Underwater acoustics: Low Frequency Active Towed Array Sonar (90’s)

Low-Frequency Active Towed Array Sonar (90’s)

Modern conventional submarines have become much quieter in the last decades. The radiated noise levels have dropped much faster than sonar technology has improved. As a consequence, the passive detection of submarines has become increasingly problematic. Moreover, torpedo ranges have largely increased. Therefore, there is a strong need for long-range active detection of submarines.

HNLMS Bruinvis in a Norwegian fjord
HNLMS Bruinvis in a Norwegian fjord

ATAS (Active Towed Array Sonar) systems are good candidates to fulfil this need. These sonars use low-frequency active transmissions such that long detection ranges can be achieved. Also, they can be towed at variable depths and can thus be deployed in the most favourable acoustic layer. An ATAS consists of a (very) powerful low-frequency source and a receiving hydrophone array. The former is far from trivial since compact high power acoustic sources are hard to design and became available only recently. The transmitted acoustic energy is received on the array after reflection on the target. The receive array must be as long as possible, to obtain a sufficient signal-to-noise ratio for remote targets and accurate bearing determination.
After years of research (amongst others at TNO-FEL) the first experimental ATAS systems were developed around 1990.

Initially, good detection results were obtained in deep (blue) water. In the same period, however, the cold war ended. This meant that most navies interests changed from deep water oceans to shallow and littoral waters. The first generation ATAS systems were not well adapted for this purpose. In shallow water, the time and space to make manoeuvres are often limited, such that long-duration tracking procedures must be avoided. Therefore it is of the utmost importance to solve to bearing ambiguity (the famous left/right problem) in one single ping. Another enormous problem is the use of high source levels in shallow water. This causes high amounts of bottom reflections (reverberation) leading to excessive false alarm rates. Further problems for ATAS in shallow water are the usually very bad signal-to-noise ratios. Apart from the unavoidable background noise of the sea, the system has to deal with flow noise that results from towing the system through the water and with tow ship noise.

The Active Low Frequent twin array system
The Active Low Frequent twin array system

All these ATAS problems are addressed in the Underwater Acoustic Group of TNO-FEL in several projects in the ’90s. The specific array data processing techniques are one of the specialities of the group. The research was concentrated around TNO’s ALF (Active Low Frequent) sonar, which has been taken to sea successfully for many years. However, also other ATAS systems are studied, mostly in close co-operation with foreign partners.

A highlight of the ATAS research was in the fall of 1998 when TNO-FEL went to sea with two different (semi)operational ATAS systems to experimentally evaluate their detection performance in a shallow water area near the Norwegian coast. The ATAS system was towed by the HNLMS Tydeman. The trials were made with (a unique situation) two submarines, one Dutch Walrus class submarine and a Norwegian Kobben class submarine (KNoMS Stord). At first, the flow noise was measured at different speeds and the hydrodynamic stability of the towed sonars was tested.  Next, experiments with a calibrated noise source attached to the Walrus class submarine took place. With these passive tests, the bearing accuracy of the towed sonar arrays could be determined.
After these experiments, active sonar detection experiments followed. With two submarines, one submarine could be positioned in a noise-limited and the other in a reverberation (clutter) limited environment. Later on, shallow water tests were made.

The successful results of this research by TNO-FEL (and the Norwegian FFI) supported the Royal Netherlands Navy in the procurement process of the ATAS sonars for the M-frigates.