Knowledge Base
A/D converter
A device that changes an analog signal (such as voltage) into a digital signal (discrete data values.)

abaft
Toward the rear (stern) of the boat.

abeam
At right angles to the keel of the boat, but not on the boat.

aboard
On or within the boat.

above deck
On the deck (not over it - see Aloft)

abreast
Side by side; by the side of.

accelerometer
A sensor that measures acceleration. Acceleration can be due to gravity or changing motion. Acceleration is measured in units of earth’s gravity (G’s) or meters per second squared.

accuracy
In navigation, a measure of the error between the point desired and the point achieved, or between the position indicated by measurement and the true position [compare with precision]

additional secondary factor
In Loran-C, a correction in addition to the secondary phase factor correction for the additional time (or phase delay) for transmission of a low frequency signal over a composite land-seawater path when the signal transit is based on the free-space velocity.

address field
For sentences in the NMEA 0183 standard, the fixed length field following the beginning sentence delimiter "$" (HEX 24). For approved sentences, composed of a two-character talker identifier and a three-way character sentence formatter. For proprietary sentences, composed of the character "P" (HEX 50) followed by a three-character manufacturer identification code.

adrift
Loose, not on moorings or towline.

aft
Toward the stern of the boat.

aground
Touching or fast to the bottom.

ahead
In a forward direction.

aids To navigation
Artificial objects to supplement natural landmarks indicating safe and unsafe waters.

AIS
Universal Shipborne Automatic Identification System (AIS). AIS operate primarily on VHF radio frequencies, with a coverage range of 20 to 30 nautical miles. AIS provides a means to automatically exchange information between ships and with hore stations, information that can be used for identification purposes or for monitoring and tracking the movements of ships. Ship information updates range from between every 2 seconds to 6 minutes, depending upon the speed of the ship. Information broadcast by each AIS equipped ship and an information from land such as from Vessel Traffic Services (VTS) is automatically received by each AIS equipped ship within VHF radio range.

alee
Away from the direction of the wind. Opposite of windward.

aloft
Above the deck of the boat.

amidships
In or toward the center of the boat.

anchorage
A place suitable for anchoring in relation to the wind, seas and bottom.

apparent wind
The speed and relative direction to from which the wind appears to blow with reference to a moving point (also called Relative Wind).

approved sentence
An NMEA 0183 sentence that has been approved for general use by the NMEA general assembly and is listed in this standard and attached Appendices.

arrival alarm
An alarm signal issued to indicates arrival at or at a pre-determined distance from a waypoint (see Arrival Circle).

arrival circle
An artificial boundary placed around the destination waypoint of the present navigation leg, the entering of which will signal an arrival alarm.

arrival perpendicular
Crossing of the line which is perpendicular to the course line and which passes through the destination waypoint.

ASCII
American Standard Code for Information Interchange. A 7 bit wide serial code describing numbers, upper and lower case alpha, characters, special and non-printing characters. See American National Standards Institute documents ANSI X 3.15, ANSI X 3.16 and ANSI 3.4.

astern
In back of the boat, opposite of ahead.

athwartships
At right angles to the centerline of the boat; rowboat seats are generally athwart ships.

atomic time
Time obtained by counting cycles of a signal in resonance with certain kinds of atoms.

autopilot
An automatic device for steering a vessel so as to maintain its heading in an intended direction. Mechanical means are used to steer the rudder. A navigation system is often connected to correct for track errors, or to select new destinations.

aweigh
The position of anchor as it is raised clear of the bottom.

azimuth
The horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from a reference direction, usually measured from 000° at the reference direction through 359°.

 
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