User's Manual
Glossary
164 SPSx50 Modular GPS Receiver User Guide
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 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 GPS data collected on a rover with
data collected simultaneously at a base station. Because it 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 has been collected by
postprocessing.
differential GPS See real-time differential GPS.
Dilution of Precision See DOP.
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:
PDOP = HDOP + VDOP
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 satellites. Normally set to 10 degrees
to avoid interference problems caused by buildings and trees, and multipath errors.