GLOSSARY OF MARINE NAVIGATION absolute temperature. Temperature measured from absolute zero which A is zero on the Kelvin scale, 273.16°C on the Celsius scale, and 459.69°F on the Fahrenheit scale. The sizes of the Kelvin and Celsius degree are equal. The size of a degree on the Fahrenheit abaft. , adv. In a direction farther aft in a ship than a specified reference scale equals that on the Rankine scale. position, such as abaft the mast. See also ABAFT THE BEAM, AFT, ASTERN. absolute value. The value of a real number without regard to sign. Thus, the absolute value of +8 or -8 is |8|. Vertical lines on each side of a abaft the beam. Any direction between broad on the beam and astern. number indicate that its absolute value is intended. See also FORWARD OF THE BEAM. abampere. , n. The unit of current in the centimeter gram-second electro- absorption. The process by which radiant energy is absorbed and con- magnetic system. The abampere is 10 amperes. verted to other forms of energy. See ATTENUATION. abeam. , adv. In a line approximately at right angle to the ship’s keel- absolute zero. The theoretical temperature at which molecular motion ° ° opposite the waist or middle part of a ship. See also BROAD ON ceases, 459.69 F or -273.16 C. THE BEAM. abyss. , n. A very deep area of the ocean. The term is used to refer to a par- aberration. , n. 1. The apparent displacement of a celestial body in the ticular deep part of the ocean, or to any part below 300 fathoms. direction of motion of the earth in its orbit caused by the motion of abyssal plain. See under PLAIN. the earth combined with the finite velocity of light. When, in accelerate. , v., t. To move or cause to move with increasing velocity. addition to the combined effect of the velocity of light and the acceleration. , n. 1. The rate of change of velocity. 2. The act or process motion of the earth, account is taken of the motion of the celestial of accelerating, or the state of being accelerated. Negative acceler- body in space during the interval that the light is traveling to the ation is called DECELERATION. earth from the luminous body, as in the case of planets, the phenom- enon is termed planetary aberration. The aberration due to the acceleration error. The error resulting from change in velocity (either rotation of the earth on its axis is termed diurnal aberration or daily speed or direction); specifically, deflection of the apparent vertical, aberration. The aberration due to the revolution of the earth about as indicated by an artificial horizon, due to acceleration. Also called the sun is termed annual aberration. The aberration due to the BUBBLE ACCELERATION ERROR when applied to an instru- motion of the center of mass of the solar system in space is termed ment using a bubble as an artificial horizon. secular aberration but is not taken into account in practical astron- accelerometer. , n. A device used to measure the accelerations of a craft, omy. See also CONSTANT OF ABERRATION. 2. The conver- resulting from the craft’s acceleration with respect to the earth, gence to different foci, by a lens or mirror, of parallel rays of light. acceleration of gravity, and Coriolis acceleration. In a single lens having spherical surfaces, aberration may be caused accidental error. See RANDOM ERROR. An error of accidental nature. by differences in the focal lengths of the various parts of the lens: (Not to be confused with MISTAKE.) rays passing through the outer part of the lens come to a focus accretion. , n. Accumulation of material on the surface of an object. nearer the lens than do rays passing through its central part. This is accuracy. , n. 1. In navigation, a measure of the difference between the termed spherical aberration and, being due to the faulty figure of the position indicated by measurement and the true position. Some lens, is eliminated by correcting that figure. A lens so corrected is expressions of accuracy are defined in terms of probability. 2. A called an aplanatic lens. Aberration may also result from differ- measure of how close the outcome of a series of observations or ences in the wavelengths of light of different colors: light of the measurements approaches the true value of a desired quantity. The shorter wavelengths (violet end of the spectrum) comes to a focus degree of exactness with which the true value of the quantity is nearer the lens than light of the longer wavelengths (red end of the determined from observations is limited by the presence of both spectrum). This is termed chromatic aberration, and is practically systematic and random errors. Accuracy should not be confused eliminated over a moderate range of wavelengths by using a com- with PRECISION, which is a measure of the repeatability of the posite lens, called an achromatic lens, composed of parts having observations. Observations may be of high precision due to the different dispersive powers. quality of the observing instrument, the skill of the observer and the aberration constant. See CONSTANT OF ABERRATION. resulting small random errors, but inaccurate due to the presence of ablation. , n. Wasting of snow or ice by melting or evaporation. large systematic errors. Accuracy implies precision, but precision abnormal. , adj. Deviating from normal. does not imply accuracy. See also ERROR, RADIAL ERROR, abrasion. , n. Rubbing or wearing away, or the result of such action. ABSOLUTE ACCURACY, PREDICTABLE ACCURACY, abroholos. , n. A squall frequent from May through August between Cabo RELATIVE ACCURACY, REPEATABLE ACCURACY. de Sao Tome and Cabo Frio on the coast of Brazil. achromatic lens. See under ABERRATION, definition 2. abrupt. , adv. Steep, precipitous. See also BOLD. aclinal. , adj. Without dip; horizontal. abscissa. , n. The horizontal coordinate of a set of rectangular coordinates. aclinic. , adj. Without magnetic dip. Also used in a similar sense in connection with oblique coordinates. aclinic line. The magnetic equator; the line on the surface of the earth absolute. Pertaining to measurement relative to a universal constant or connecting all points of zero magnetic dip. natural datum. acoustic depth finder. See ECHO SOUNDER. absolute accuracy. The ability of a navigation or positioning system to acoustic navigation. See SONIC NAVIGATION. define an exact location in relation to a coordinate system. absolute gain. See ISOTROPIC GAIN (of an antenna). acoustics. , n. 1. That branch of physics dealing with sound. 2. The sound characteristics of a room, auditorium, etc., which determine its absolute humidity. The mass of water vapor per unit volume of air. quality with respect to distinct hearing. absolute motion. Motion relative to a fixed point. If the earth were sta- tionary in space, any change in the position of another body, relative acoustic sounding. See ECHO SOUNDING. to the earth, would be due only to the motion of that body. This acquisition. , n. The selection of those targets or satellites requiring a would be absolute motion, or motion relative to a fixed point. tracking procedure and the initiation of their tracking. Actual motion is motion of an object relative to the earth. acre. , n. A unit of area equal to 43,560 square feet. 717 718 GLOSSARY OF MARINE NAVIGATION across-the-scope echo. See CLASSIFICATION OF RADAR ECHOES. afterglow. , n. 1. The slowly decaying luminescence of the screen of the active satellite. 1. An artificial satellite which transmits an electromag- cathode-ray tube after excitation by an electron beam has ceased. netic signal. A satellite with the capability to transmit, repeat, or re- See also PERSISTENCE. 2. A broad, high arch of radiance or glow transmit electromagnetic information, as contrasted with PASSIVE seen occasionally in the western sky above the highest clouds in SATELLITE. 2. As defined by International Telecommunications deepening twilight, caused by the scattering effect of very fine par- Union (ITU), an earth satellite carrying a station intended to ticles of dust suspended in the upper atmosphere. transmit or re transmit radio communication signals. aged ridge. A ridge of ice forced up by pressure which has undergone active tracking system. A satellite tracking system which operates by considerable weathering. transmission of signals to and receipt of responses from the satel- age of diurnal inequality. The time interval between the maximum lite. semimonthly north or south declination of the moon and the actual motion. Motion of an object relative to the earth. See also maximum effect of the declination upon the range of tide or the MOTION. speed of the tidal current; this effect is manifested chiefly by an acute angle. An angle less than 90°. increase in the height or speed difference between the two high additional secondary phase factor correction. A correction in addition (low) waters or flood (ebb) currents during the day. The tides occur- to the secondary phase factor correction for the additional time (or ring at this time are called TROPIC TIDES. Also called DIURNAL phase delay) for transmission of a low frequency signal over a com- AGE. posite land-water path when the signal transit time is based on the age of parallax inequality. The time interval between perigee of the free-space velocity. moon and the maximum effect of parallax upon the range of tide or ADF reversal. The swinging of the needle on the direction indicator of the speed of the tidal current. See also PARALLAX INEQUAL- an automatic direction finder through 180°, indicating that the ITY. station to which the direction finder is tuned has been passed. age of phase inequality. The time interval between new or full moon adiabatic. , adj.