View Full Version : Effects of Depth on Sonar
Hello everyone
I haven't been here for years, perhaps one or two remember me from past Sub Command (and SCX) times. Yesterday I played Dangerous Waters again and it was great fun!
Now I wondered about sonar performance and depth, because frankly I can't remember how that worked in-game. How does your own depth and the depth of your targets affects detectabilities (both ways). Does depth actualy matters at all? Of course disregarding the obvious layer effects and surface noise.
What are the best depths to detect ships and subs at various depths (for each SSP)? What are the best depths to stay undedected?
And while surface noise will degrade own sonar performance at shallow depth, will it actualy cover my ownship noise?
In a enviornment without layer, will it actualy matter wether I drive at 400 or 1200ft (in game)?
My tests showed that besides the layer effects and surface noise, depth has no effect. It's 2D problem. Depth is only used to detect on what side of the layer you are.
All the results are here:
http://www.subsim.com/radioroom/showthread.php?t=117814
Thanks, fascinating read.
Molon Labe
02-21-08, 09:59 AM
http://www.subsim.com/radioroom/showthread.php?t=124973
Sonoboy's tests showed a link between detecting platform depth and detection range, when the contact was at the surface and the detecting platform was below the layer in a surface duct SSP. My interpretation of his data is that DW increases modeled transmission loss at higher velocity points on the SSP. I doubt this is only applicable to cross-layer detections.
I have accidentally attributed this data to Dr. Sid in the past, so I apologize to Sonoboy.
EDIT: I can't say I fully understand Dr. Sid's data on the shadow zone (the half on the SNR vs range is simple enough though) but my best guess is that it shows a "longer" shadow zone at depths closer to the layer--that the shadow zone extends closer to the detecting platfrom at shallower depths. That is completely consistent with Sonoboy's data showing a significant correlation between depth and detection range.
Yes .. the shape of the front 'edge' of the shadow zone can be seen on my data too, but not clearly. Sonobuoy made it clear.
But this only extends the layer mechanism a bit. And it is shadow zone SHAPE alone. It does not affect transmittion loss. It stays constant, until you enter shadow zone, then it becomes zero.
Also the front slope of the shado zone is quite steep, I doubt it could be used much.
Molon Labe
02-21-08, 02:44 PM
Yes .. the shape of the front 'edge' of the shadow zone can be seen on my data too, but not clearly. Sonobuoy made it clear.
But this only extends the layer mechanism a bit. And it is shadow zone SHAPE alone. It does not affect transmittion loss. It stays constant, until you enter shadow zone, then it becomes zero.
Also the front slope of the shado zone is quite steep, I doubt it could be used much.
I've been applying it to active sonar evasion over the last few months, and found that it can make the difference between getting in range for a USET shot or having a ton of ASROCs shot at you.
Hm .. I see .. yes, it's better to go to depth just below the layer, than deeper. :up:
I have been using the 'just bellow the layer' approach a couple of times and it works quite well.
What I still wonder is how exactly does shallow depth affect sonar dedection. I know that the increased ambiet noise close to the surface decreases my own sonar performance. But does it also help to mask my signature? Will I be less detectable by lets say a surface ship when operating at PD compared to 250ft?
On a similar note I wonder how the acoustic condition areas on the nav map work exactly. As we know, the different shades of blue on the nav map do not idicate depth but rather acoustic conditions. The question is, where in calculation of the game do they take effect? Will they effect the sensor only (by reducing effectivity) or also the source (by masking emissions)? What about the in between? Let's say both sensor and source are in good acoustic conditions but in between them is a region of bad ones. Is the travel of sound trough that region of bad acoustic conditions taken into account by the game?
My last question is unrelated but I don't want to open a new topic just for that. When using the active sonar of the FFG I noticed that some seconds after the ping I saw a second "wave" traveling up the display. I guess this is some sort of bottom bounce effect, where some sound waves have taken a longer route that took longer. What effect was it exactly that I have been observing here? I can't remember the exact SSP, it was in about 250ft deep water and general bad acoustic conditions.
Now that is old HOAX ! Color on the map is combination of depth and 3d shading. It has nothing to do with sound propagation. I really wonder where this idea came from.
As for the second wave, it should not work like this, since the reciever can't tell bottom bounce from direct reflection. Bottom bounces should just create false contact little far away from the real contacts.
Anyway I have never seen anything like 'second way' :huh:
Now that thing about the map colour is funny, I think that has been around since 688(I) H/K.
On the active sonar thing, I actualy saw a second "false" contact a bit further out. And I also saw a second run of screen refreshing going over the screen when the primary ping reached the far end of the screen.
Try to post screen, or save.
And btw. what platform you were in ? Since you play since 688 .. are you aware that as the ping reaches far end usually next ping is sent ? :-D
It was on the FFG, single ping mode. I still remember the mission, will try to recreate it later and make a savegame.
It's funny, but I have not been able to recreate the second ping thing. Played the mission again and drove to the aproximatly same spot on the map, but this time all seemed normal.
But while playing mission 2 of the perisher campaign I noticed something strange that seemed to be in contrast to the previous testings of the shadow zone. I created a test mission to check it out further:
Latest LWAMI
Ownship is 688(I) at 5 kts and various depth
Oponent is 688(I) at 5 kts and 350ft
SSP is CZ with layer at 390ft, seastate 3, rock bottom and 10'000ft water depth.
Goal was to check out the boundarys of the shadow zone depending on depth. What I did is observe the broadband sphere sonar and note the range at wich I would pick up the other sub. Since the shadow is about completely block sound, SNR and sensor sensitivity should not play a role.
Here are the results:
http://cid-f33d8b1019e6f4ac.skydrive.live.com/self.aspx/Public/dw.JPGhttp://x7buza.bay.livefilestore.com/y1pm3An14az1yXUSsvXlz8TOxLPSzevazm6J7TkcyISJ1qITL_ 7oQjH43N2ox-DVYAlZvzs1fu9LJYlTYpp9lDmqQ/dw.JPG
-What is strange are the variations between 400 and 550ft.
-And what is realy strange is the immense drop in detection distance when ownship is bellow 800ft.
I did some more tests with the speed of the enemy sub at 32 knots to verify that the shadow zone is realy about cutting all sound and not dependent of SNR. Since changes to the mission meant I could not play from the same savegame as with the first test, the layer depth varied a bit and therefore the results were not exactly the same. But they seemed to generaly support the first results. At great depth I would not dedect the other sub even at 32kts until range was down to about 1.2 NM, then he would pop up on broadband in all his brightness.
This is all rather confusing and seems to contradict many tests that were done here previously. Anyone can make sense out of that?
Molon Labe
03-23-08, 11:35 AM
Easy. Sonoboy's tests were in a surface duct SSP. Yours are in a CZ SSP. Apples and oranges.
But does Surface Duct or CZ realy matters in ranges bellow 8 NM? What is the difference in behaviour?
Regardles of the previous tests, what is the explanation of what happened here?
Molon Labe
03-23-08, 12:07 PM
Usually, the point of doing tests like yours is to find out what the model is. I'm not aware of any testing on the shape of the shadow zone in CZs, so you're breaking new ground. I did a little bit of testing with sonoboys a while back and noticed that generally cross-layer detections were a bit shorter in a CZ than an SD, but that didn't make any attempt to account for the depth of the buoy relative to the layer.
As for explaining it...I really can't. I'm looking at the SSP shape and I'm quite surprised that you're getting longer ranges near the layer boundary than you are when you're deeper. I would have expected exactly the opposite.
But one thing I would expect is a difference because in the CZ SSP once the sound gets below the layer it is going to curve out instead of down. I would expect the shadow zone shape to end up being similar, but it should (in my judgment) but pushed back further out from the source.
Pretty strange .. will try this too. Me previous test did not show anything like that.
Molon Labe
03-23-08, 02:01 PM
I'd like to suggest to do future tests with narrowband. Reading the broadband can be quite subjective, especially for quieter contacts like submarines. It might take a few mintues of looking at the intermediate interval to realize you even have a contact present. With NB you get the line right away as long as you have the right bearing displayed.
With LWAMI you get contacts on the sphere on broadband before narrowband. Also since the shadow zone is "cutting off" sound, the contacts pop up on broadband quite bright and noticeable. But perhaps the SNR methode of the FFG towed array would still be the best methode.
BB is quite OK on these ranges.
I did the same test.
The problem near near the indicated layer depth (400ft in my case) seems to be that sub must be at about 415ft to feel the effect. This depth difference does not seem to be depending on the range and it was 15 ft for LA in my case. If you were 10ft bellow indicated layer depth, it should work like you have found.
I could not reproduce the deep problem. I can measure nice curve down the crushing depth, no anomalies.
Make sure the target sub did not slow down for some reason, it happened to me once, with no obvious reason. If it still happens, post the save.
Molon Labe
03-23-08, 05:01 PM
BB is quite OK on these ranges.
I did the same test.
The problem near near the indicated layer depth (400ft in my case) seems to be that sub must be at about 415ft to feel the effect. This depth difference does not seem to be depending on the range and it was 15 ft for LA in my case. If you were 10ft bellow indicated layer depth, it should work like you have found.
To feel what effect?
I think if the layer is at 400ft and the sub has an indicated depth of 415 feet, it's possible that the sonar entity is above the layer. 415 feet is the depth of the keel, and 15 feet is what...two decks? How many decks does a 688I have? More than two I'd guess...
Dr. Sid, try this mission:
http://x7buza.bay.livefilestore.com/y1pm3An14az1yWNi07W1Xq9rAchV7WvSE4YEeqr7Lv8izXnR9k YTn2ydXCXln4CXCRK95SIcgxZM6KD_YNBUPZDqw/detectiontest.mu?download
I attached the mission and not the savegame (which is abou 4mb large). So layer depth might varie, but I don't think it should make a difference. If it doesn't work I will uploade a savegame.
What you should see is a 688(I) screaming in at 32 knots and poping up on broadband at 1.4 NM.
LA diameter is 33 feet (10 meters). So yeah, sphere center is about 15ft from keel. That should be it.
Bohaha ! You have discovered second layer ! :up:
Check SSP in that game of yours. There is speed maximum, which DW marks as layer depth. Allright. And then, with covergence zones type SSP, the speed goes down and up again. Theres is minimum on that curve. In your case it's neatly on 800ft. As you got under 800ft, the signal was really bad.
With my tests this minimum was at 2000ft, I could not get there, so it did not show.
Why was that different ? Because of different latitude it seems ! In real this minimum, called deep thermocline, gets generaly shallower with higher latitude, and DW DOES simulate this !
This calls for another bunch of test and it can have MAJOR effect in latitudes above lets say 60 degrees.
Btw. in real, this minimal speed attracts sounds and forms so called deep sound channel. If listener is at this depth it should get GAIN on detection range especially if the sound source too is in this depth. That especially must be tested. I sugest about the same latitudes as your mission has, which is 67N.
Now this game is not dead ! It's not fully understood yet ! :arrgh!:
Edit: Anyway I don't see how this is realistic. As much as my sound simulation shows, it's not. Deep channel does not block sound.
You are right, the deep sound channel is actualy above 1000ft in that scenario. I didn't notice that because I used to think that the deep sound channel is unreachable in DW anyway.
It explains why dedection ranges were so small. In cross sound channel situations, the sound got cought in the channel. Still it seem to be a bit of an abstraction, as the sound got cut off in DW at a very specific depth, while the deep sound channel should be a rather fuzzy (the thermocline has a sharp edge in the SSP, the sound channel a relaxed curve).
Also I there doesn't seem to be any effect of actualy being inside the sound channel, where dedection ranges should be very long.
Still this is a rather important discovery. And it means that the deeper diving subs in DW actualy have a reason to dive deep and exploit the sound channel (even if it just works as an additional layer).
Molon Labe
03-23-08, 05:58 PM
Bohaha ! You have discovered second layer ! :up:
Check SSP in that game of yours. There is speed maximum, which DW marks as layer depth. Allright. And then, with covergence zones type SSP, the speed goes down and up again. Theres is minimum on that curve. In your case it's neatly on 800ft. As you got under 800ft, the signal was really bad.
With my tests this minimum was at 2000ft, I could not get there, so it did not show.
Why was that different ? Because of different latitude it seems ! In real this minimum, called deep thermocline, gets generaly shallower with higher latitude, and DW DOES simulate this !
This calls for another bunch of test and it can have MAJOR effect in latitudes above lets say 60 degrees.
Btw. in real, this minimal speed attracts sounds and forms so called deep sound channel. If listener is at this depth it should get GAIN on detection range especially if the sound source too is in this depth. That especially must be tested. I sugest about the same latitudes as your mission has, which is 67N.
Now this game is not dead ! It's not fully understood yet ! :arrgh!:
Interesting.... I'm guessing the Month will have some effect on that too...can't imagine why else it's listed in the Acoustic Conditions setting.
That sound channel depth caught me by surprise too. I'm also used to seeing them around 2000ft...or deeper. So it seems like what might be going on is that the sound channel, instead of being modeled as an effect that happens gradually, centered around the 800ft minimum SV, is a discrete layer beginning at the minimum SV. Anyone want to put the contact at that depth and see if the det range is a crapload longer than it is for a contact outside of the channel?
Edit: Anyway I don't see how this is realistic. As much as my sound simulation shows, it's not. Deep channel does not block sound.
Like the other SSP features, it's a simplification rather than a realtime ray trace. But I'm thinking back to what you said about the sound paths being the same both ways. The sound channel tends to trap sound inside it; that should also mean that sound outside it has a hard time staying inside it (the sound that goes down thru it and then curves back up without being captured notwithstanding)
Anyone want to put the contact at that depth and see if the det range is a crapload longer than it is for a contact outside of the channel?
In my understanding the sound source does not need to be in the sound channel, as the sound gets cought in the channel eventualy anyway. But the listener needs to be inside.
Judging from my previous results and without any more specific tests I would say that you are right and we don't actualy have a sound channel modeled but just an additional layer at minimum soundspeed depth.
In real, the sound is trapped inside it if it is emited near the sound axis. If it is well above the sound channel, it will just go throught. Well better .. it's matter of the angle. Some part of the sound will always be trapped inside.
Depth of the minimal speed affects the shape of the front edge of the shadow zone, but just a bit, and actually in the opposite direction (it moves the shadow zone away from the sound source as the depth increases). It by no means can create second sound blocking layer.
Month also does affect ice amount at different places. Check the files IceWorld01.Dgd to IceWorld12.Dgd in DW's world directory.
ASWnut101
03-25-08, 09:49 PM
Well, this thread gave me some motivation, so I did some tests about latitude and months. As it turns out, the month only does seem to affect the amount of ice generated in the polar regions.
However, latitude seemed to play a big part in the depth and structure of the deep sound channel. I conducted each test five times for accuracy.
In test one, I was centered at the center of the grid (0 deg latitude, 0 deg longitude). The DSC consitantly seemed to be at around 2000-2100ft in depth, with an overall low minimum SV at the DSC axis.
In test two, I was about 500nm SSW of Iceland, in the North Atlantic. The DSC still rose a bit to around 1800-1900ft in depth. The SV at the axis was also a little higher on average when compared to test one.
In test three, I was 260nm E of Jan Mayen (the tiny island north of Iceland). This was weird. There was no sound channel axis at all. Minimum SV occured at the surface, which rose quickly to layer depth. At layer depth, the SV's rate of increase dropped dramatically, but never actually went negative (the SV starts decreasing). It began a gradual increase from layer depth to the bottom of what the XBT probe could see, causing a gradual curve on the SSP chart. Here's a drawing of what it looked like:
http://i175.photobucket.com/albums/w142/ASWnut/SSP.jpg
-ASWnut
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