Sonar Performance

There is a strong tendency in the military observer world to accept manufacturer’s claims at face value and, worse, to assume that those claims are absolute and unvarying even as real world conditions vary.  One common example of this tendency is the discussion of sonar performance.  Observers tend to believe that a manufacturer’s maximum range claim is gospel, is always achievable regardless of real world factors, and every object within the claimed range will be unerringly detected.  Of course, nothing could be further from the truth.

Let’s look a bit closer at sonar performance.

I assume that readers have a basic understanding of sonar types and mechanism of operation.  Sonobuoy Tech Systems website provides a good summary of the various sonobuoy/sonar types. (3)

Passive sonar simply listens to all the noise that impinges on its receiver.  It is up to the operator to determine which bits of noise are naturally occurring and which, if any, are manmade.  So, for passive sonobuoys, a target submarine is detected when its noise level rises above that of the background noise for a sufficient period of time and at a sufficient level to be recognized by the sonar analyst (human or software).  Complicating factors include thermoclines, overlapping noise sources such as other ships or biologics, sound reflecting objects, flow noise from water movement over and past objects or over the sea floor, wave noise, channeling phenomena, etc.

In the real world, a single buoy cannot even determine a reliable distance range for the sub. The reasons for this are 1., that the source level is usually not known and 2., that the sound propagation circumstances could differ significantly between positions that may be located only some hundred meters apart. For example, in good conditions, a 100 dB source level could be heard over a distance of several kilometers while the same source level may only be heard over a distance of a few hundred meters at a position where the conditions are poor. (1)

We see then, that passive sonobuoy detections provide a target bearing only – no range, course, or speed.  Over time, and with multiple detections, range, course, and speed can be developed.  Of course, since sonobuoys are fixed locations, either the target has to move or additional sonobuoys have to be deployed to obtain the cross-fixes necessary to develop the target data.

Active sonar suffers from many of the same problems although, in this case, the source noise level and timing is known.  Active sonars provide bearing and range, under good conditions – but not target course or speed.  Over time, with multiple detections, course and speed can be developed.

While sonar detection ranges are classified, we can get some hints from public source data although such hints are exceedingly rare.

DelBalzo and Stangl modeled active sonobuoy performance using detection ranges of 2-8 nm and, in another scenario, 0.6-1.6 nm. (2)

SSQ-15 (B-Size) active sonobuoy (circa 1960) had a range of 2500 yds. (4)

It is important to understand that manufacturer’s claims are valid only under perfect conditions with all physical and environmental factors being favorable in the extreme and no acoustic countermeasures being employed.  It is also necessary to understand that the claims are based on laboratory tests and software simulations.  To the best of my knowledge, no manufacturer has their own modern naval submarine for use in testing.  At times, they may be allowed to take part in Navy sponsored testing but even that, to the best of my knowledge, is rare and only involves submarine surrogates, not real submarines.  DOT&E has repeatedly reported on the lack of fidelity of submarine surrogates so that offers the manufacturer little realistic information.

A further complicating factor is acoustic countermeasures. 

One such measure is the now ubiquitous anechoic tile that all modern submarines are coated with.  Designed to absorb sound, the US Navy believes that a submarine with anechoic tiles has a good chance to remain undetected even by active sonar at all but very short ranges.

A similar measure is the use of bubbles which trap and ‘deaden’ noise.  This can be either air bubbles injected into the water, as in the Navy’s Prairie/Masker system for surface ships or embedded in paint-like coatings which are applied to vessel hulls.

What this means for the purpose of this discussion is that claimed sonar detection ranges are unrealistically optimistic since none take into account acoustic countermeasures.

We see, then, that manufacturer’s sonar detection claims are unrealistic to the point of uselessness.  Divide a manufacturer’s claims by a factor of 10 and you might begin to approach the realistic sonar range performance.

Considering all factors, it seems likely that realistic sonar detection ranges against modern submarines are something on the order of 0-5 miles for active sonar and zero to dozens of miles for passive sonar although we have to note that passive detection at extended ranges requires just the right combination of factors.

The commonly held belief that a single active sonar ping will instantaneously provide a 100% perfect, target quality data picture for many dozens of miles around is pure fantasy.  This belief was epitomized in some of the comments in the previous ASW corvette story (see, “Shallow Water ASW Story”) wherein commenters objected to the use of active sonar by ASW corvettes in the story, believing, incorrectly, that a single ping by the corvette would instantly provide perfect and complete target data with a submarine torpedo launch occurring seconds later.

All of this discussion should also reinforce the negative ‘review’ of the Sea Hunter in a recent post in which I expressed the utter disbelief that a small, low powered Sea Hunter sonar could continuously track a modern submarine when our very best, large, high powered, ship mounted sonars operated by the best analysts backed by state of the art software could not (see, “The Sea HunterMyth”).

__________________________________

(1)“Submarine Tracking by Means of Passive Sonobuoys”, Kristian Johansson and Per Svensson, FOA-R-97-00440-505-SE, June 1997, ISSN 1104-9154, Div. of Command and Control Warfare Technology, SE-581 11 Linkoping, Sweden,

https://www.foi.se/download/18.3bca00611589ae7987881/1480076259197/FOA-R--97-00440-505--SE.pdf

(2)“Design and Performance of Irregular Sonobuoy Patterns in Complicated Environments”,

Donald R. DelBalzo, Technology Solutions Group, and Kevin C. Stangl, OPNAV N874, 0-933957-38-1 ©2009 MTS,
http://www.dtic.mil/dtic/tr/fulltext/u2/a527878.pdf

(3)Sonobuoy Tech Systems website,

https://www.sonobuoytechsystems.com/products/

(4)“The Evolution of the Sonobuoy from World War II to the Cold War”, Roger A. Holler, U.S. Navy Journal of Underwater Acoustics, Jan-2014,

http://www.navairdevcen.org/PDF/THE%20EVOLUTION%20OF%20THE%20SONOBUOY.pdf

Anechoic Tiles

In its quest for ever quieter submarines, the Navy began affixing sound absorbing tiles (anechoic tiles) to submarines in 1980.  The tiles are an inch or so thick and are made of different layers, materials, and void spaces designed to deaden specific frequencies of sound. (1) 

Interestingly, anechoic tiles were first used on German U-Boats at the end of WWII.

While effective, the tiles have a disturbing tendency to fall off during patrols.  Tile loss reduces the effectiveness of the silencing and increases flow noise due to the “holes” left on the submarine’s surface.  The rough edges of these holes likely create additional turbulence and noise just as any rough edge or projection on a hull would.  Addressing a photo of a ragged looking USS Mississippi on return from patrol, former submariner Bryan Clark stated,

Bryan Clark, a senior fellow at the Center for Strategic and Budgetary Assessments and a former Navy submariner, said the amount of acoustic coating missing on the Mississippi "could create enough flow noise to be a sound problem at even relatively slow speeds. Also, there is enough tile missing that it could reduce the coating's ability to absorb sonar energy and make the submarine easier to find with active sonar." (3)

The Navy has formed various study groups to address the problem and have claimed success multiple times but tile loss remains a problem as this photo of USS Virginia in 2018 shows.

USS Virginia - Note the patches of missing tiles.

One alternative approach to anechoic tiles is being developed by researchers at the University of Michigan.  They are developing a “superhydrophobic (water repellent)” paint-like coating presumably filled with bubble voids which reduces drag thereby increasing speed, fuel efficiency, and quietness. (2)  The challenge, as with anechoic tiles, is to make the coating durable.

Similar research at the Université Paris Diderot in France has examined the use of microscopic bubbles in thin coatings. (1)  The bubbles dissipate acoustic energy.

In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.

To make submarines invisible to the sound frequencies used in sonar, larger bubbles are needed. Still, the researchers predicted that a 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible. (1)

While the experimental results are encouraging, the challenge, again, is to produce an easily applied, durable coating.  This is technology that is worth keeping track of in the future.

____________________________________

(1)Live Science website, “Thin 'Bubble' Coatings Could Hide Submarines from Sonar”, Charles Q. Choi, 4-Feb-2015,

https://www.livescience.com/49689-stealth-coatings-hide-submarines.html

(2)Popular Mechanics website, “Navy Testing Superhydrophobic Hull Coatings For Submarines”, Kyle Mizokami, Jul 5, 2018,

https://www.popularmechanics.com/military/research/a22063315/navy-testing-superhydrophobic-hull-coatings-for-submarines/

(3)Military.com website, “Navy Subs Still Show Issue with Stealth Coating”, The Honolulu Star-Advertiser By William Cole, 2018,

https://www.military.com/daily-news/2017/03/06/navy-subs-still-show-issue-stealth-coating.html

Sonar Fails - Navy Orders More

DOT&E has panned the sonar that the LCS MCM module was trying to use.  So, what does the Navy do?  Order more of them, of course!  From the defense.gov website,

“Raytheon Co., Portsmouth, Rhode Island, is being awarded a $20,406,692 modification to previously awarded contract (N00024-14-C-6302) to exercise options for the procurement of four AN/AQS-20A sonar, mine detecting sets.”

So, the AQS-20A sonar which doesn’t meet specifications, costs $5.1M each.  Seems like a wise use of taxpayer money.  I cannot believe how screwed up Navy procurement is.

The AQS-20A sonar was the small sonar that was to be towed by the now cancelled (or maybe not cancelled) RMMV which has, itself, failed miserably.  You can read any DOT&E report for details on the sonar’s failings, if you’re interested.

T-AGOS

One of the vital but relatively unknown vessels of the Navy is the T-AGOS ocean surveillance ships.  The Navy operates a small fleet of these vessels under the Military Sealift Command.  This post offers a brief look at these vessels and their function.

Function

As you might imagine, getting detailed information on the functionality of these ships is difficult.  The vessels are broadly tasked with surveillance of submarines.  Presumably this includes both tracking of submarines and development of detailed acoustic profiles of submarine classes and specific submarines.  Their primary “weapon” is the SURTASS surveillance towed array sensor system which is intended to monitor submarines at very long ranges. 

Additional functions include oceanographic and hydrographic data collection.

SURTASS

SURTASS is the mobile version of the well known SOSUS system.  In its baseline configuration, the array was passive only but now includes a vertical array active low frequency (LFA) source.  This allows operation in a bistatic mode.  USS Impeccable began operating the LFA in 2004.  Smaller versions of the LFA have been developed for the Victorious class vessels. Other versions of SURTASS are being developed for enhanced shallow water submarine detection.

The horizontal array is 4900 ft long and is towed at a depth of 500-1500 ft.

Ships and Characteristics

The Navy currently operates four T-AGOS ships of two classes.  The three-ship Victorious class was begun in 1986 and the one-ship Impeccable class was started in 1993.  Additional ships were planned in the Impeccable class but were cancelled for budget reasons.

The ships are built with SWATH (Small Waterplane Area Twin Hull) hulls for stability at low speed and in heavy weather.

The current list of active T-AGOS vessels is:

Victorious T-AGOS-19

Able T-AGOS-20

Effective T-AGOS-21

Loyal T-AGOS-22

Impeccable T-AGOS-23

Here are a few physical characteristics for the Victorious class.  The Impeccable class is similar, though larger.

Length:                       234 – 281 ft

Displacement:           3400 – 5400 tons

Speed:                       10 – 13 kts;  3 kts when towing

Crew:                          20 mariners, 5 techs, and up to 15 Navy crew

USS Impeccable

Impeccable Incident

There is a demonstrable and common sense need to provide protection for these ships when they operate near unfriendly countries.  Impeccable was invoved in a famous incident of harassment by Chinese vessels in 2009 that illustrates the need for protection.  From a Naval War College report comes this summary of the incident (1).

“[Impeccable] … was engaged in lawful military activities in China’s claimed EEZ. On 8 March 2009, five PRC vessels—a navy intelligence ship, a government fisheries-patrol vessel, a state oceanographic patrol vessel, and two small fishing trawlers—surrounded and harassed Impeccable approximately seventy-five miles south of Hainan Island in the South China Sea. The fishing trawlers maneuvered within twenty-five feet of Impeccable and then intentionally stopped in front of it, forcing Impeccable to take emergency action to avoid a collision. The U.S. government protested the PRC’s actions as reckless, unprofessional, and unlawful. China responded that Impeccable’s presence in China’s claimed EEZ had been in violation of Chinese domestic law and international law. Impeccable returned to the area the next day under escort of a guided-missile destroyer, the USS Chung-Hoon (DDG 93).”

Note that Impeccable did not have effective protection during the incident.  Failure to provide protection is a recipe for disaster.  You’ll recall the Chinese forcedown and subsequent seizure and looting of the EP-3 aircraft some years back.  Without ready protection, we run the risk of losing a valuable and highly secret vessel.

LCS

The Navy continues to search for a mission the LCS can be effective at.  Adapting a LCS to the T-AGOS mission would be a possibility.  With at least a minimal level of self-protection as well as some useful speed, the LCS-AGOS could operate with a degree of defensive capability that the T-AGOS does not possess.  Of course, the LCS is inherently endurance-challenged but modifications ought to be possible to greatly increase the range and endurance.  This is not a perfect option but might be a way to get more use out of an otherwise fairly useless platform.

In summary, the T-AGOS vessels provide a very valuable function in the Navy’s subsurface warfare effort and, though less glamorous, deserve increased attention, upgrades, and protection.  All in all, an interesting class of ship!

(1)https://www.usnwc.edu/getattachment/d11a2362-fa30-4742-8ec4-c8bed2025114/Close-Encounters-at-Sea--The-USNS-Impeccable-Incid.aspx