Terrestrial Navigation Navigation technique is a method of determining position and velocity of a vehicle. A navigation system is also referred to as navigational aid. Navigational aid is a device that determines position and velocity. Terrestrial navigation systems are radio positioning systems that use land-based transmitters or reference points for the calculation of position information. Some of the terrestrial based positioning systems which use techniques developed during Second World War are still in use. They are long range aid to navigation (LORAN), dead reckoning (DR), and inertial navigation systems (INS). 2.2.1 Dead Reckoning Dead reckoning is the process of estimating present position by projecting course and speed from a known past position. It is also used to predict the future position by projecting course and speed from a known present position. Dead reckoning measures either the change in position or the velocity and integrates it. This is added to the previous position in order to obtain current position. The speed or distance tracked is measured in the body coordinate frame, so a separate altitude measured is required to obtain the direction of travel in the reference frame. For a 2D navigation, a heading measurement is sufficient, whereas for 3D navigation, a full 3 component measurement is needed. The navigator uses dead reckoning in many ways, such as: to predict landfall, sighting lights and arrival times, to determine sunrise and sunset, to evaluate the accuracy of electronic positioning information, to predict which celestial bodies will be available for future observation. The most important use of dead reckoning is to project the position of the ship in the immediate future and avoid navigation hazards. Usually these techniques are combined with one or more position fixing techniques in an integrated navigation system to get the benefits of both systems.

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