Anti-Submarine Warfare

"Captain, my MAD has detected a moving fusion plant underwater. Probably a submarine, but I am unable to classify it. I am tranfering location data to your display."
"Thanks Sam, "Captain Davies responded to the AI as if it was a person, and as far as he was concerned it was. The new data appeared in his VR display - a fuzzy green region showing the approximate position of the sub, along with other data such as contact strength. The position of the aircraft with him, his copilot Lieutenant Richards, and Sam, was shown as a black dot roughly 50 kilometers from the green region.
"Sam, give me a brief lidar pulse in LPI mode to identify the target then prepare a shadowing course that keeps us between 20 and 60 kilometers away."
"Aye captain," there was a brief pause, and Davies noted a brief flash on his display as Sam pulsed the blue-green lasers across the suspect region, "contact is an Indonesian Mongisidi class arsenal submarine. Position and depth locked." Davies checked his display. A dotted blue line showed the 200-mile military exclusion zone around Atafu atoll. The contact (now a green point) was just crossing it. "Sam, launch all sonobots under Richard's command, then start a continuous lidar track of the target. Lets give them a scare...

The ability to detect and neutralise submarines has been a crucial naval capability since the first world war. As modern naval vessels are increasingly staying underwater to avoid hypersonic missiles and space based weapons, sub-hunting has only increased in importance. The tools of modern sub-hunting are magnetic anomaly detectors, blue-green (BG) lidar, and active and passive sonar. All have their advantages and limitations.

• Magnetic anomaly detectors (MADs) are long ranged passive sensors, that can detect an enemy sub at any depth and even under ice or through geographic features. Their key limitation is applicability - modern subs are made of non-ferrous material, so rather than detecting the metal of their hull, the MAD is detecting the powerful magnetic fields of an operating fusion reactor. They provide only an approximate location, so cannot be used to target unguided weapons fire.
• BG-Lidar provides a precise target location that can be used to target weapons, and also provides the targets shape and size. The limitations are a relatively short range (especially if the enemy sub is operating deep) and the fact that it is an active sensor and could alert the target that it has been detected. They do have LPI (low-probability of intercept) mode, but with only a small bandwidth to work width (the blue-green) this is less effective that usual.
• Passive sonar remains a key element of sub-hunting. Modern subs are extremely quiet (partially due to MHD propulsion and turbine-free reactors) but advances in sonar (particularly AI-signal processing) have kept pace. Passive sonar provides an approximate target location, but unlike MADs they work even on non-nuclear subs.
• Active sonar has broadly the same limitations as BG-lidar, being precise and providing much information, but short ranged. It is harder to stealth against than BG-lidar, but is almost certain to alert the target.

The most common sub-hunting units are aircraft, surface warships, and hunter-killer submarines (SSKs), all of which are usually assisted by sonobots. Sonobots are autonomous cybershells, ranging from simple NAI controlled sonar bouys, to SAI controlled miniture helicopters with their own weapons. Teams of infomorphs are usually employed, sometimes forming a true gestalt intelligence. In addition to any infomorphs in cybershells there will also be a NAI operating each sensor system, and one or two LAI or SAIs coordinating.

In a typical sub-hunting operation the searching unit will have been given an area to search, presumably one where hostile submarines are likely to be. The unit will search the region several times in a methodical fashion using passive sensors, making progressively tighter sweeps each time. Assuming nothing is known about the enemy sub (nuclear or not, level of sound baffling, etc.) the first two sweeps will be MAD and passive sonar only, the next two will be assisted by BG-lidar in LPI mode. Once a subject has been found it must be identified. A target found by passive sonar will have a preliminary identification, but this will be confirmed by a brief BG-lidar scan in LPI mode.