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K GPS Glossary of Terms
MiLLennium Command Descriptions Manual 241
Coarse Acquisition (C/A) Code — a spread spectrum direct sequence code that is used primarily by
commercial GPS receivers to determine the range to the transmitting GPS satellite. Uses a chip rate
of 1.023 MHz.
Communication protocol — a method established for message transfer between a talker and a listener which
includes the message format and the sequence in which the messages are to be transferred. Also
includes the signalling requirements such as bit rate, stop bits, parity, and bits per character.
Control segment — the Master Control Station and the globally dispersed reference Stations used to manage
the GPS satellites, determine their precise orbital parameters, and synchronize their clocks.
Course — the horizontal direction in which a vessel is to be steered or is being steered; the direction of travel
through the air or water. Expressed as angular distance from reference North (either true, magnetic,
compass, or grid), usually 000° (north), clockwise through 360°. Strictly, the term applies to
direction through the air or water, not the direction intended to be made good over the ground (see
track). Differs from heading.
Course Made Good (CMG) — the single resultant direction from a given point of departure to a subsequent
position; the direction of the net movement from one point to the other. This often varies from the
track caused by inaccuracies in steering, currents, cross-winds, etc. This term is often considered to
be synonymous with Track Made Good, however, track made good is the more correct term.
Course Over Ground (COG) — the actual path of a vessel with respect to the Earth (a misnomer in that
courses are directions steered or intended to be steered through the water with respect to a reference
meridian); this will not be a straight line if the vessel's heading yaws back and forth across the course.
Cross Track Error (XTE) — the distance from the vessel’s present position to the closest point on a great
circle line connecting the current waypoint coordinates. If a track offset has been specified in the
GPSCard SETNAV command, the cross track error will be relative to the offset track great circle
line.
Cycle Slip — when the carrier phase measurement jumps by an arbitrary number of integer cycles. It is
generally caused by a break in the signal tracking due to shading or some similar occurrence.
Dead Reckoning (DR) — the process of determining a vessel’s approximate position by applying from its last
known position a vector or a series of consecutive vectors representing the run that has since been
made, using only the courses being steered, and the distance run as determined by log, engine rpm,
or calculations from speed measurements.
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 the final destination of a voyage.
Differential GPS (DGPS) — a technique to improve GPS accuracy that uses pseudorange errors at a known
location to improve the measurements made by other GPS receivers within the same general
geographic area.
Dilution of Precision (DOP) — a numerical value expressing the confidence factor of the position solution
based on current satellite geometry. The lower the value, the greater the confidence in the solution.
DOP can be expressed in the following forms.
GDOP - all parameters are uncertain (latitude, longitude, height, clock offset)
PDOP - 3D parameters are uncertain (latitude, longitude, height)
HTDOP - 2D parameters and time are uncertain (latitude, longitude, time)
HDOP - 2D parameters are uncertain (latitude, longitude)
VDOP - height is uncertain
TDOP - clock offset is uncertain
Doppler — the change in frequency of sound, light or other wave caused by movement of its source relative
to the observer.