Eyes and Ears Underwater

The Indian Navy awaits the nine ATAS which were cleared by DAC in 2017

Palak Gupta

Since radar does not work underwater and water is an acoustic medium, Sound Navigation and Ranging (Sonar) acts as the eyes and ears of a naval vessel. The applications of Sonar include detection, tracking and destroying enemy ships, submarines and underwater mines, thus playing a vital role in anti-submarine warfare (ASW). Sonars can be vaguely broken down into two categories: active and passive. Connected to a large sonic database, passive sonar ‘listens’ without transmitting and hence does not give out the identity and location of the user.

Advanced Active cum Passive Integrated Sonar System (HUMSA NG); Integrated Submarine Sonar (USHUS)

In a bid to give impetus to the submarine hunting capability of the Indian Navy, the Defence Acquisition Council (DAC) in 2017 had approved the procurement of nine Active Towed Sonar System (ACTAS) for warships to the tune of Rs 450 crore. The clearance bears enormous operational significance for the Indian Navy.

These ACTAS will be fitted on the indigenous Kamorta class ASW corvette and will be procured from German firm Atlas Elektronik which is a naval/marine electronics and systems company.

Under the deal, the first six systems will come from Germany and the remaining three will be manufactured in India in cooperation with Bharat Electronics Limited (BEL).

This (a batch of nine ACTAS) is an additional requirement. Six ACTAS have already been fitted on the ships that have been taken on contract, said Cmde Anil Jai Singh (retd).

In 2014, the Indian ministry of defence (MoD) had signed a contract with Atlas Elektronik for the delivery of six ACTAS for the Indian Navy.

The Indian Navy mainly uses hull mounted Sonar system, the kind which can be said to have its own set of limitations. Hull mounted Sonars are fitted to the hull of the warship and can be operated round the clock without requiring launch time.

“Below the hull, there is a dome and inside the dome, we have antennae of hull mounted Sonar,” explained Cmde Anil Jai Singh, a submariner himself. The limitation of hull mounted Sonar is its lack of flexibility of depth. A submarine can move up and down and position itself depending on the offensive or the defensive role it wishes to play, and odds are it may go undetected, he added.

Hull mounted Sonars also have higher Electromagnetic Interference/ Electromagnetic Compatibility (EMI/EMC) requirements to be able to work with the ship’s electronics. This is where ACTAS play a vital role in the Indian Navy’s ASW capabilities.

According to Atlas Elektronik, ACTAS is a low-frequency ASW Sonar system that operates simultaneously in active and passive modes and provides high-resolution target detection. The ACTAS detection ranges is said to exceed the weapons range of hostile submarines, hence denying the enemy an offensive capability.

The ACTAS can detect, track and classify submarines, torpedoes, surface vessels, including speed boats.

It is pertinent to mention that once a submarine dives into waters, the depth and the temperature of water are measured continuously on every level. Earlier, it was done manually, however, nowadays it is an automatic task and is generated by computers.

ACTAS is a cutting-edge technology and defence sources maintain that it is particularly paramount for the Indian waters (which are usually warm.)

ATAS is especially vital in the Arabian Sea. In the Indian waters, which are warm and shallow, the returning signal often gets lost. In the Indian context, water is warm on the surface and cools rapidly as the naval vessel dives deeper, the sharp ‘temperature gradient’ refracts Sonar waves, bending them away from the sensors and hence, the vessel cannot detect the submarine upon not receiving the returning signal.

Stressing on the significance of towed array Sonar, Cmde Jai Singh said, “It can be streamed out. The cable (sonar antenna) goes into waters. It is some 100 metres behind the ship and hence, it is not affected by the sound of the ship or (propellers.)”

As submarines move through waters, various sounds are produced and noise is generated from internal operating systems. Thus, there is a need to detect sounds from external sources.

A towed array depth Sonar is the most important ASW sensor on a surface ship. Although it takes time to launch, it offers advantages of reduced interference from the ship as it operates at a distance from the ship and at different depths, enabling it to operate in optimal water conditions.

It comprises hydrophones and with the help of a cable is tugged at a considerable distance behind a submarine. These hydrophones are often kilometers away, thus ‘greatly improving the signal to noise ratio.’

The towed array Sonar is effective in the detection and tracking of even faintest of the sounds such as the low noise-emitting submarine threats or seismic signals.

These Sonars overcome the limitations posed by hull mounted Sonars, that it is it can be adjusted to the required and chosen depth.

The towed array Sonar also has superior resolution and a wider range than the hull-mounted.


Passive Towed Array Sonar

The Indian Navy ships use Passive Towed Array Sonar (PTAS) sourced from French company Thales. The demerits of PTAS are the speed of the towing body and the need to protect the cable from being damaged or cut off.

The Indian Navy on 18 November 2016 formally inducted four types of indigenously developed Sonars — Abhay, HUMSA-UG, NACS and AIDSS. The systems were designed and developed by Naval Physical and Oceanographic Laboratory (NPOL) a Defence Research and Development Organisation (DRDO) laboratory based in Southern Naval Command, Kochi.

Abhay: It is a compact hull mounted Sonar for small ships and shallow water crafts. Induction of Abhay, according to the written reply given by the then minister of state for defence Subhash Bhamre, enables indigenous Sonar system to be installed on small ships, thereby enhancing the ASW surveillance capability of the fleet to smaller vessel, like shallow water crafts, light and patrol vessels, ‘which was hitherto limited to frigates and destroyers.’

HUMSA-UG:  It is an upgrade of the hull mounted Sonar array (HUMSA) series of Sonar systems for ships. Here the capabilities of the indigenously-developed sonar system HUMSA have been upgraded by minimising the existing hardware and addressing technology obsolescence issues.

It has an additional new receiver electronics and an ultra-cool power amplifier which aims to reduce maintenance by addressing the obsolescence issues.

Advanced Panoramic Sonar Hull Mounted (APSOH) was the first Sonar from NPOL that was inducted into the Indian Navy. Its success prompted further developments, resulting in HUMSA. “These indigenously developed Sonars keep getting updated, upgraded and developed further with time as per the requirement, keeping pace with the advancement in technology.”

NACS: The Near Field Acoustic Characterisation system for ship Sonars provides an “operationally efficient means to determine the frequency-dependent 3-D transmission and reception characteristics of the hull mounted Sonar aiding in the optimum performance and maintenance of the Sonar.”

AIDSS:  The Advanced Indigenous Distress Sonar System for submarines is used to signal that a submarine is in distress and thereby enable quick rescue. It is an alarm system designed to transmit Sonar signals of pre-designated frequency and pulse shape in an emergency situation, so as to attract the attention of a rescue vessel in the vicinity.

Another contribution by the DRDO to boost the navy’s capability is the state-of-the art Sonar suite USHUS 2, an Indian developed Sonar system for Russian Kilo class submarine.

According to DRDO, USHUS-2 is an integrated Submarine Sonar System which both physically and functionally substitutes the MGK-400 and MGK-519 Sonars on four EKM class of submarines.

USHUS-2: It is a state-of-the-art upgrade of NPOL designed Sonar USHUS in terms of technology and Sonar capabilities. USHUS is operational onboard five of the nine frontline EKM submarines of the Indian Navy. USHUS-2 Sonar suite is tailored for the remaining four EKM class of submarines. It includes Passive Sonar, Active Sonar, Intercept Sonar, Obstacle Avoidance Sonar and Underwater Telephony.



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