Quote:
|
Originally Posted by Deathblow
When the water conditions are bottom limited (increasing sound speed with increasing depth) then its best to stay at the bottom right? Because the active sonar pings are going to curve upwards? as well as the returns? If my understanding of rudimentary sonar is right, that will make it harder for active sonars to detect me at the bottom, while making it easier for passive sonars to detect me right?
|
Short answer: It depends.
You also have it backwards. In a bottom-limited environment, the temperature of the water cools as you travel deeper, causing the sound speed to drop with depth. It's typical of shallow water. That means the sound energy travels DOWNWARD until it hits the bottom. Whether the sound continues to travel depends on whether the bottom tends to absorb sound or reflect it. If you get a bottom bounce, then the reflected sound and direct path sound create an interference pattern that can result in a "poor man's convergence zone." If you don't get a bottom bounce, then the direct path sound is mostly absorbed into the bottom and detection ranges are extremely limited.
What you need to do to minimize the probabilty of you being counterdetected as you approach a bad guy depends on whether there is a bottom bounce or not. If there's a bottom bounce, I'd stay deep. If there's no bottom bounce, then it probably doesn't matter much where you are. His first detection will most likely be your periscope through binoculars.
Increasing sound speed with depth is typically a surface duct environment. That means that sound emitted downward is refracted back upward where, depending on the sea state, it can bounce back downward and maybe back up again. Depending on how good the duct is, the transmission loss is relatively flat and looks like cylindrical spreading.
To avoid detection here is actually relatively difficult because of the duct. Go deep, stay slow to minimize Doppler and keep your bow on the emitter. If you're not within the limiting lines of approach of the target, I wouldn't approach him. Snap some photos as he goes by and send them back to the fleet.
It's also possible that in certain shallow water surface ducts, the sound speed increases with depth, but not enough to really make me care about it. That's really a kind of "littoral mush" where sound energy stays going in almost a straight line so basically, if it's not direct path, you'll never pick it up. You get direct path just with cylindrical spreading. That's only a little better than bottom limited with no bottom bounce.