Knowledge Base
D/A converter
(Digital to Analog Converter, also abbreviated DAC) Converts a digital signal (discrete values) to an analog voltage.

data field
In an NMEA 0183 sentence, a field that contains a data value.

dead ahead
Directly ahead.

dead astern
Directly aft.

dead reckoning
The process of determining the position of a vessel at any instant by applying to the last well-determined position (point of departure or subsequent fix) the run that has since been made, usually based on the recent history of speed and heading measurements.

Decca chain
A group of associated stations of the Decca Navigator System. A Decca chain normally consists of a master and three secondary stations. Each station is balled by the color of associated stations. Each station is called by the color of associated pattern of hyperbolic lines as printed on the chart, i.e., red, green, and purple.

Decca navigator system
A short to medium range low frequency (70-130 kHz) radio-navigation system by which a hyperbolic line of position of high accuracy is obtained. The system is an arrangement of fixed, phase locked, continuous wave transmitters operating on harmonically related frequencies and special receiving equipment located on a vessel. The operation of the system depends of phase comparison of the signals from the transmitters brought to a common comparison frequency with the receiver.

deck
A permanent covering over a compartment, hull or any part thereof.

delimiter
In the NMEA 0183 standard, a character or characters used to separate fields or sentences. The following delimiters are used in the NMEA 0183 standard: Field delimiters:*ASCII "$" (HEX 24) for address field, *ASCII "," (HEX 2C) for data fields, *ASCII "*" (HEX2A) for checksum field, Sentence delimiters: carriage return and line feed (HEX 0D0A) [note: is not required preceding the first sentence transmitted]

depth sounder
An instrument that determines the depth of water by measuring the time interval between the emissions of a sound and the return of its echo from the bottom.

destination
The immediate geographic point of interest to which a vessel is navigating. It may be the next waypoint along a route of waypoints or a final destination of a voyage.

deviated
Term used to indicate that a compass has been corrected for Deviation

deviation
The deflection of a compass needle caused by local magnetic influence.

DGNSS
Differential GNSS, the use of GNSS measurements, some or all of which are differentially corrected.

DGPS
Differential GPS, the use of GPS measurements, which are differentially corrected.

dinghy
A small open boat. A dinghy is often used as a tender for a larger craft.

displacement
The weight of water displaced by a floating vessel, thus, a boat’s weight.

displacement hull
A type of hull that plows through the water, displacing a weight of water equal to its own weight, even when more power is added.

dock
A protected water area in which vessels are moored. The term is often used to denote a pier or a wharf.

dolphin
A group of piles driven close together and bound with wire cables into a single structure.

Doppler speed log
An instrument which measures the relative motion between a vessel and the reflective sea bottom (for bottom return mode) or suspended particulate matter in the seawater itself (for water return mode) by measuring the frequency shifts between a transmitted and subsequently echoed acoustic or electromagnetic signal.

draft
The depth of water a boat draws.

drift
The speed of a current.

DSP
Digital Signal Processor. This is a microprocessor optimized for digital signal manipulations.

 
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