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I GPS/GLONASS Glossary of Terms
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.
Digital — Generally, information is expressed, stored and transmitted by either analog or digital means. In a
digital form, this information is seen in a binary state as either a one or a zero, a plus or a minus. The
computer uses digital technology for most actions.
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
- estimated uncertainty for all parameters (latitude, longitude, height, clock offset)
PDOP
- estimated uncertainty for all 3D parameters (latitude, longitude, height)
HTDOP - estimated uncertainty for all time and 2D parameters (latitude, longitude, time)
HDOP
- 2D parameters are uncertain (latitude, longitude)
VDOP
- estimated uncertainty for height
TDOP
- estimated uncertainty for clock offset
Distance root mean square (drms) — The root-mean-square value of the distances from the true location
point of the position fixes in a collection of measurements. As typically used in GPS positioning, 2
drms is the radius of a circle that contains at least 95 percent of all possible fixes that can be obtained
with a system at any one place.
Doppler — the change in frequency of sound, light or other wave caused by movement of its source relative
to the observer.
Doppler aiding — a signal processing strategy, which uses a measured Doppler shift to help a receiver
smoothly track the GPS signal, to allow more precise velocity and position measurement.
Double-Difference — a position estimation mechanization which uses observations which are differenced
between receiver channels and between the reference and remote receivers.
Double-Difference Carrier Phase Ambiguity (or sometimes double difference ambiguity or ambiguity, for
short) — carrier phase ambiguities which are differenced between receiver channels and between the
reference and remote receivers. They are estimated when a double difference mechanism is used for
carrier phase positioning.
Earth-Centered-Earth-Fixed (ECEF) — a right-hand Cartesian coordinate system with its origin located at
the center of the Earth. The coordinate system used by GPS to describe three-dimensional location.
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GPS/GLONASS Receiver User Manual Rev 1
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