed in the Arctic, where magnetic compasses are unreliable. Despite the short baseline (10cm) available in a sonobuoy, they have demonstrated the required heading accuracy of less than 5 by a phase interferometry approach using a pair of COTS receiver modules and their antennas.

In the UK, teams led by Thomson Marconi Sonar (TMS) and Ultra Electronics are competing to supply an Active Search Sonobuoy System (ASSS) to meet Staff Requirement (Sea/Air) 903. ASSS will equip Royal Air Force (RAF) Nimrod MRA.4 maritime-patrol aircraft, the first seven of which are due to be in service by 2005. BAE Systems, as prime contractor for the platform, is also responsible for acquiring the ASSS. The company, in conjunction with the UK Ministry of Defence (MoD), will issue an invitation to tender for full-scale development and initial production in the middle of this year followed by a "main gate" submission and contract award in 2001.

Ultra, which is now the only sonobuoy manufacturer in the UK, can draw on the expertise of overseas companies that it has acquired over the past few years. These include Hermes Electronics in Canada, the former Magnavox and Raytheon subsidiary UnderSea Sensor Systems Inc (USSI) and the sonobuoy receiver supplier Flightline Electronics.

The package that Ultra is bidding for ASSS includes the company's new Active Low Frequency Electro-Acoustic source (ALFEA); a version of the SSQ-110 impulsive source - of which USSI is one of two manufacturers - adapted for carriage by the Nimrod; the SSQ-955 High Instantaneous Dynamic Range (HIDAR) receiver buoy with an integral GPS receiver; and the SSQ-981 version of the Barra receiver buoy with a modified horizontal planar array.

The A-size ALFEA buoy, which operates in the 1-2kHz frequency range, exploits technology from Ultra's family of sonar countermeasures devices. It incorporates a GPS receiver and can generate a large number of pings (reportedly several hundred) with programmable waveforms. Ultra says that recent tests in Lake Seneca, New York State, "have shown that it can comfortably meet the designed source levels".

ALFEA incorporates a 99-channel Autonomous Function Selection (AFS) facility, together with a UHF command function to control its operation following deployment. The latter is compatible with those used for the UK SSQ-963 CAMBS (Command Active MultiBeam Sonobuoy) and US AN/SSQ-62 DICASS (Directional Command-Activated Sonobuoy System).

Upgraded

Ultra began production of HIDAR early this year. It uses the same sensor concept as the earlier SSQ-954D DIFAR G-size buoy - two dipoles, together with an omnidirectional sensor, employing null- steering techniques to improve the signal-to-noise ratio - but has otherwise been substantially upgraded. Key new features include an increase in dynamic range, from about 40dB to 80dB, resulting from the adoption of digital signal processing. HIDAR digitizes the signal at the sensor itself, then transmits the data either over a datalink employing a Gaussian minimum-shift-keying digital telemetry format or via a conventional FM radio link.

Barra deploys a horizontal planar array of 25 hydrophones mounted on five arms and generates highly directional beams to minimize the effects of bottom reverberation. It can also employ digital telemetry, which brings several additional advantages. These include the ability to handle other traffic, such as GPS position, compass data and bathythermal measurements.

Ultra has already been selected to provide the AN/AQS-970 acoustic processor and other sonics-related equipment for the Nimrod MRA.4. The company is collaborating with Computing Devices Canada on development of the AQS-970, which is derived from the AN/UYS-503 that equips the defense forces of Australia, Canada, Sweden and the US. The variant proposed for the Nimrod MRA.4 employs updated software providing clutter reduction, automatic detection and classification, and tactical aids for the operator. The last of these provides assistance in how to lay the sonobuoy field; what ping sequences to use; which active buoys to ping; what tactics to use for localization and tracking; and which techniques to employ to reduce the impact of the environment, such as bottom reverberation and ambient noise. Related developments specifically aimed at the Nimrod MRA.4 requirement involve the operator/machine interface. This would incorporate a new energy-map display, developed by Ultra and DERA, to highlight possible detections as they occur.

In February 1999, the MoD selected Ultra to provide AQS-970 technology for the Replacement Acoustic Processor (RAP) upgrade to the RAF's Nimrod MR.2s, of which there are approximately 20 in service. The RAP program is a "spend-to-save" effort that will replace the 16-channel AQS-901 processor, which has become difficult to maintain, with a 32-channel unit (a half-size implementation of the AQS-970). The MRA.4 installation will have two additional 16- channel processors giving it a total capability of processing 64 channels.

Algorithms

Ultra's contribution to the MRA.4 also includes ARR-970 32-channel sonobuoy receivers (two per aircraft), which employ technology provided by the company's US subsidiary Flightline Electronics; and a Geolocation Sonobuoy Positioning System that is for both the MRA.4 and the Canadian Forces' Aurora upgrade. The positioning system measures phase changes in the signals from each buoy at standoff ranges of up to approximately 70n miles. Ultra is writing the Kalman filter algorithms for the system with the support of the UK Defence Evaluation and Research Agency's test facilities at Funtington.

The company says that its ASSS-related software could run on other acoustic processors in addition to the AQS-970. Potential applications include the Canadian Forces' maritime helicopter program, P-3 Orion upgrades in several countries (notably Japan and the US), the German-Italian MPA 2000 program, and the SH-60R variant of Seahawk.

The US Navy (USN), like many of its counterparts, is having to re- orient its equipment and operating procedures to meet the demands of littoral warfare. The Extended Echo Ranging (EER) series of multistatic active acoustic systems acts as US naval aviation's primary sensor for detecting and tracking submerged threats under favorable environmental conditions. The baseline system uses the SSQ-110 source (see above) and the AN/SSQ-77 Vertical Line Array DIFAR (VLAD) sonobuoy as a receiver. Improved EER (IEER) introduces the new AN/SSQ-101 Air Deployed Active Receiver (ADAR). The Advanced EER (AEER) program is planned to implement further enhancements for both shallow- and deep-water ASW search, using active sources such as the Air-Deployed Low-Frequency Projector (ADLFP) operating in conjunction with ADAR.

Enhancements to the DICASS active sonobuoy focus on implementing a reliable ability to localize contacts in order to reject false targets, and to identify and track submarines. DICASS, together with the ADLFP, is a candidate to provide the precision necessary to classify and track contacts as part of the Shallow Water ASW Localization and Attack System (SWALAS). The long-range detection potential of ADLFP would also allow it to provide a significant deep-water ASW search capability as a back-up to the EER acoustic impulse sources. SWALAS will also include an improved magnetic- anomaly detector.

In parallel, the USN is continuing to develop traditional, directional passive acoustic sonobuoys to augment EER search, provide threat classification, and limit threat high-speed operations. The service plans to incorporate Beartrap functions (see below) into the DIFAR buoy which, in selected cases, could also act as an acoustic receiver for EER sources as an alternative to the more expensive ADAR. The USN says that converting the electronics from analog to digital is "highly desirable" to incorporate all of the required functions into DIFAR while retaining its low cost. Long-term goals include consolidating the functions of DIFAR, LOFAR and VLAD into a single type known as Unibuoy.

Environment

Knowledge of the environment in which a threat submarine is operating, and its impact on ASW sensors, is fundamental to success. The availability of accurate meteorological and oceanographic (METOC) predictions greatly assists a crew in making the best choice of sensors and their tactical employment. Accordingly, the USN is restructuring its Beartrap METOC data-collection program to take advantage of the fact that all ASW sensors carried by an aircraft can be calibrated and used to provide direct measurements of actual threat and environmental characteristics. The collected data can be rapidly analyzed and fed back to the operational commander for operations in progress, as well as subsequently filling databases supporting the development of new sensors and techniques.

A phenomenon known as 'acoustic-ray anomaly' has been observed under laboratory conditions for several years, and data from the Beartrap collection program has shown that it can be both detected and exploited in the open ocean. Naval Sea Systems Command has therefore initiated an investigation program to implement this over the next three to five years. The results could be used to expand the number of target characteristics that the EER sonobuoy system can exploit, thereby achieving a significant increase in detection, classification and associated ASW search rate.

Effective operation of active sonars, particularly in shallow waters, requires detailed knowledge of the background noise and reverberation field, including direction and amplitude. Current on- board or expendable measurement sensors, which measure only those parameters - such as temperature versus depth, and ambient noise - required by passive systems, are of very limited use to active operations. The US Chief of Naval Operations is funding development of a Tactical Acoustics Measurement and Decision Aid (TAMDA), scheduled for introduction to the fleet in Fiscal Year 2007 (FY07), that will exploit new ways of measuring environmental acoustic parameters applicable to both active and passive sonars. These will include monostatic and bistatic reverberation, from which other required parameters - such as bottom backscatter, depth and loss - can be derived.

The aircraft-delivered TAMDA, resembling a sonobuoy, is foreseen as including a sound source (generating a broadband probe pulse); a suspended receiving array of calibrated omnidirectional hydrophones to measure water conductivity, temperature and depth; and on-board processing algorithms. The sensor should operate at frequencies down to less than 1kHz, preferably covering a band from 50Hz to 10kHz.

The introduction of longer-life sensors that can operate in both the active and passive acoustic modes would support longer-term surveillance. Candidate technologies include full-spectrum in-buoy screening and classification, passive algorithms, power management of processing and communications, and the reliable location of sensors in the sonobuoy pattern as well as their relation to each other. InvenTek Corp is developing a long-life thermal battery for sonobuoys under a contract from the Office of Naval Research (ONR). The adoption of a fused-salt primary battery, housed in a vacuum/multifoil super-insulated container, promises to extend the typical operating time of an A-size sonobuoy from 4 to 12 hours. The reduced thickness of the insulation compared with a conventional design increases the diameter of the battery stack by 35%, permitting 200 or more ping-seconds of operation with an output power of more than 13kW.

Passive detection

Last year, the ONR issued a broad agency announcement soliciting proposals for research into technologies that could lead to the deployment of battery-powered drifting or moored autonomous sensors by 2005. These, which could be deployed from aircraft, would operate with minimal operator intervention to provide continuous surveillance over wide geographical areas. Automatic passive detection, classification and localization would take place within the sensor, generating detection alerts for relay over low-bandwidth links (including satellite communications) to remote post-processing and contact-fusion sites.

These would combine inputs from multiple sensors and integrate them with other surveillance information, to generate probable submarine tracks and signal analysis information that could then be used to cue tactical ASW forces. Alternatively, the sensors could transmit information via a link with a capacity of 1.2-9.6kbit/sec (as with a cellular telephone) directly to a tactical platform such as a maritime-patrol aircraft or a submerged attack submarine deploying a buoyant-cable antenna.

Potential sensor components of such a network include the Autonomous Drifting Line Array (ADLA), operating at depths down to more than 500m, which would comprise an array of 30-100 hydrophones operating at very low frequency. These can act as either a near-linear horizontal aperture or one that is tilted and changes shape under the influence of ocean currents, to form a curved multidimensional aperture. The array geometry is sensed continuously to support beam- forming.

The ADLA would operate in bistatic or multistatic mode with coherent or incoherent acoustic sources. The former could include Low- Frequency Active (LFA) SURTASS, Long-Endurance Low-Frequency Active Surveillance (LELFAS) and its air-deployed lightweight variant, and the ADLFP. LELFAS, which is the subject of an ONR-sponsored advanced technology demonstration running until FY01, is anchored to the sea floor and deploys a vertical array of transducers. A high-density power source - which includes a thermo-electric generator, combustor, turbine, and a flywheel able to store 50kW - has a capacity of 260kWh and can operate for 15,000 ping-seconds. Incoherent or broadband impulsive echo ranging for ADLA would be provided by the SSQ-110 sonobuoy or a similar broadband VLF source. Typical coherent multistatic arrangements would include two to 10 sources and five to 50 receivers.

The technology necessary for the airborne delivery of such networks from standoff ranges is already widely available. AeroVironment, on behalf of the ONR, has conducted trials of an A-size sonobuoy fitted with an Expendable Glider (X-Glider) range-extension package occupying only 11% of its volume. The flexible wings wrap around the buoy during storage, then extend to confer a glide ratio of 10:1. The incorporation of a GPS-based navigation system permits autonomous flight