Underwater navigation

(navigation) The navigation of a submerged vessel. Also known as submarine navigation.

The process of directing the movements of submersible vehicles, and divers, from one point to another. The development of improved submersible vehicles, coupled with advances in saturated diving, has resulted in new requirements for underwater navigation. Various methods which have proved successful include acoustic transponder systems, dead reckoning, surface-referenced navigation from a support ship, homing, and various combinations of these. The choice of the navigation system depends on such factors as precision required, area to be covered, availability of surface vessels, sea state under which they are expected to operate, and for crewed vehicles the redundancy necessary for safety. See also Diving; Submarine; Underwater vehicle.

Acoustic transponders

The most accurate undersea navigation systems use acoustic transponders to precisely determine position with respect to points on the sea floor. A minimum of two transponders are normally dropped from a surface or submerged vehicle to establish a bottom-tethered array. The transponders can be interrogated by surface vessels, submersible vehicles, or divers. When the navigation solution is computed on the surface vessel, the submerged vehicle also carries a transponder, and the position of the submerged vehicle is computed relative to the bottom-tethered array.

Dead reckoning

An undersea vehicle needs sensors to show distance traveled and direction of travel for dead-reckoning estimation of position. The most promising sensor for operation near the sea floor is a Doppler sonar. Ground speed, fore-aft and athwartship, can be determined from the Doppler shift in frequency of signals returned from the sea floor. Pulse-type and continuous-wave Doppler sonars are available. The pulse type, which makes use of a frequency tracker circuit to lock on the received pulse, appears to offer the best accuracy, and it has the ability to operate to 600 ft (180 m) above the sea floor. See also Sonar.

In addition to Doppler sonar, some specialized submersibles carry inertial dead reckoning in the form of a small, stabilized platform. See also Dead reckoning; Inertial guidance system.

Other methods

Most submersible vehicles carry additional navigation sensors. The horizontal obstacle sonar (HOS) which is a constant-transmission frequency-modulated sonar (CTFM), is normally used to detect objects ahead of the submersible. This sonar also has a transponder channel for determining bearing and range to specially designed transponders. An altitude-depth sonar provides vertical navigation by furnishing depth and altitude off the bottom. Finally, a vertical obstacle sonar (VOS) determines heights of objects in the path of the vehicle. Both horizontal and vertical obstacle-avoidance sonars are useful in under-ice navigation. See also Homing; Piloting.

An acoustic tracking system allows the monitoring and vectoring of the position of a vehicle from a surface vessel, where space and weight are not at a premium, and Loran C, the Global Positioning System, Omega, or other radio techniques can be used to determine the ship's own position in geographic coordinates. Surface tracking systems are of two types: ultrashort baseline, employing orthogonal line hydrophone arrays; and short baseline, employing four separate hydrophones. See also Marine navigation.

Submarines must operate over wide areas of the ocean and often under highly secure and covert conditions; therefore, navigation techniques which depend upon sonic or electromagnetic emissions, or which restrict movements of the vehicle to either near-bottom or near-surface regions, are judged to be too restrictive. The ability of inertial navigation systems to operate without frequent recourse to external position updates makes them prime candidates for submarine navigation. The need for covert and secure long-duration navigation has resulted in the evolution of a navigation configuration that uses a pair of electrostatic gyroscope navigators (ESGNs) as the primary elements of an integrated navigation subsystem. The very stable, low-drift characteristics of the electrostatic gyroscope result in an inertial navigation system with a low and highly predictable error growth. Weapons system accuracy is further enhanced by velocity derived from both gyroscopes and secure correlation velocity sonar techniques to give a direct measure of the submarine ground speed. Vertical deflection maps, which are used for vertical-axis tilt compensation of the gyroscopes, are generated from combined satellite and oceanographic surveys of the Earth's gravity field. The relatively rare position resets are selected from either Global Positioning System satellite or bathymetric sonar data. The passive electromagnetic-log, which measures vehicle velocity relative to the water and is therefore subject to ocean current errors, is used primarily to damp the gyroscope computational (Schuler) oscillations. The various data are processed in a central navigation computer which uses statistical estimation algorithms. See also Estimation theory.

Navigation has always been a difficult problem for divers. Some of the same sonars developed for submersible pilotage offer an answer for a diver. A horizontally swept constant-transmission frequency-modulated sonar is well matched to the excursions of saturated divers, who are limited in allowable vertical movement. By communication with the habitat, divers who stray too far or lose their orientation can be guided back to safety.