CSI Wireless Seres, Справочное руководство

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Seres Reference Manual
6
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.