Feature Articles—November 2009 Issue
Submarine Hunting In Shallows— A Two-Stage ApproachA New Application of a Multibeam Echo Sounder System For Use in Anti-Submarine Warfare
By Markus Schäfer
System Engineer
Multibeam Systems
and
Matthias Conrad
System Engineer
Sonar Systems
L-3 ELAC Nautik
Kiel, Germany
The main tasks of modern non-nuclear or diesel-electric submarines are surveillance, reconnaissance and intelligence. For these types of missions, the submarine operates in the shallow waters of coastal areas for longer times.
Furthermore, submarines on intelligence missions generally have no permission for weapon employment, so they pose little threat to hunting surface vessels. Though in military terms the threats these subs pose is normally “white,” their nonthreatening, stationary nature can cause considerable difficulty for ships attempting to detect them using traditional anti-submarine warfare (ASW) sonar methods.
Submarines are already difficult to detect in shallow and littoral areas due to a high reverberation level, which is the sum of the reverberation occurring at the sea bottom and the sea surface or by scattering layers in the insonified water volume. This reverberation, in combination with unwanted echoes from underwater structures such as rocks and shoals, interferes with the target echo of the submarine and makes detection challenging.
However, when submarines are stationary, they cause even more significant problems for detection with conventional sonar methods. These types of systems typically work only in the noise-limited area, or the area between the ship and the first bottom bounce of the sonar beams. In this area, it is possible to detect a submarine with a conventional anti-submarine sonar system if the submarine is in the water column and not hiding below thermal or salt layers. In the reverberation-limited area, however (the area behind the first bottom or surface bounce of the transmitted sound), detection is much more challenging, since the reverberation is more or less correlated to the transmission signal, making the target echo nearly impossible to discriminate from the reverberation, particularly when the submarine is lying on the seafloor or remaining static in the water column.
To counter this problem, this article presents a two-stage approach to detecting submarines on surveillance missions in the littoral environment, combining a conventional ASW sonar with a multibeam system. Submarines lying at the bottom or hovering in the water column can be detected by means of a multibeam echo sounder using water column imaging (WCI) and side scan technology. Once the submarine recognizes that it is has been detected by the multibeam sonar, it will be forced to stop its mission and escape from the operational area, allowing it to be more easily detected by ASW sonar based on its Doppler shift, which is not impacted by reverberation.
Advantages of A Multibeam System
Since the purpose of the multibeam system is to scan for nonmoving submarines directly below the vessel, the operational procedure is comparable to mine hunting operations or seabed surveys. A survey area has to be defined and survey lines have to be calculated with expected or known water depths, coverage and other parameters. These survey lines have to be laid to reach 100 percent bottom coverage, and the hunting warship sails along them.
One technology that can be used to perform this aspect of the operation is the SeaBeam 3050, an L-3 ELAC Nautik mid- and shallow-water multibeam bathymetric sonar system that features a transmission technique that compensates for vessel pitch, roll and yaw motions for full coverage without gaps.
The SeaBeam 3050 generates sonar data for wide-swath contour charts, backscatter data for seabed sediment classification, raw data for WCI and side scan data for side scan images with a maximum coverage of 3,500 meters and a maximum water depth of 3,000 meters.
The new multiping technology of the SeaBeam 3050 allows a higher maximum survey speed without losing either 100 percent bottom coverage or high data density by creating two swaths per ping cycle.
The main hardware components of the SeaBeam 3050 are a transceiver unit, a transducer array (split in projector array and hydrophone array for different beam widths) and an operator station (as a standalone PC or integrated in a console). The projector array and the hydrophone array are arranged in a Mills cross configuration and split into multiple modules. This allows for the customization of the required along-ship and across-ship beam widths.
The transceiver unit contains the transmitter and receiver electronics. It consists of the transmitter amplifiers, the transmit beam former and the sonar controller board, which provides the interfaces to the other units and mainly handles all control tasks within the transceiver unit. Furthermore, the transceiver unit contains the necessary elements for signal conditioning and sonar processing. This includes the digital down conversion and the receive beamformer as well as information processing, such as the bottom depth finder, sound velocity correction and alignment correction.
The operator station provides the main machine interface to the operator. This means the station shows the operator various depths, backscatter amplitudes, side scan data, water column image, position and other information relevant to the mission.
The side scan mode and the WCI mode are best suited to detect a submarine lying on the seafloor or drifting in the water column. Once detected with the multibeam sonar system, the submarine has no choice but to leave the area, as it must comply with the rules of engagement and is not allowed to attack the ship. When it begins to move, the advantages of conventional ASW sonar come to the fore.
Advantages of ASW Sonar
An active ASW sonar is best suited for detecting moving submarines by processing the target Doppler shift, which is not negatively impacted by the reverberation level in shallow waters.
For an omnidirectional ASW sonar system installed on a nonmoving surface ship, the reverberation is centered on the frequency of the transmitted signal and will have the same frequency in all bearings; therefore, as the target moves, the Doppler effect will bring its echo signal out of the range of the reverberation frequency. As long as the surface ship is stationary, the Doppler shift of the target will allow the sonar to track it.
When using long, narrowband continuous wave pulses in combination with low side lobe levels, the reverberation will only occur in this narrow band. Furthermore, a frequency spread in the reverberation frequency occurs that is directly dependent on the transmission pulse length.
The formula fs=1/τ states the frequency spread in hertz for a defined transmission pulse with pulse length of τ seconds.
At low sea states, this frequency spread may decrease. The mentioned frequency spread follows a Gaussian distribution (one Σ) and is centered on the transmission frequency.
The returning energy will be detected by the ASW sonar and processed in the frequency domain by using narrowband matched filters.
After processing the target Doppler shifts, the submarine echo away from the spread reverberation signal and the detection situation becomes noise limited instead of reverberation limited.
To increase the detection probability, adaptive target selection algorithms and automatic target tracking algorithms are used. Formula fs=1/τ shows that the transmission pulse length has an important influence on the frequency spread: Longer pulses will have a positive influence on detecting targets with low target Doppler shift.
One technology that can be used for this purpose is the ASW 8000, a multifunctional sonar for surface ships that is used for localization of underwater contacts, providing an overview of the underwater situation and passive signal detection. The sonar consists of the following major elements: a cylindrical transducer array, a transducer matching box, a transmit/receive decoupling connection box, a transmitter unit, a receiver and processing unit and a sonar controller unit. The display of the sonar data and the control of the sonar are done via multifunction consoles that are part of the combat management system (CMS).
Detection and localization of submarines or other objects is done in a horizontal sector of 360 degrees depending on the installation of the transducer array. The vertical coverage of the system is approximately 17 degrees. Preformed beams are used in the horizontal plane, allowing for horizontal bearing and range measurement in the active mode. The passive mode is used for receiving underwater signals and determining the bearing of these signals.
The active panorama mode is used for active detection and bearing and distance determination of underwater targets within the situation awareness, like conventional submarines. The active information processing consists of the functions range, bearing, level and Doppler shift.
For all threshold detections, the range is calculated based on the known sound speed and travel time. The information processing continually calculates the bearing for each threshold detection. The system forms 64 horizontal beams and offers an additional cursor beam for higher bearing measurement accuracy.
By control of the preformed horizontal beams, it is possible to measure the range, speed, Doppler shift and bearing of detected objects. Use of a target-tracking algorithm allows simultaneous tracking of several objects. The trackers are initialized manually by the operator. The continuously calculated target information is displayed and forwarded to the CMS.
The console needs an audio unit, including a headset and a volume control device, to make the selected channel audible. The actual echo signal strength of the cursor beam is transferred into audible tones, which are fed into a digital/analog (D/A) converter. The analog output of the D/A converter is amplified for presentation to the operator’s headphones.
In order to increase the performance of the audio channel, the audio signal processing contains an additional independent beamformer.
In the passive mode, no sonar pulses are transmitted, but the noise signals emitted by the target are detected. The actual energy is detected by an energy detector and is incoherently integrated to reduce signal fluctuations.
Conclusions
There is no general approach for submarine hunting in littorals, and all sonar systems have their advantages and disadvantages. But as the submarine threat shifts from the open ocean to the shallows and from active, moving submarines to stationary ones, ASW tactics must take these challenges into account.
As discussed above, submarines with low or zero target Doppler shift are difficult to detect with ASW sonar alone. If reverberation dominates because of a low or zero target Doppler shift, frequency modulated pulses and pulse compression techniques can be used to detect the submarine, but again, in this case, a multibeam sounder using side scan imaging and WCI can achieve much better detection results.
Therefore, with a two-stage approach—the combination of a multibeam echo sounder like the SeaBeam 3050 and a sonar like the ASW 8000—the detection probability is increased.
Markus Schäfer is a system engineer for multibeam systems at L-3 ELAC Nautik. He graduated from the University of German Armed Forces in Hamburg as an engineer. He joined the German navy in 1993 and spent 13 years as a naval officer.
Matthias Conrad is a system engineer for sonar systems at L-3 ELAC Nautik. He graduated from the University of Applied Science in Kiel, Germany, as an engineer. He joined the German army in 1994 and served as an officer for eight years.
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