Geneq SXBlue II Technical Reference Manual page 147

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 a charged layer of the atmosphere responsible for
the Northern Lights. Charged particles from the sun ionize this portion of the
atmosphere, 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 can lead to an incompletely corrected error
source that could degrade positioning accuracy with increased distance from the base
station.
SBAS systems (WAAS, EGNOS, MSAS, GAGAN, etc) use a different approach, using a
network of reference stations in strategic locations to take measurements and model the
real-time ionosphere. Updates of the ionospheric map are sent on a continual basis to
ensure that as the activity of the ionosphere changes with time, the user's positioning
accuracy will be maintained. Compared to using a DGPS beacon, the effect of
geographic proximity to a single reference station is minimized resulting in more
consistent system performance throughout all locations within the network.
Correction Latency
The latency of differential corrections to a lesser extent affects the achievable positioning
accuracy at the remote receiver since the magnitude of SA was turned to zero in year
2000. Latency is a function of the following:
• The time it takes the base station to calculate corrections
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