Vector Sensor Reference Manual 130
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, such as
that of radiobeacons. 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 decoration
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 it 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, is not as simple a task to
correct. The ionosphere is 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 will see a general cooling trend of the ionosphere over the next
few years thus reducing 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 that 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’s possible that the state of
the ionosphere differs between the remote user and the single reference station. This can lead to
incompletely corrected error source that could degrade positioning accuracy with increased
distance from the base station.
WAAS and SBAS use a different approach, using a network of reference stations in strategic
locations to take measurements and model the real-time ionosphere. Updates 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.
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