AndrewJ
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Joined: 1/5/2014
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Hello folks,
I found a very interesting report while browsing around, called "Probing the Ocean for Submarines. A History of the AN/SQS-26 Long-Range Echo-Ranging Sonar." You can download it here: https://apps.dtic.mil/sti/pdfs/ADA538018.pdf (Apologies if people have posted this already.)
Written by one of the people responsible for the development of the system, the report details the development and introduction into service of the AN/SQS-26 active sonar, from its origin in the mid-1950s, through to the mid-70s. The AN/SQS-26 was to be the main sonar on the Knox-class ASW frigates and other ships in that time period.
The report is written in mostly non-technical language, so you don't need to be a physicist to understand it, and it has a lot of very clear descriptions of the general operating principles of the sonar, including the different modes of operation (surface layer/direct path, bottom bounce, convergence zone), and physical and environmental factors influencing sonar performance. Perhaps most interesting to a CMO gamer is Chapter 9, on Fleet Performance. This gives multiple examples of the sonar's performance in actual real-world use, both in planned ASW exercises and against passing NATO and Soviet submarines. It's a fascinating look at actual sonar operations that most of us will never have the chance to see.
There were some things in here that I already 'sort-of' knew, more that I didn't, and this report helped confirm which ideas were correct, and clear up misconceptions where I was wrong. If you don't have a chance to go through all 266 pages, here's some of the highlights and factoids that seemed interesting to me.
Coming out of WWII, sonars had a big problem trying to detect submarines below the layer. Sonar pulses travelled well in the surface duct above the layer, but could only travel down below the layer at reasonably close distances. This left a large 'shadow zone' where it was difficult to find subs.
If you direct a sonar beam obliquely downwards you can get it to bounce back up off the bottom of the sea and up into that shadow zone. This fills the gap between the direct-path surface layer detections and the distant convergence zone, and gives you long-range detections that are reasonably independent of submarine depth. This new bottom bounce technique was a major part of the SQS-26.
Wartime QB sonar and Postwar QHB - effective range about a mile against shallow targets.
SQS-4 - about 2.5 miles vs shallow targets, only about 1 mile vs targets under the layer.
SQS-23 - direct path to about 10 miles
SQS-26 - direct path out to about 10 miles, bottom bounce contacts out to 25 miles! CZ contacts further out to as much as 40 miles, in final versions.
Active sonar works in convergence zones, provided the sonar has the power to reach that distance, and the SQS-26 was one of the first to be able to do so reliably. It was easier to get convergence zone contacts in the Mediterranean, where the convergence zones tended to be closer to the ship, than out in the big oceans where they were further away. Low frequency research test rigs in 1955 showed you could get active sonar contacts as much as 100 miles away in the third convergence zone, if your gear and conditions were right, although those test systems were much too big for operational ASW platforms. CZ contacts aren't greatly affected by target depth, since the sound is passing through the layer at a steep enough angle that it is not disturbed.
Active sonar works just fine against surface ships, which have about the same sonar echo as a submerged submarine. There are multiple examples of the active sonar being used to track other ships, even out in the convergence zone, which is a great non-radar way to keep tabs on friends and enemies. He even provides examples of ships on exercise using their active sonars to maintain formation with each other while remaining radar-quiet, twenty miles apart at 10 knots in the CZ.
Bottom bounce detection is not only affected by the condition of the bottom (no surprise), but also by wind-speed. The bounced sonar beam rising from the sea-floor will reflect off the surface of the ocean around the target. If the sea is rough, a significant amount of backscatter will be generated, making it harder to pick out the echo, and thus reducing the sonar's performance.
Prairie Masker isn't just about hiding your propellor noise from the enemy. It also gives a significant boost to your sonar system, by reducing the amount of self-generated noise that reaches the sonar.
Active sonar searches can be slow, particularly in older WWII era systems, where narrow beams meant it took about 4 minutes for a single 360-degree sweep. The original prototype SQS-26 only had a 30-degree search beam, and it wasn't until testing in the mid-1960s that they opened it up to 120 degrees.
Variable Depth Sonars, 300 to 500 feet down, can get a performance boost from long-range refraction sound channels to increase detection ranges. However, it turns out that these aren't found everywhere, and generally don't exist within 30 degrees of the equator. The performance of a VDS there isn't much better than a hull sonar.
CZs do not provide a uniform boost in signal strength. They are strongest in the center, and weaker at the inner and outer edges. And they vary in size, a lot. In the Mediterranean they are typically around 20 miles, in the Pacific 28 miles, and in the North Atlantic 35-40 miles.
The sonar equipment itself is only part of the equation, and I hadn't really appreciated just how crucial knowledge of the environment was to getting good sonar performance, and how little knowledge we really had at the time. Choosing the optimal bottom bounce settings required good knowledge of the seafloor characteristics, and thus there was an extensive survey program to map these. There were even plans made to route shipping across areas where bottom bounce sonar performance was optimal, to enhance the detection of nearby enemy subs. Another environmental issue was reverberation, which varied significantly with time of day, season, and location. These were big variations - 10 to 15 dB - and couldn't be ignored. It turned out that causes were often biological: e.g., the highly reflective gas-filled swim bladders of swarms of juvenile fish! Since you could predict fish locations by what they ate (zooplankton), and you could predict those by what they ate (phytoplankton), efforts were made to map plankton concentrations. So environmental surveys and geological mapping were a crucial part of ASW efforts.
It wasn't just the electronics, etc., which had an impact on sonar performance. Structures like the sonar dome had a major effect on performance. Initially, the dome of the SQS-26 was made of painted steel. However, the high-powered sonar pulses and flexing of the dome face had the effect of knocking the paint off the dome, leaving rough surfaces and leading to corrosion and fouling which greatly reduced the sonar performance. In the early 1970s they introduced a new pressurized dome design, with wire-reinforced rubber windows, and performance shot up. With the steel dome, convergence zone contacts weren't possible much beyond 30 miles, which was a problem in the Atlantic where the zones were further out, often between 35 and 40 miles. When the rubber dome was introduced, strong CZ detections were demonstrated out to 40 miles. The rubber dome also improved flow-noise characteristics, such that a rubber-dome Knox at 22 knots was just as effective a search platform as one at 15 knots. By contrast, a steel-dome Knox was 20dB noisier at those speeds, and completely unable to echo-range into the convergence zone.
The Mediterranean has hugely variable sonar conditions. There is no surface duct in the 5 warmer months of the year, meaning direct path ranges are typically limited to as little as 3/4 of a mile!
The SQS-26's ability to bottom-bounce and use convergence zones meant it could get useful long-ranged contacts at 10 to 25 miles, year-round. Further complicating the Mediterranean situation is its chemical composition, which causes significantly higher attenuation of sonar signals than in the Atlantic or Pacific.
The Fleet Performance chapter has all sorts of interesting bits of information. Submerged Foxtrot detection: 20 miles in CZ in Med. Multiple examples of vectoring other aircraft and ships onto distant CZ contacts. Subs finding it difficult to judge when they had been detected by the active sonar. Long CZ detection ranges against Soviet subs reduce the ability of the sub's escorts to interfere with the surveillance. US SSN detections in the Med from 12 miles up to as much as (briefly) 28 miles. In the Pacific, contacts on a US SS in the second CZ at 60 miles. In the North Atlantic, reliable CZ contacts on subs below the layer at 35 miles. Tracking an Echo-II in the CZ for 13 hours. Etc. etc.
The AN/SQS-26 was a very complex system, with multiple modes of operation, and crew competence played a huge role in how well it performed. He mentions how he felt that only 1 in 10 crews could really get the best out of the finicky early versions of the system. Later in the eighties, when passive was in and active was out, some crews had almost no experience with the active portion of the sonar.
And plenty more... Well worth the read, if that sort of thing interests you.
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