cellular modems
A wireless adaptor that connects a laptop computer to a cellular phone system
for data transfer. Cellular modems, which contain their own antennas, plug into
a PC Card slot or into the USB port of the computer and are available for a
variety of wireless data services such as GPRS.
CMR/CMR+
Compact Measurement Record. A real-time message format developed by
Trimble for broadcasting corrections to other Trimble receivers. CMR is a more
efficient alternative to RTCM.
CMRx
A real-time message format developed by Trimble for transmitting more
satellite corrections resulting from more satellite signals, more constellations,
and more satellites. Its compactness means more repeaters can be used on a
site.
covariance
A statistical measure of the variance of two random variables that are observed
or measured in the same mean time period. This measure is equal to the
product of the deviations of corresponding values of the two variables from
their respective means.
datum
Also called geodetic datum. A mathematical model designed to best fit the
geoid, defined by the relationship between an ellipsoid and, a point on the
topographic surface, established as the origin of the datum. World geodetic
datums are typically defined by the size and shape of an ellipsoid and the
relationship between the center of the ellipsoid and the center of the earth.
Because the earth is not a perfect ellipsoid, any single datum will provide a
better model in some locations than in others. Therefore, various datums have
been established to suit particular regions.
For example, maps in Europe are often based on the European datum of 1950
(ED-50). Maps in the United States are often based on the North American
datum of 1927 (NAD-27) or 1983 (NAD-83).
All GPS coordinates are based on the WGS-84 datum surface.
deep discharge
Withdrawal of all electrical energy to the end-point voltage before the cell or
battery is recharged.
DGPS
See real-time differential GPS.
differential correction
Differential correction is the process of correcting GNSS data collected on a
rover with data collected simultaneously at a base station. Because the base
station is on a known location, any errors in data collected at the base station
can be measured, and the necessary corrections applied to the rover data.
Differential correction can be done in real-time, or after the data is collected by
postprocessing.
differential GPS
See real-time differential GPS.
DOP
Dilution of Precision. A measure of the quality of GNSS positions, based on the
geometry of the satellites used to compute the positions. When satellites are
SPS985 GNSS Smart Antenna Getting Started Guide 30
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General