The U.S. Navy is big on sonobuoys again. Another 65,000 were recently ordered. While sonobuoy technology has not changed a lot recently, the way sonobuoys are used has. This is all about taking advantage of new software, and cheap, but massive, computer power, to provide an edge against non-nuclear (and very quiet) submarines operating in coastal waters. The most effective way of hunting down subs is via helicopters or maritime patrol aircraft equipped with homing torpedoes and sonobuoys. The latter are nearly a meter (three feet) long, 120mm in diameter and weigh about 18 kg (40 pounds). They are used once, by dropping them into the water. The U.S. has two basic types of air dropped sonobuoys. The AN/SSQ-101 floats upright, sending sonar signals into the water, and transmitting any data bouncing back, to the aircraft overhead.
The AN/SSQ-36B collects other data (bathythermograph, conductivity, temperature, and depth), which it transmits. The data analysis systems look for faint patterns left by submarines slowly moving through coastal waters. This takes into account the underground geography, and the activity of undersea animals and plants. The number and pattern of deployed sensors would also be taken into account. This is the area where quiet subs have suddenly become most vulnerable.
New pattern analysis software was developed, and quiet diesel-electric subs from allies were used to test and refine the system. The data analysis computers are located in the aircraft, nearby ships, or even back at land bases (via data transmission via satellite). Once the sub is located, the aircraft or helicopter drops a homing torpedo, that will seek out (with the help of the pattern analysis) and attack the sub.
The U.S. has been scrambling to develop the ability to detect the new generation of quiet diesel-electric subs. You won't hear any official pronouncements about progress in this area, for obvious reasons. There is progress, and just how much won't be publicly known until these new anti-submarine methods are used in combat. But the increased orders for sonobuoys, and the known progress in sonobuoy data analysis, is an indicator of where the best solutions are being found.