Underwater Acoustics: Low-Frequency Active Sonar (LFAS)
Low-Frequency Active Sonar (LFAS) for submarine detection
In cooperation with the Royal Netherlands Navy, TNO-FEL started with the definition and construction of an experimental new type of sonar for submarine detection in 1991. In 1994, this Low-Frequency Active Sonar (LFAS; ALF in Dutch) was delivered by the French company Thomson Sintra ASM and tested at sea. The LFAS sonar system consists of a streamlined body (height approx. 2.5 m and a weight of approx. two tons), a towing cable and a hoisting installation installed at the oceanographic research vessel of the Royal Netherlands Navy, the HNLMS Tydeman (A906). The towing body contains a very powerful acoustic transmitter and is towed underwater using a towing cable. The drag body also contains electronics for data transmission and signal processing. Two receiver arrays are attached to the drag body: acoustic antennas in the form of long plastic hoses containing a row of underwater microphones. The LFAS emits an acoustic signal that may be reflected by a submarine. The echo is received by the acoustic antennas. The direction of the echo and the length of time between transmission and reception can be determined by properly processing the received signals. The position of the submarine can then be calculated. LFAS sends out low-frequency signals because these propagate well in water and allow submarine detection at large distances.
In June-July and October 1994, the first LFAS trials at sea took place on board the HNLMS Tydeman to test all functions of the sonar supplied by Thomson Sintra ASM and the TNO-FEL developed system parts. During these tests, the strength of the transmitter was measured and the stability of the drag body was investigated. These tests were successful. During both test periods, a lot of experience was gained with the hoist used to extend or retract the source with the towed sonar receiver arrays.
In 1995, the first LFAS expedition was carried out in close collaboration with four ships of the Royal Netherlands Navy (the oceanographic research vessel HNLMS Tydeman (A906), the torpedo work ship HNLMS Onverschrokken (M886), HNLMS Mercuur, the submarine HNLMS Dolfijn, and the minehunter Zr.Ms. Zierikzee) and completed successfully. In addition to the LFAS towed by the HNLMS Tydeman, entirely new echo repeater/transponder systems developed by TNO-FEL were installed at the HNLMS Mercuur and HNLMS Dolphin.
It was the first time that the LFAS system attempted to detect a submarine at a large distance under different operational conditions in the North Atlantic Ocean. This included aspects such as target strength, propagation of the transmitted signal in the ocean, reflections of sound from natural objects, and the position and shape of the receive arrays during towing. The LFAS has been tested in this expedition mainly concerning the deep water properties where the source power is very important. It became clear that LFAS can improve the long-range submarine detection capability in the future. During the experiment, much attention was also paid to measurements for the development of position and shape-corrected beamformers that may enable the Royal Netherlands Navy to guarantee optimal sonar performance during manoeuvres.
Preliminary performance forecasts based on model calculations of an LFAS in the Atlantic Ocean show detection ranges for submarines up to the first Convergence Zone (CZ). The CZ is a long-distance ring-shaped region, several kilometres wide, where the propagation conditions are such that the probability of detection is considerably larger than outside the ring. Important parameters for detection are the reflection properties of the target, reflections from the seabed, sea surface, and the plankton layer present in the Atlantic Ocean (reverberation), the ever-present background noise in the sea, the sound propagation and the hydrodynamic behaviour of the towed sonar arrays. With the LFAS, it is possible to verify or determine these parameters experimentally.
In 1995, a more strategic long-term study started in the field of new array technology and sonars for use in shallow waters in close collaboration with TNO and Thomson Sintra ASM. More specifically, the performance of cardioid hydrophone arrays will be examined, the performance of a new type of sensor, a so-called panic velocity sensor instead of a normal hydrophone, and possibly the use of thin-line arrays.