Now most submersibles are connected with a support ship on the surface. This connection is an armored cable measuring an inch or two in diameter and weighing up to 10 tons and it transmits power and navigational commands to the submersible, as well as sends sensor data and television images back to the support ship. Cables allow submersibles to transmit data at a great speed, but they limit the range of territory studied and have many disadvantages in operation.
Autonomous underwater submersibles can move freely. Controlled by on-board microprocessors or by acoustic signals transmitted by a ship on the surface, battery-operated submersibles can cover much greater areas. They can operate under ice and in very deep water. Such three-ton unmanned crafts can submerge to the depth of almost 20,000 feet and stay there for up to seven hours. High quality images of the ocean bottom can be transmitted to the support ship in three to four seconds (because of the slow speed – about 5,000 feet per second through water – acoustic data transmission is much less quick than signals sent via cable which travel at the speed of light).
But even these most advanced submersibles have definite disadvantages: batteries are heavy, data transmission is slow and computer programs are primitive. Future submersibles may overcome those difficulties. Some may be propelled by nuclear power or by fuel cells that use oxygen from the sea water. Many of them will rely on signal-compression techniques to speed up acoustic data links. Computerized systems will enable some submersibles to repair damaged telephone cables or oil platforms. If research work in this field continues to expand at its present rate, the number of radically different kind of more efficient crafts will appear very soon.