Sonar Systems for Vessels as Small as Webber Class WPCs

Photo credit: NavyRecognition. Thales CAPTAS 1

As expected, the EuroNaval 2014 trade show is offering some interesting products. This one might be of interest if the Coast Guard ever decides to get back into the ASW mission. Thales, maker of some of the most highly respected sonars in the world, is offering both hull mounted and towed active/passive sonars for vessels as small as 300 tons. The towed sonar is the CAPTAS 1, joining two previous members of a family of systems that share common technology. The Largest of these, the CAPTAS 4 is being evaluated for the ASW module for the LCS.

Towed Array Sonar–A Tool for Drug Interdiction

The use of “Self Propelled Semi-Submersibles” (SPSS), semi-submersibles that run with only a snorkel above water, and true submersibles by drug smuggling organizations represents a serious challenge to efforts to interdict drug trafficking. SPSS and snorkels are difficult radar and visual targets. They can be detected, but sweep widths are dramatically reduced. True submersibles are invisible to radar and only rarely visible from the air, when the water is clear and calm, and even then, only at relatively short distances.

Source: “Introduction to Naval Weapons Engineering

There is a way to detect these vessels at ranges greater than typical radar ranges against even normal surface contacts. Passive Acoustic Towed Arrays routinely make detections beyond the radar horizon. Direct path detection ranges vary a lot. They are typically inside the radar horizon, but there are common phenomenon that make detection at extended ranges possible. These include the deep sound channel, bottom bounce, surface ducting and convergence zone (all explained here. You can skip the math and go down to “Propagation Paths” that starts about a third of the way down). These conditions frequently allow detection at well over 60 miles. These conditions are also recognizable and predictable. Passive acoustic detection is not as unambiguous as radar, in that passive sonar does not provide range, but it does give other information radar does not, that can aid in classification. In the past, success with passive systems has been highly dependent on the skill of the operator, but reportedly newer systems like the Multi-Function Towed Array (MFTA), planned for the Littoral Combat Ship, provide computer assistance that makes them much more intuitive. Range can sometimes be inferred from knowledge about the water conditions, and it can also be deduced by target motion analysis. Whatever information can be gleaned from the passive contact can be used to cue a helicopter to search a specific area.

Because this sensor is passive, it permits searches without betraying own ship’s position. It is not too hard to believe that drug runners who can build submarines are sophisticated enough to use Electronic Support Measures (ESM) to detect and identify Law Enforcement units  based on their radar emissions. A towed array gives the commander the option of searching a large area while going passive electronically.

It would be reasonable to expect that the Navy would pay for these systems, just as they did sonar systems in the past, so that they could be available for military operations. The Offshore Patrol Cutters should incorporate provision for this system in their design. (Previous related posts here, here, and here.)

National Security Cutter as Navy Patrol Frigate

Navy Times’ “Scoop Deck” asks what the Navy will do “After the frigates are gone” and suggest that variants of the National Security Cutter (NSC) might be a better solution than the Littoral Combat Ship (LSC).

Back in March, Defense News also suggested that the NSC might be the Navy’s best option.

This has been an on going discussion for a long time, fueled no doubt by Northop Grumman’s desire to sell more ships. But the suggestion has been taken seriously. In July 2009, the Congressional Budget Office Study did a study that included an upgraded 20 NSCs as an option to 25 of the LCS.

That study suggested that these 20 NSCs be upgraded as follows:

“For approximately $260 million, the Navy could replace the Close-In Weapon System (CIWS) currently used on the national security cutter with the SeaRAM Mk-15 CIWS. Unlike the former system, which consists of a rapid-firing gun designed to engage subsonic antiship missiles at close ranges, the SeaRAM CIWS would incorporate a rolling airframe missile on the same physical space but provide the ship with the ability to engage supersonic antiship cruise missiles out to 5 nautical miles. The SeaRAM system includes its own sensor suite—a Ku band radar and forward-looking infrared imaging system— to detect, track, and destroy incoming missiles.

“An additional layer of antiship missile defense could be provided by installing the Mk-56 vertical launch system with Evolved Sea Sparrow Missiles (ESSMs) along with an Mk-9 Tracker/Illuminator system to detect, track, and engage antiship missiles. The ESSM can engage supersonic antiship missiles at a range of nearly 30 nautical miles. Installing 20 sets of a 12-cell launching system (which would carry 24ESSMs), buying the missiles, and integrating the weapons with the ships would cost about $1.1billion.”

So these upgrades would cost $1.360B/20 ships or $68M/ship

With many more critics than supporters, there is a lot speculation that the Navy will not build anywhere near the 55 LCSs currently planned. The black-eye lean manning is getting in the Navy lately, and the fact that the LCSs are designed for lean manning with no apparent option for growing the crew, is adding to criticism of its limited weapons and poor endurance. The Coast Guard is looking smart for providing the NSCs and OPCs with both realistic crews and room for growth.

If the government wanted to open an option for the future, it might be smart to increase the CG buy of NSCs to 12, to make up some of the shortage of ship days that is certainly in our future and direct that the last 6 be made as a “B” class with a weapons fit including the systems sited above, a towed array sonar, and all necessary space and equipment for support of two MH-60Fs, with the marginal cost paid out of the Navy budget. The nation would have an additional capability and the Navy would have have a ready option in a mature design, that could take on the functions of the FFGs.

A Relatively Painless Submarine Detection Capability

If the Coast Guard should ever again decide it needs a submarine detection capability, there may be a way to add it to vessels as small as the 87 ft WPBs.

The Navy is currently fielding a new version of it’s  ASW helicopter, the MH-60R, and it’s new dipping sonar is proving much more effective than it’s predecessor.  The complete sonar system can weigh less than 600 pounds.

The Soviets also used dipping sonars, but not just on helicopters. They used them on small surface craft as well. These vessels would work in teams using a sprint and drift tactic.

The same transducer might also be hull mounted with relatively little impact. There is also the possibility that with relatively minor modifications it could be made into a towed variable depth sonar. A combination of hull mounted transducer and variable depth sonar working off the same console could offer some advantages.

Certainly not very effective for chasing nucs, and I’m not suggesting we need a big program to  look for Narco subs, but, should the need arise, it could be work against the ultra quiet but slow moving diesel electric subs that might lurk in the high noise areas of the littorals.