widely spaced relative to each other, the DOP value is lower, and position
precision is greater. When satellites are close together in the sky, the DOP is
higher and GNSS 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 precision 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 GNSS. EGNOS is the European equivalent of WAAS, which is available
in the United States.
elevation
The vertical distance from a geoid such as EGM96 to the antenna phase center.
The geoid is sometimes referred to as Mean Sea Level. In the SPS GNSS
receivers, a user-defined sub gridded geoid can be loaded and used, or for a
small site, an inclined vertical plane adjustment is used as an approximation to
the geoid for a small site.
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,
atmospheric issues, and multipath errors.
ellipsoid
An ellipsoid is the three-dimensional shape that is used as the basis for
mathematically modeling the earth’s surface. The ellipsoid is defined by the
lengths of the minor and major axes. The earth’s minor axis is the polar axis and
the major axis is the equatorial axis.
EHT
Height above ellipsoid.
ephem-
eris/ephemerides
A list of predicted (accurate) positions or locations of satellites as a function of
time. A set of numerical parameters that can be used to determine a satellite’s
position. Available as broadcast ephemeris or as postprocessed precise
ephemeris.
epoch
The measurement interval of a GNSS receiver. The epoch varies according to
the measurement type: for real-time measurement it is set at one second; for
postprocessed measurement it can be set to a rate of between one second and
one minute. For example, if data is measured every 15 seconds, loading data
using 30-second epochs means loading every alternate measurement.
feature
A feature is a physical object or event that has a location in the real world, which
you want to collect position and/or descriptive information (attributes) about.
Features can be classified as surface or non-surface features, and again as
SPS985 GNSS Smart Antenna Getting Started Guide 31
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General