9 minute read
Yes or No: Probability and Confidence in ASW
By LCDR Tom Phillips, USN (Ret.)
ASW is tough. It is tougher if you are stupid. It is IMPOSSIBLE if you are stupid and don’t know it. Bishop’s Law #5 – You can’t deal with uncertainty by ignoring it.
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ASW is a game of probabilities. We use probabilistic terms all the time but perhaps do not think of them as uncertainties. Modern ASW seems to be steadily trending toward more and more binary: zero or one… yes or no. Absolute.
Confidence Levels
Right on the face of it, we have four confidence levels, NONSUB, POSSUB, PROBSUB and CERTSUB. POSSUB and PROSUB possible and probable, clearly probabilistic terms.
Confidence levels CERTSUB and NONSUB sound very declarative, very binary, and don’t sound as probabilistic as POSSUB or PROBSUB. But look closer. CERTSUB might not be so CERT, as breaking waves, pods of dolphins, flotsam and jetsam, spurious radar contacts, have ALL-too-often been declared CERTSUB during real ASW. The less experienced against REAL submarines that ASW forces are, the less real ASW they have seen, and when the shooting starts, the less actual combat they have had, the more nervous they are, the more “things” will be declared to be submarine contacts, when the danger is real. (I can vividly still recall my old man telling me about sailing out of the Chesapeake Bay into Torpedo Junction in 1942, and the blizzard of “periscopes” detected: every breaking wave was a feather. U-boats had sunk 397 ships there in six months.) If you have never seen the threat submarine, when can you declare it to be CERTSUB? When you see floating tennis shoes and nuclear reactors? Even an oil slick after an attack can be a deception - such tricks almost as old as ASW itself.
NONSUB. You are declaring that what somebody thought might be a submarine, something hard to find, something you can not even see, something YOU DID NOT FIND, is, therefore NOT THERE….. based on your extensive and exhaustive investigation….. You are declaring that you have proved a negative. There is no submarine here. Are you saying YES, you have stopped beating your wife? Or NO, you have not stopped beating your wife? Traditionally, NONSUB could only be declared if you found something there not a submarine which explained the alarm. Say for example, sighting Moby, with a 50-Hz ac-powered prosthetic flipper provided by the early Greenpeace folks after that unfortunate incident with the Pequod.
Probability Terms
Like the forward pass, where there are three outcomes relative to it (disregarding flag on the play); completion, an incompletion, or an interception, there are three outcomes which are relevant to the business of searching a Datum:
Detection: The obvious. Missed Detection: A sub was there but you missed him; False Alarm: A sub was NOT there but you thought you had one. They are all probabilities.
Probability of Detection versus Probability of Missed Detection
They sum to 100%. Probability of missed detection is 1 minus probability of detection. If, given the submarine is really there, and probability of detection is, say, 75% (0.75) then probability of missed detection must be 25% (100% minus 75%, or 1.0 - 0.25).
Probability of Missed Detection and Probability of False Alarm
If you do not detect, there are the two other outcomes, missed detection and false alarm. They do NOT sum to 100%. They are in opposition to each other, but in a complex mathematical relationship (not simple addition or subtraction summing to 100% In fact, they will not sum to 100%), unlike the simple summing-to-100% relationship of detection and missed detection. RELAX! we are NOT going to the mathematical proof here, or elsewhere if I can help it. Both missed detections and false alarms are undesirable, but they are inescapable in real life ASW.
An ASW Conundrum:
Minimize False Alarms Versus Minimize Missed Detections
You can minimize false alarms by raising the threshold of what is to be defined as “detection.” No calls unless Helen Keller can make the call. Every call will be a valid submarine contact. You’ll never be wrong that a called detection is not a submarine, since your criteria is so high that there are virtually NO FALSE ALARMS, but you will miss a lot of submarines, and that can be fatal to ships, people, mission, strategy, objectives, etc. A BGO. It is an ASW conundrum. In theory, at the extreme, you can eliminate false alarms, but logically, you can see that there has to be consequently more missed detections.
You can minimize missed detections. To be damn sure you don’t miss the sub, you must call POSSUB at anything which COULD remotely be a whiff of a submarine. In theory, you will not miss a valid detection, but clearly, many will later be resolved (or declared) to be NONSUB, and technically fall
into the category of having been a false alarm, but be careful of that dangerous term, for if you want to minimize false alarms, you will increase missed detections, and a missed detection is far more likely to be fatal, whereas a false alarm is a bother, a necessary bother, can be annoying and frustrating, and use up a lot of resources (the cost of doing business in the real world), but is not usually fatal. However, collapsing a screen on a false contact can, and has, opened a path for the real submarine to penetrate to attack – fatal. A smart BG will not collapse the inner zone screen on a detection…… which puts pressure on the Romeo and highlights the classification flaws of its sonar.
Remember, to properly declare a possible detection as NONSUB, which by definition then becomes a false alarm, is a positive outcome, and a successful conclusion to an ASW effort. Many POSSUBS will be determined to be NONSUB, and that’s ALL RIGHT…. If you ARE right.
Is there some middle ground between these two extremes? It depends on:
WHERE Are You Doing Your ASW?
Way out there beyond the near field of the defended force, we can accept missed detections because, there is a tomorrow. If we miss out there, there is hopefully another chance later as the threat submarine progresses from distant, to middle, to inner, to TDZ, to in our knickers.
In the inner zone, there is no tomorrow. Our back is against the wall. We cannot allow a submarine to get by us. We simply can not afford a missed detection. (Perhaps there will be revenge, but revenge is a bitter pill.) We CAN drive the probability of a missed detection down to virtually zero, but only by accepting many false alarms. Many false alarms means many investigations and a lot of activity for days, perhaps weeks. Our resources must be plentiful enough and our tactics must be FAST enough, and robust enough to deal with this reality. In a serious situation – war, or crumbling edge – we will have to sustain a lot of flight time, standing sentry around the clock. Sustained, numbing, flight ops. Why? Because in the inner zone, seconds count and no alert posture can possibly suffice. You have to already be in the air. If you are not already airborne, you are not a contributor to ASW inner zone defense.
The Tyranny of Close-in ASW.
This critical insight means a LOT of time in the air, a lot of energy, wear and tear on equipment, crew rest stress, stores expended, from the moment when a hostile submarine COULD be out there as we go to harm’s way. If we wait until we believe he is PROBABLY there, and he surprises us, we lose…
I have been there: Based on our intel, one fine exercise in the ECS, we estimated when the submarine would intercept and saved ourselves until we thought we would need to be airborne. The submarine attacked while we were beddy-bye in our racks, lounging in the ready room, conserving ourselves and our resources: we lost the fight before we were even in it. We KNEW he COULD be there but we decided he PROBABLY wouldn’t be. Never forget, he gets a vote and will be creative. We postulated what he would likely do in the conditions (water depth, merchant traffic, beach news, and a whole bunch of factors I can not talk about here, or I’d have to kill you. That was NOT HIS VOTE.
BREAK BREAK
Most of us do not truly experience this on range, because we KNOW the “sub” is there. We get the occasional false contact, and evaluate it sometimes as NONSUB, and at other times, yes, attack it. Driven To Classify with Ordnance:
That’s what the RN did in the Falklands. Their situation is eerily like ours today. They only had ASW helicopters and ships, and four distant submarines in associated support (Where? Sorry, that answer is classified at the “burn-beforereading” level and present deponent does not even want to talk about it).
So, their two carriers were defended in their inner zone and middle zone by helicopters and ships. Wherever the RN submarines might have been, they were NOT near the carriers, that’s for sure and, to ANY who call themselves an ASW professional, it better be a BGO why.
The results? They shot ALL the torpedoes they HAD - 200 of them, over the four weeks - and hit no submarine – it was a rough time for the whale population of the South Atlantic.
Why? Because they COULD NOT AFFORD a missed detection. Imagine how everything would have changed had one RN carrier been torpedoed by an Argentine diesel submarine. Admiral Sandy Woodward, RN, the commander down there, said such a hit would have won the WAR for Argentina.
The Argies claim they never even heard a torpedo at any time during the entire war, ALTHOUGH they conducted several torpedo attacks on RN ships but their torpedoes failed. Some serious food for thought for the Romeo carrier community as well as expeditionary too.
“The U.S. Navy report on the Falklands stated: The Royal Navy, long believed to be the best equipped and trained Navy in the free world in the field of shallow water ASW, was unable to successfully localize and destroy the Argentine submarine San Luis, known to be operating in the vicinity of the task force for a considerable period” (30 days)
“Woodward wrote of the “inescapable truth that the Argentine commanders failed inexplicably to realize that if they had hit Hermes, the British would have been finished. They never really went after the one target that would surely have given them victory.” Does any of this sound like it might apply to us in a modern situation in a limited “war?” Short of war, who is holding who at risk as a way to influence the decision to start shooting?
A BIG problem for us? We don’t exercise these conditions. All those NONSUB outcomes from false alarms, are annoying, burn time and resources, and can cause all sorts of angst and force poor decisions as the pressure builds. So there can be a lot of pressure from the uninitiated (not understanding the numbing endurance and patience carrier ASW requires) trying to economize all that energy usage and all those false alarms. That can be a very dangerous thing in ASW. One missed detection in the inner zone can well be fatal.
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