Unmanned systems are vessels that sail on the surface or under water with no crew, and robotic vehicles that do dangerous work for armies. Unmanned surface vessels sail on the sea. Unmanned underwater vessels travel below it. Robotic ground vehicles carry loads, clear mines or enter buildings where a soldier would be at risk.
Each machine needs three things. First, a body that can survive its environment, such as salt water, waves or rough ground. Second, a link to the operator, which is hard at sea. Radio does not travel well through water, and a ship far from shore is out of range. So vessels use mesh and tactical radios and satellite terminals and modems. Underwater vessels cannot use ordinary radio while submerged. So they follow a plan alone, send short messages as sound through the water, or come to the surface to report. Third, software that steers it. Autonomy software lets a vessel follow a route and avoid obstacles when the link breaks.
Consider a small surface vessel that patrols near the port of Visakhapatnam. It follows a route of 12 nautical miles and carries a camera and a sonar. A nautical mile is the distance used at sea, about 1.85 km. If the vessel loses contact for ten minutes, it sails back by itself. Such vessels also face attacks on their computers. The topic is covered under maritime cyber security.
The idea matches an aerial drone, and it comes from the same levels of autonomy. A vessel can stay at sea much longer than a crewed boat, and no crew is put at risk. In India, armies and navies buy these machines through a public tender, and many makers are small and young.