SX BLUE GPS iSXblue II+ GNSSTM, Руководство пользователя

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SXblue Series User Manual 90
considerable, such as when using 300 kHz DGPS radiobeacons. Consequently,
some of the errors associated with GPS at the base station differ somewhat from
those at the remote user’s location. This spatial decorrelation of errors can result
in a relative position offset from the absolute coordinates of the remote receiver.
This offset may be as much as one meter for every 100 km (62 miles) between the
base station and remote receiver.
The causes of decorrelation are:
•
GPS satellite orbit errors (significant)
•
Ionospheric errors (potential to be most significant depending on level of activity)
•
Tropospheric errors (less significant)
GPS satellite orbit errors are typically a greater problem with local area differential
systems. The decorrelation effect is such that the satellite’s orbit error projects onto
the reference receiver and remote receiver’s range measurements differently. As
the separation between the receivers increases, the orbit error will not project onto
the ranges in the same manner, and will then not cancel out of the measurement
differencing process completely. SBAS networks, with the use of multiple base
stations, are able to accurately compute the orbit vector of each satellite. The
resulting corrector is geographically independent, so minimal decorrelation occurs
with respect to position within the network.
The ionosphere and the troposphere both induce measurement errors on the
signals being received from GPS. The troposphere is the humid portion of the
atmosphere closest to the ground. Due to its humidity, refraction of GPS signals
at lower elevations can distort the measurements to satellites. This error source
is rather easily modeled within the GPS receiver and doesn’t constitute a
significant problem.
The error induced by the ionosphere is more significant, however, and is not as
simple a task to correct. The ionosphere is the charged layer of the atmosphere
responsible for the Northern Lights. Charged particles are ionized by solar
radiation resulting in an electrically active atmospheric layer. This charged activity
affects the GPS signals that penetrate this layer, affecting the measured ranges.
The difficulty in removing the effect of the ionosphere is that it varies from day to
day, and even hour to hour due to the sun’s 11-year solar cycle and the rotation of
the earth, respectively. During the summer of 2001, the sun’s solar cycle reached
an 11-year high and going forward we saw a general cooling trend of the
ionosphere over the few years that followed, thus with reduced ionospheric activity.
Removing the effect of the ionosphere depends on the architecture of the
differential network. DGPS radiobeacons, for example, use a more conventional
approach than WAAS or SBAS in general. DGPS beacons make use of a single
reference station, which provides real-time GPS error corrections based upon
measurements that it makes at its location. It is possible that the state of the
ionosphere differs between the remote user and the single reference station. This