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”).

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(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