GPS ACCURACY
As described earlier, the Model 3440 Plus gauge can be equipped
with an optional GPS receiver that determines the location (latitude
and longitude) of the gauge. This information is stored with each
gauge measurement. The GPS receiver used in the Model 3440 Plus
has Wide Area Augmentation System (WAAS) capabilities, which
provides accuracy to within 3 m (10 ft). However, the accuracy is
dependent upon the user’s location and other factors as described
below.
The global positioning system is a satellite-based navigation system
that consists of 24 satellites and a network of ground stations that
monitor and control those satellites. The satellites orbit the earth at
an altitude of approximately 11,000 miles and constantly transmit
signal information back to earth. A GPS receiver uses this
information to determine its location.
To determine its latitude and longitude, a GPS receiver must receive
the signals from at least three satellites.
On average, the receiver used in the gauge is accurate to within
15 m (approximately 50 ft) when receiving GPS data alone.
However, the WAAS capabilities can increase the accuracy to
within 3 m (10 ft), as shown in Table I–1.
The accuracy of GPS information can be affected by a number of
atmospheric forces and other conditions. The Wide Area
Augmentation System (WAAS) corrects for these factors by placing
GPS receivers at 25 known, precisely surveyed locations, called
reference stations, across the United States. The reference stations
determined a measured distance to each satellite using the signals
received from the satellites. For each satellite, the stations compare
the measured distance to the actual range (as calculated from its
known position) to determine a differential correction for each
satellite
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