CSI Wireless Seres, Reference Manual

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Seres Reference Manual
5
1.4.1 Proximity of the Reference Station
The distance between a remote user and the reference station can
sometimes be considerable, such as when using 300 kHz DGPS
radiobeacons (not supported by the Seres). 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, 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