Troxler Enhanced RoadReader Plus 3451 Manual Of Operation And Instruction page 23

Two master stations, located on either coast, collect data from the
reference stations to create a GPS correction message. This message
is then broadcast through two geostationary satellites that are in a
fixed position over the equator. The GPS receiver applies the
correction factors contained in the correction message to increase
the accuracy of its measurements.
Note that the signals from the WAAS satellites may not be available
to gauge users in locations where trees, mountains, and other large
objects obstruct the view of the horizon. On other occasions, the
GPS receiver may not be able to access the signals from the three
satellites required to determine a position. As noted earlier, the
gauge stores location information with the results of each
measurement. The measurement display also denotes the quality of
the location fix as follows:
 If WAAS information is available during a gauge measurement,
the fix quality for that measurement will be denoted as DGPS
fix.
 If a GPS location is determined, but the WAAS information is
unavailable, the fix quality will be denoted as GPS fix.
 If the GPS receiver cannot determine a location, the latitude and
longitude will be denoted as 0.
The Wide Area Augmentation System (WAAS) is
currently functional only in the United States.
If the gauge is moved a long distance between uses, the
GPS system must be allowed to initialize. In some
instances, initialization may take as long as 30 to 45
minutes from the time the gauge is powered on with the
GPS enabled. Note also that the gauge must be
positioned such that the GPS receiver can receive signals
from the GPS satellites as described above. If the GPS
does not initialize within 45 minutes, contact your
Troxler representative.
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