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Fundamental ASW Lessons from Basic Training scenarios

 
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Fundamental ASW Lessons from Basic Training scenarios - 2/19/2018 11:58:39 PM   
Primarchx


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With Silent Service due to release in early March I thought it would be nice to review ASW warfare from the perspective of sub, surface and air platforms using the Basic Training tutorial scenarios. We'll start looking at the engagement from the ship perspective. The USS Stockdale is a DDG 103 Truxtun (Burke) Class Destroyer, circa 2015.

It has the following ASW Sensors ...

An AN/SQS-53C(V)1 Hull Active/Passive sonar operating in the LF band. It shows nominal range of 40nm and early 1990s tech. Also the TB-37/U MFTA passive towed array sonar operating in the VLF band. It shows a nominal range of 70nm and early 2010s tech. Notice that the hull sonar can provide search range, speed and heading info as well as the general Underwater Search capability? That means precise info can be delivered from using the hull sonar in active mode. The TASS can only provide Underwater Search, meaning that heading, speed, depth and range all have to be derived from TMA (target motion analysis) over time.

The Stockdale has 3 ASW weapons. Two triple Mk 32 324mm torpedo tubes holding Mk54 LHT Mod 0 torpedoes facing the port and starboard arcs. Eight RUM-139C VLA (Mk54) VL ASROC weapons in the Mk41 VLS. And a SH-60B maritime patrol helicopter carrying Mk54 LTH Mod 0 torps. Each weapon system uses the Mk54 LHT torpedo as it's terminal ASW tool, the difference between them being the range they can engage from. Typically the MK 32 torp tubes are a close defense weapon, the VLA providing a bit of standoff range (9nm) and the SH-60B providing the longest range for ASW operations for the ship (100+nm).



From this we can see the Mk54 torp itself has a 4nm anti-sub range, a kinematic range of 6nm, a Sub PoH of 85% and can engage targets down to -1510ft. Additionally the torp has Search Pattern, BOL and Re-Attack Capabilities. When dropped from a helo, deployment range is typically somewhere in the realm of 0.3nm from an altitude of 1000' or less. Finally, its' seeker is active & passive, has a 1nm range and operates in the HF band.

Let's take a look at the map display rings around the Stockdale.



The first one to note is the dark green inner circle. This is the longest range organic ASW weapon mounted, the RUM-139C VL ASROC, at 9nm. The next is the first Convergence Zone, the shaded annular zone starting at 25nm and stretching to 30nm, with a second CZ starting at 52nm and stretching to 57nm. Finally we have the outer light green ring, maxing out at 70nm. We can infer that the TASS has the ability to extend to two CZs in the current water environment based on this graphic. It is likely that both the TASS and hull-mounted sonar can hit the first CZ based on range values.

Now the Stockdale moves forward quickly into contested waters at 20kts, hunting for a sub.



Contact! A conventional sub at bearing 187 degrees, estimated 12nm range. The stated range estimate is just that, an estimate, and no speed, heading or depth info is available. Looking at the contact trace we can infer a few things. First, it's a direct path contact that begins at 7.3nm and stretches to the inner limit of the first CZ. Selecting the target, we can see it was only detected by the Stockdale's TASS. The TASS trails below the thermocline while the hull sonar is above (obviously). This may indicate that the contact is below the layer at this range.

Now what to do? First off I'll cut speed to Creep (5kts) to reduce self noise. In reality changing course could reduce the TASS' effectiveness until it straightens again, but I'm not sure how or if such degradation happens in Command. Given the range I'll turn a bit more perpendicular to the target to slow down closure. Checking Stockdale I see that the change in Passive Sonar signatures is just +0.5-1dB by changing the aspect. I figure it's worth it. At the same time I launch my ASW-configured SH-60B helo.



After the turn is completed the Seahawk is now up, contacts continue via the TASS (so no maneuver penalty), the range estimation has shortened to 7.5-12nm. Could be a potential VLA shot but I'll let the helo handle it. Moving the Seahawk in at 200' and Loiter speed (55kts) the bearing shifts a bit but the range estimate shrinks to a 9.8-11nm zone. Laying a couple of sonobuoys should firm up the contact, toot-sweet.



There it is! A North Korean Romeo below the thermocline making 2kts on a course of 310. It's just out of VLA range from the Stockdale. So it's up to the Seahawk to get the kill. Swinging around, the helo makes a low pass and drops a Mk54 at close range, then follows it up with an active sonobuoy at both deep and shallow depths to follow the sub as it evades.



Saw sub. Sunk same.

< Message edited by Primarchx -- 2/20/2018 10:24:46 PM >
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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 12:38:55 AM   
Primarchx


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Got pics updated.

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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 1:09:43 AM   
Primarchx


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Debrief:

1) The Stockdale established contact with the NK Romeo using it's towed array but not it's bow array. A couple of reasons come to mind. First is that the Romeo was cruising below the thermocline, where the towed array normally streams. This means there could be no reduction of acoustic strength between the sub and sensor due to the layer. Another is that the TASS operates in the VLF band, where the Romeo has a base signature of 115dB vs the hull sonar's LF band where the Romeo has a 85dB base signature. This is the comparative difference in volume of a garbage disposal vs a live rock band.

2) TMA occurred quickly aboard the Stockton. Probably a combination of the modern tech of the TB-37/U MFTA, crew quality (Normal) and the strong signature of the Romeo at that near range to the ship during the tracking.

3) The SH-60B was the tool of choice. The VLA and point-defense Torpedo Tubes have their place for close encounters, but when given the option, DON'T close range with a sub. The helo has all the tools to track down and kill a well-localized sub contact while having a good degree of protection from counter-attack.

< Message edited by Primarchx -- 2/20/2018 12:07:36 PM >

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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 3:52:13 AM   
apache85

 

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Very nice!

The sensor analysis is particularly good. Thanks

_____________________________


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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 3:01:09 PM   
Primarchx


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Another thing to note is that the destroyer was moving at max cruising speed, 20kts. Not an optimal speed with regard to self-noise, as you can see it is cavitating (CAV), which effects how easy it is to pick up with passive sonar. Also it's own sensors are degraded by this same noise and interference at high speed.

A slower, constant speed or the use of sprint-drift throttling would provide better use of the passive sonar. You can see the change in quality of the sub contact as the ship slows to 5kts.

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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 3:05:06 PM   
BrianinMinnie

 

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Hi Primarchx,

Great work on those samples.

"Checking Stockdale, I see that the change in Passive Sonar signatures is just +0.5-1dB by changing the aspect"

Where do we find this info? (the change in Passive signatures)

Does it update vs whatever contact we have selected, as we maneuver the ship? or is it in the ship database as various dB statistics per database listing?

Also, is there any way to tell how loud the ship that you’re currently maneuvering in dB's is, helping to determine if you’ve been detected by the sub? or if you’re in a sub, if you’ve been detected by enemy combatants?

Any help is appreciated.

Thanks


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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 4:18:19 PM   
Primarchx


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quote:


"Checking Stockdale, I see that the change in Passive Sonar signatures is just +0.5-1dB by changing the aspect"

Where do we find this info? (the change in Passive signatures)

Does it update vs whatever contact we have selected, as we maneuver the ship? or is it in the ship database as various dB statistics per database listing?


It's in the signatures section of the platform's Data Base entry, spread out between Front, Side and Rear. I see these as baseline figures that are adjusted by environmental and performance changes. As far as I can tell they are static figures in the DB.

quote:

Also, is there any way to tell how loud the ship that you’re currently maneuvering in dB's is, helping to determine if you’ve been detected by the sub? or if you’re in a sub, if you’ve been detected by enemy combatants?


I wish! It's been something I've asked for in the past, as well as rating how 'strong' the incoming passive datum is. Without that info you really just have to make educated guesses and stick to SOPs when it comes to this ... see my Sub Ops - The Thin Blue Line post about those (http://www.matrixgames.com/forums/tm.asp?m=3876405.

If you're driving a sub and make a passive contact to another sub you really have few cues as to whether or where they might also see you. So in most cases the first thing you do is slow to Creep speed to reduce your noise and hopefully sharpen your passive sensors. Then you want to stay as far away and at as low a speed as you can but also work in a way that you can improve your contact info.

A few tricks here are that you can usually determine a CZ contact from a closer, direct-path contact since a CZ contact usually originates at the inner edge of a CZ and radiates away from there. You will probably NOT get a very localized position from a CZ contact except that it originates in one of CZ rings, though it's significantly more likely to be in the first CZ than subsequent ones. I'll often drop a pair of RPs as a guide along the bearing line beginning at the edge of the first CZ and ending at the outer edge.

Next discover what sensors are holding the contact. Recognize that a VDS or towed array is always deployed BELOW the Layer. Flank and bow arrays are of course in the same depth location as your sub. As you hold the contact move slowly between the various depths and see if the contact is improved or lost/gained by your passive sensors (and if so, which ones are doing what). In my experience this includes moving from Above the Layer to Shallow depth (if they are significantly different) as I've found contact on Shallow targets has improved even when my sub is Above the Layer. Making these depth changes may assist you in determining what depth your target is lurking with an educated guess.

So now let's assume an unidentified sub with no other available info than it's bearing and contacting sensor (TASS) has been spotted, with an uncertainty area that begins at the edge of your first CZ and spiking out from there. You can make two basic initial assumptions - it's somewhere in the first CZ and it's below the layer. You reduce speed, lay initial RPs to show the initial contact bearing and alter course in the contact's direction and begin to alter depth, but only maintain contact with the TASS. You notice aspect changes as you move towards it, indicating movement across your bow in one direction or another. Your data on the contact may improve (type, class, speed, bearing, depth), if you're lucky.

If it's truly a CZ contact your sensors will very likely lose the contact once it falls out of the CZ. This is probably a time for a dash to close distance, since CZs are usually the same distance and width for all passive sensors that can utilize them. This can be complex but I typically dash at the shallowest depth I can to avoid cavitation. This is because hull sensors have less range than towed arrays (which we know lurk below the Layer). Estimate a location you think you would reacquire the target and then move as fast as you prudently can to get there. Be sure to do a quick ESM sweep before dashing, just make sure no MPA are around dropping sonobuoys.

Let's say you make it to your dash destination with no incident. Slow back down to Creep, check the surface for ESM and then slowly move between the depths, to see if you can reestablish contact. It may take a little while, or not happen at all, depending on the target's course and speed. But let's assume you soon do pick up the contact. This time it's your bow sonar while above the Layer and the spike ends before the first CZ. You're close! But are you too close?

Now it's time to refine your data, maintaining the contact but keeping your distance. Dip into the Layer a bit and see where/if you lose the contact. This will help you determine how strong the contact is. If you lose it as soon as you go into the layer, odds are good it's not very close and if you maintain the contact with the bow sonar even on the other side of the Layer, it's gonna be close. Using Command's TMA and doing a bit of your own you can try to slip behind the target and get out of their bow sensor arc. Be aware that if you let a faster target do this you may have to dash to get into weapon parameters which could expose you to another enemy.

Once you've formed up the target's info you're ready to begin engagement. But that's another story!

< Message edited by Primarchx -- 2/20/2018 4:22:43 PM >

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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 5:46:21 PM   
Gunner98

 

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This is really good stuff - thanks for posting!

Very tricky business, and your insights are very useful.

B

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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 6:26:41 PM   
Primarchx


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quote:

ORIGINAL: Gunner98

This is really good stuff - thanks for posting!

Very tricky business, and your insights are very useful.

B



Thanks, Gunner! Hope to have a sub v. sub overview up soon.

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RE: Fundamental ASW Lessons from Basic Training: Warshi... - 2/20/2018 10:55:01 PM   
mikkey


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Very interesting, thanks Primarchx!

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