Introduction
GR-5 Operator’s Manual
1-4
• Control - Ground stations located around the Earth that monitor
the satellites and upload data, including clock corrections and
new ephemerides (satellite positions as a function of time), to
ensure the satellites transmit data properly.
• User - The community and military that use GNSS receivers to
calculate positions.
Calculating Absolute Positions
When calculating an absolute position, a stationary or moving
receiver determines its three-dimensional position with respect to the
origin of an Earth-Center Earth-Fixed coordinate system. To calculate
this position, the receiver measures the distance (called pseudo-
ranges) between it and at least four satellites. The measured pseudo-
ranges are corrected for clock differences (receiver and satellites) and
signal propagation delays due to atmospheric effects. The positions of
the satellites are computed from the ephemeris data transmitted to the
receiver in navigation messages. When using a single satellite system,
the minimum number of satellites needed to compute a position is
four. In a mixed satellite scenario (GPS and GLONASS), the receiver
must lock onto five or more satellites to account for the different time
scales used in these systems and to obtain an absolute position.
Essential Components for Quality Surveying
Achieving quality positioning results from the GR-5 requires an
understanding of the following elements:
• Accuracy - The accuracy of a position that is delivered by a
GNSS receiver primarily depends upon the observed satellite
geometry (Geometric Dilution of Precision, or GDOP) and the
measurement (ranging) errors.
– Differential positioning techniques (DGPS and RTK) can be
used to almost completely remove all major GNSS error
sources, such as atmospheric and orbital errors.
– The more satellites that are in view, the stronger the signal,
the lower the GDOP number will be, leading to the highest
positioning accuracy. For DGPS and RTK operations, it is
important to consider that the GDOP is dependent on the
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