Keeping the fleet stealthy – the acoustic ranges used to support the Royal Navy

As part of a series covering UK naval infrastructure, here we look at the little-known facilities used to evaluate the acoustic and magnetic signatures of the navy’s ships and submarines.

Sound advice

The RN invests a great deal in ensuring the nose emitted from its submarines and frigates is minimised. When new vessels enter service or have been in a long period of upkeep or maintenance, the effectiveness of these silencing efforts needs to be measured and evaluated. The data gathered from these evaluations allow the operators to understand the particular sound signature the vessel emits into the water and which items of machinery are responsible for creating sounds at particular frequencies. Even ships or submarines of the same class may have subtly variable characteristics as each vessel is unique. Each may received slightly different equipment over time and been subject to different stresses and wear and tear, etc. This knowledge will help inform which machinery may be run and at what speed in different tactical scenarios. The evaluation may also show up deficiencies in equipment that needs further rectification to reduce noise and vibration before the vessel is ready for operations.

It is also necessary to test and calibrate the performance of active and passive sonars in benign and quiet waters before being used operationally. Additionally, torpedoes and underwater decoys need to be trialled in controlled conditions where their performance can be accurately monitored before they can be certified for frontline use.

LTPA

In order to conduct this testing regime, a series of ranges have been established around the UK. There are small ranges on the South coast off Portsmouth, Portland and Plymouth but the key sites are situated in North West Scotland. These sites were formerly operated by the Defence Evaluation and Research Agency (DERA) and remain MoD-owned but are operated by QinetiQ, DERA’s privatised successor. In 2003 the MoD signed a £1.3Bn Long Term Partnering Agreement (LTPA) with QinetiQ who provide experimentation, test, evaluation and training support to the MoD at 15 land, sea and air ranges. In 2019 QinetiQ agreed to invest a further £190m at various sites including a provision to upgrade the ranges for testing the Dreadnought and Astute class submarines.

Magnetism

Besides sound signatures, the magnetic signature of warships and submarines are also minimised using internal degaussing coils and external deperming coils. This is primarily to reduce the ship’s magnetic field in order to make them less vulnerable to mines, torpedoes or sensors that rely on magnetic detection. To evaluate the magnetic signature of vessels Qinetiq operates several degaussing ranges in the UK that measure a ship’s magnetic field as it passes over cables on the seabed.

At the naval bases, there are small Harbour Entrance Ranges (HER) that automatically gather the magnetic signatures of vessels as they enter or leave harbour. At Devonport the range is at Eastern Kings and Faslane is served by a range at Rosneath. There are also Open Sea Ranges (OSR) in deeper water at Pier Cellars off Plymouth (20m depth), Portland (9m) and Barons Point on the Clyde (30m).

The OSRs are operated from control buildings ashore and connected by cables to a large coil lying on the seabed. The vessels make passes over the coil and the strength of the electromagnetic field is adjusted to either neutralise or bias the magnetic field of the steel hull and its ferrous components. The vessel’s magnetic signature is continually monitored and the process is repeated until the correct signature is achieved.

The silent lochs

Loch Goil is a tributary of Loch Long, home of RNAD Coulport where Trident missiles are loaded onto the nuclear deterrent submarines. 82 meters deep and sheltered by surrounding hills and distant from underwater noise sources, it is the ideal environment for measuring sound in water. The main facilities buildings are located at the head of the loch and include a control room, workshop, offices, conference room and accommodation for around 30 personnel. The 0.5 km2 range covers the upper half of the loch and is primarily used to evaluate acoustic signatures of ships and submarines (both surfaced and submerged) while moored between four buoys.

Sound data is collected by hydrophones suspended from buoys placed around the vessel under test as well as from a series of fixed hydrophones located on the bottom of the loch. Submarines may also be bombarded with calibrated sound sources, simulating active sonar to measure their Target Echo Strength (TES) so as to determine the effectiveness of their hull shaping and anechoic tiles in absorbing sound waves.

Loch Goil is also used for other static underwater trials and in 2012 HMS Astute conducted an ESCAPEX. Submerged to 28m depth in the loch, volunteer submariners successfully conducted ascents from the Forward Escape Tower (FET) to prove the new escape system on the boat.

The main image above shows HMS Richmond on the Loch Goil range in May 2022. As the only Type 23 frigate so far to have completed the Power Generation Machinery Upgrade (PGMU) with new diesel generators in acoustic enclosures, she will have a significantly different, and hopefully reduced, sound signature compared to her sister ships. Improved stealth was not the main reason for the upgrade but an important benefit that can be assessed using measurements taken on the ranges. As a static range, radiated noise assessment at Loch Goil is focussed on that from sonars, together with auxiliary machinery and generators.

To assess the noise from main propulsion systems, the much larger range at Loch Fyne is used allowing surface ships to be evaluated while steaming at speeds up to 20 knots and submerged submarines up to 14 knots. The instrumented area is about 5km in length, 1km wide and about 140 meters deep. The RN’s large nuclear submarines rarely use this range but it is occasionally used by smaller conventional NATO boats. Placed in the bottom of the loch, there are an extensive array of hydrophones covering various target aspects. Two sets of lights, one located at Leanach and another on Strone Point provide navigational reference points for vessels underway on the range to keep them on the correct course relative to the hydrophones. Target vessels can be accurately tracked using a Differential Global Positioning System (DGPS) and coordinated from the range control building on the eastern shore of the loch.

Underwater ranging

The third noise range in Scottish waters is the Rona Noise Range is situated between the Isle of Skye and the mainland. It is primarily used by submarines as the only UK range that has the space to allow submerged boats to reach full speed. The test area is around 12km long by 4km wide and has an average depth of 235m. The Hebridean Islands and the Scottish mainland provide shelter on three sides which helps isolate the range from the ambient noise of the ocean.

The range is equipped with sensors comprising 8 noise measurement hydrophones and 9 underwater tracking hydrophones. These are connected by cables to the terminal buildings at the northern end of the island of Rona, where data is recorded and analysed. The underwater signatures of submarines are especially sensitive information and is highly classified.

To the South of the Rona Range and is the most sophisticated naval range in the UK. The British Underwater Test and Evaluation Centre (BUTEC) provides real-time tracking in three dimensions above and below the water and is used in the trials, testing, calibration and development of submarines, UUVs, torpedoes, underwater weapons, decoys, sonars and above-water systems. BUTEC was established in the 1970s and has seen significant investment since, including further modernisation as part of the Qinetiq-MoD LTPA contract amendment announced in 2019.

Local bye-laws prevent public access, limit fishing in the area and the airspace up to 1,500 feet over the range can be restricted when required. The deep water, soft sea bed, shelter from wind and swells together with low levels of ambient ocean noise make the site ideal. The instrumented inner sea area is about 10km long and 6km wide, having a depth of up to 200m.

Sophisticated sensor arrays are installed in the inner sound and monitored from the Applecross range terminal buildings. BUTEC is supported by the logistics and administrative depot at Kyle of Lochalsh which has a harbour used by the range vessels operated by Serco.

Royal Navy submarines regularly visit BUTEC to certify crews and equipment in weapon and operational skills. Inert weapons clearance (Spearfish torpedo and TLAM) trials for the Astute class boats were conducted at BUTEC. For more sophisticated submarine tactical trials and development including test-firing Trident missiles, the RN makes use of the US Atlantic Undersea Test and Evaluation Center (AUTEC). Located in the Tounge of the Ocean basin in the Bahamas, this vast facility is 90 km long by 28km wide and 1,800m deep in places.

Development of sonars is facilitated at BUTEC with a floating laboratory and includes towed array calibrations, mine warfare development, hydrophone calibration and measurements of target echo strength and beam pattern trials on submarines and ships. There is a sonobuoy test facility (STF) at BUTEC which was recently upgraded in partnership with Ultra Electronics. The STF certifies that production samples meet their specifications, and supports the development of new sonobuoy types. Qinetiq also maintains underwater acoustic targets, torpedo-sized vehicles that simulate the noise and echo strength of a submarine for ASW training purposes.

Summary

The geography of Scotland provides the optimal environment for these ranges with deep, sheltered sea lochs, conveniently located for the Faslane submarine base and main submarine operating areas of the North Atlantic. In the event of independence, besides the enormous expense, it would be difficult to relocate these facilities as there are no equivalent sites in the rest of the UK that can match the geography of Scotland. There is a small noise range at Grove Point off Portland but located in relatively shallow water of 22 metres with just 3 hydrophones and exposed to ambient sounds of the busy English Channel.

The facilities described above are another example of low-profile but critical infrastructure needed to maintain an effective navy, especially through acoustic hygiene that can provide a battle-winning advantage in underwater warfare.