User's Guide

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simultaneously at a base station. Because the base
station is in 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 has been collected by postprocessing.
Differential GPS
See real-time differential GPS.
DOP
Dilution of Precision. A measure of the quality of GPS
positions, based on the geometry of the satellites used
to compute the positions. When satellites are widely
spaced relative to each other, the DOP value is lower,
and position accuracy is greater. When satellites are
close together in the sky, the DOP is higher, and GPS
positions may contain a greater level of error.
PDOP (Position DOP) indicates the three-dimensional
geometry of the satellites. Other DOP values include
HDOP (Horizontal DOP) and VDOP (Vertical DOP), which
indicate the accuracy of horizontal measurements
(latitude and longitude) and vertical measurements
respectively. PDOP is related to HDOP and VDOP as
follows:
PDOP² = HDOP² + VDOP²
Dual-Frequency GPS
A type of receiver that uses both L1 and L2 signals from
GPS satellites. A dual-frequency receiver can compute
more precise position fixes over longer distances and
under more adverse conditions because it compensates
for ionospheric delays.
EGNOS
European Geostationary Navigation Overlay Service. A
satellite-based augmentation system (SBAS) that
provides a free-to-air differential correction service for
GPS.
EGNOS is the European equivalent of WAAS, which is
available in the United States.
Elevation Mask
The angle below which the receiver will not track