A sonar array that is towed behind submarines, and maybe surface ships as well. This gets the equipment away from the noise of the sub or ship, so it is more effective. If a sub with a towed array makes a turn, it has to wait for the array to straighten out again before it works properly.

Towed arrays also solve the critical problem of not being able to use onboard sonar to see what's happening behind your propeller. They are also towed behind helicopters occasionally, as they have better endurance and sensitivity than sonobuoys and can be reused. Not the same thing as a dipping sonar. Used in this way, they can be quickly deployed from shore or from a helo-capable ship.