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Appendix E - Glossary | 195

TRUE HEADING

Direction in relation to true north. True heading is compass heading corrected for

MAGNETIC DEVIATION and VARIATION. The true heading used by the navigator

for navigation calculations is a composite of magnetic heading input from the

NMEA compass, magnetic variation as calculated by the navigator, and the values

entered into the compass deviation table.

TRUE NORTH

Direction along the meridian of the observer to the north pole.

TRUE WIND ANGLE (TWA)

Similar to APPARENT WIND ANGLE, but compensated for the motion of the vessel.

TWA and AWA are equal if the vessel is not moving.

TRUE WIND DIRECTION (TWD)

The direction of the wind over ground, expressed as an angular distance from

north clockwise through 360°.

TRUE WIND SPEED (TWS)

The wind speed relative to either ground or water rather than to the moving

vessel.

UNCERTAINTY

In the navigator, an indication of the expected accuracy expressed as the radius of

a circle around the calcu lated (displayed) position. The uncertainty is expressed in

meters or feet. The calculation of uncertainty is based on the HDOP value.

UNIVERSAL TIME COORDINATED (UTC)

Greenwich mean time corrected for polar motion of the Earth and seasonal

variation in the Earth’s rotation.

UPDATE

See POSITION UPDATE.

UTC

See UNIVERSAL TIME COORDINATED.

UTM

See UNIVERSAL TRANSVERSE MERCATOR.

UNIVERSAL TRANSVERSE MERCATOR

Alternative grid system used in harbor areas and for land applications instead of

latitude and longitude.

VDL

VHF Data Link.

VDOP (Vertical Dilution of Precision)

An indicator of the accuracy in altitude. See DILUTION OF POSITION.

VELOCITY MADE GOOD (VMG)

The speed by which the vessel is moving in the upwind direction. When tacking,

the optimization should be based on VMG (assuming that TWD is expected to be

fairly constant). See also WAYPOINT CLOSURE VELOCITY.

VHF

Very High Frequency – A set of frequencies in the MHz region.

VISIBLE SATELLITE

A satellite whose orbit has placed it above the earth’s horizon (elevation angle >0°)

from the location of the navigator and is, therefore, available for acquisition. The

navigator will not use satellites with an elevation angle < 5°.

VMG

See VELOCITY MADE GOOD.

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