Tom Clancy hypothesis on sonar performance...

[goldorak]

Tom Clancy in one of his books posited the following :

A chinese destroyer is pinging an american sub. Now the distance between the 2 is so great that the destroyer has no usefull echo. On the other hand the american sub uses the reflected ping to "illuminate" chinese subs that are nearby as well as the chinese destroyer. So it finds them on sonar.

Is this thing possibile or is good ol' Clancy just pulling the leg on us ?
I think it has no merit, because subs normally carry anechoic materials so as to attenuate and absorb incoming sound waves. If a sub existed that could reflect almost all incoming energy of a ping and use it to find subs in the neighborhood, well it wouldn't be a stealthy sub at all.

What do you guys think ? Just give general considerations on wether Clancy is telling something plausible or just plain horse****.

[Dr.Sid]

Anechoic means it reflects less. But not nothing.

This method could work. Only problem would be that the 688 would be visible to the Chinese sub as well. Sure it all depends on geometry of the situation and sensitivity advantage.
But if the Chinese sub was close to the destroyer, it would be painted and visible, while the US sub could still be far enough to avoid detection.
On the other hand, if the Chinese sub was far away from the pinger, let's say slightly behind it's active range, it could pick the echo of US sub without being much exposed.

Anyway, this approach would be better used with buoys. They can be passive (thus cheaper) and they you can use explosives or strong surface vessel to paint the area.

But those are interesting tactics. I wonder how much it is used too.

[goldorak]

Originally Posted by Dr.Sid

Anechoic means it reflects less. But not nothing.

Yes of course, another thing is, the reflected ping doesn't have an isotropic propagation. Most of its energy goes into the direction between the emitter platform and the american sub. So any stray sub that wiould be out of this "axis" would not receive sufficient energy to be "visibile" on the american sonar. Do I have the physics wrong and the reflected ping instead would also be propagated isotropically, its energy being distribuited evenly in every direction ?

This method could work. Only problem would be that the 688 would be visible to the Chinese sub as well. Sure it all depends on geometry of the situation and sensitivity advantage.
But if the Chinese sub was close to the destroyer, it would be painted and visible, while the US sub could still be far enough to avoid detection.
On the other hand, if the Chinese sub was far away from the pinger, let's say slightly behind it's active range, it could pick the echo of US sub without being much exposed.

I'm still not conviced that a sub could be illuminated by this reflective ping if its position is way out of the line (bearing) between the chinese destroyer and the american sub.

Anyway, this approach would be better used with buoys. They can be passive (thus cheaper) and they you can use explosives or strong surface vessel to paint the area.

But those are interesting tactics. I wonder how much it is used too.

I think the 2 kinds of situations are different. In Clancy's description the us sub uses a ping generated by another vessel (an enemy vessel) to "paint" the area. But doing so obviously alerts the enemy units to the presence of the american sub, since the ping actually pinged on the us sub in the first place. Explosive echo ranging, and passive sonobuoys don't use a reflected ping to paint the area. The explosions generate the sound waves (equivalent to a normal ping) and then the sonobuoys simply listen to the echo.

[Dr.Sid]

Oh I see .. the reflection itself is almost surely too weak to paint anything. In Clancy it is said that pinger can be detected at 5 times range as the echo. That could be even more difference with today's anechoic coatings.
From this you can say that echo is about 5 times weaker then ping.

Also if you wanted to paint anything with your own echo, you would suffer the same pinger's problem .. you would be detected sooner then you get the echo.

Reflection would not be any much directional. Most common reflection comes from curved surface of the boat, and it will act as spherical wave. That means less directional, but also much less energy reflected in any specific direction.

Anyway the ping itself can paint. Yes, sonar use directed beams, but if you look for a target, you look in all directions (omnidirection or rotational id DW's FFG). Only when you track or investigate specific contact you can use narrow beam.

You could skip beaming in direction of your ally, but then you don't know exactly where he is and comunication with sub is always a problem. Also you could tell to the listener 'hey I'm not pinging in this direction, maybe something is there'. Listener could see where you are pinging from amount of reverbation (especially bottom reverbation) from your ping. Reverbation generaly works same way as light beam in fog.

[Pisces]

Originally Posted by goldorak

Tom Clancy in one of his books posited the following :

A chinese destroyer is pinging an american sub. Now the distance between the 2 is so great that the destroyer has no usefull echo. On the other hand the american sub uses the reflected ping to "illuminate" chinese subs that are nearby as well as the chinese destroyer. So it finds them on sonar.

I'm not getting a clear picture. Does the distant chinese destroyer also reflect the (allready reflected) ping back to the american sub. (in other words a three-way trip ping) That sounds very unlikely because of range attenuation. Anechoic tiles certainly makes this worse for the us sub.

Or does the us sub emit a pulse at the exact same time the Chinese (destroyer) pulse is reflected on them to mask it. (would require supperb timing) That still doesn't help them, it would infact help detection by the distant chinese destroyer because it receives more intensity. But most likely make the closer chinese sub detect them first.

I'm no submariner, but sounds like boo-boo to me.

[goldorak]

Originally Posted by Pisces

I'm not getting a clear picture. Does the distant chinese destroyer also reflect the (allready reflected) ping back to the american sub. (in other words a three-way trip ping) That sounds very unlikely because of range attenuation. Anechoic tiles certainly makes this worse for the us sub.

Or does the us sub emit a pulse at the exact same time the Chinese (destroyer) pulse is reflected on them to mask it. (would require supperb timing) That still doesn't help them, it would infact help detection by the distant chinese destroyer because it receives more intensity. But most likely make the closer chinese sub detect them first.

I'm no submariner, but sounds like boo-boo to me.

No, it goes like this (in this scenario there is only 1 ping and this ping gets reflected, no other active ping is sent in the water) :

1 - chinese destroyer emits a ping
2 - the ping arrives on the american sub
3 - the chinese destroyer being to far does not receive the echo from the ping that it sent moments before
4 - the american sub does not emit any active ping, it basically uses the reflected ping to paint any contacts in the neighboorhood including the chinese destroyer
5 - american sub now know the position of everyone and proceeds to attack and sinks all chinese opposisition (ok this part i made up )

It still find it very improbable and Clancy most assuredly just made up all this to make his story more interesting.
But I don't think that such things happen in real life.

[Dr.Sid]

These are more likely:

-Chinese destroyer pings
-US subs reflects something, but too little for the destroyer to pick it up
-Chinese sub is closer and gets the echo, but reflects something too
-US gets echo of Chinese sub

or

-Chinese destroyer pings
-US subs reflects something, but too little for the destroyer to pick it up
-Chinese sub is also too far away, somewhere near the destroyer, and it is painted too
-US sub knows everything

or

-Chinese destroyer suspect US sub in specific area, does directional ping
-US subs reflects something, but too little for the destroyer to pick it up
-Chinese sub is a closer, and can detect the echo, while being aside anough not to be painted enough
-US sub knows about destroyer, and can estimate it is at safe distance, but it does not know about the Chinese sub

[Kazuaki Shimazaki II]

Originally Posted by goldorak

Tom Clancy in one of his books posited the following :

That must be that horrid book SSN...

A chinese destroyer is pinging an american sub. Now the distance between the 2 is so great that the destroyer has no usefull echo. On the other hand the american sub uses the reflected ping to "illuminate" chinese subs that are nearby as well as the chinese destroyer. So it finds them on sonar.

It is mathematically possible, assuming a ridiculously insensitive Chinese sonar unit versus a ridiculously sensitive American sonar unit combined with the right geometry and some lucky ducting effects that will reduce propagation loss.

Unfortunately, since that's basically the essence of Clancy's entire assumption in pretty much every one of his books, especially SSN (everyone that's not American is deaf, while Americans are all seeing), it can work - in Clancyverse.

[Richard G]

This won't work.

Active range is calculated (by a computer) by clocking the difference between active transmission, and the return signal. (sound speed*time/2=range).

The U.S. sub has no way of knowing the start time of the active transmission from the chinese transmiter.

I did work with a now declassified very low freq active sonar that worked between 2 ships. One ship xmitted, the other ship recieved the signal, but both ships were in close proximity to each other, and datalinked.

[Dr.Sid]

It will work. You only won't have range. But you will have bearing, and you will know something is there.

[TLAM Strike]

Originally Posted by Dr.Sid

It will work. You only won't have range. But you will have bearing, and you will know something is there.

And if you have multiable sensors along the hull (like the old PUFFS or the modern WAA) or in conjunction with a TA you can get range by getting the bearing from each sensor abit one that is kinda vague.