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Until recently, GPS users here and elsewhere were
accustomed to position errors in the magnitude of 100 meters. That is, the
reading the machine gave could be off as much as 100 m from the true location
due to a policy of the United States military known as Selective Availability
(SA). SA is the intentional degradation of Global Positioning System (GPS)
signals to the public.
At midnight on May 1, the United States
government discontinued Selective Availability. The decision to remove SA is the
latest measure in an on-going effort to make GPSmore responsive to civilian and
commercial users worldwide.
With SA discontinued, users in the
province are now able to achieve up to 10 times better accuracy than before or,
in other words, an improvement in position accuracy to approximately 10
meters.
What does the removal of SA
mean to GPS users in the province? Will users need to differentially correct GPS
data now that SA is no longer in effect? The answers to these questions depend
on your GPS application.
For mapping purposes or area verification, where the
user requires better than 10 meter accuracy, discontinuing SA has a minimal
effect because Differential GPS (which uses observations made at a known
location to correct for observations made at an unknown location) will still be
required to remove the longer term errors in the system such as atmospherics,
multipath, receiver noise, orbit and satellite clock errors. Differential GPS
was also effective in eliminating errors associated with SA.
An example of this is provided which identifies a GPS
file collected with Trimble’s Geo 3 mapping grade receiver. Both the
differentially corrected file (yellow polygon) and the uncorrected file (blue
polygon) were exported to ARCView and combined into one coverage. Although the
shapes of the polygons are similar now that SA is no longer in effect, the
clipped section in the top corner clearly shows a difference between the
boundaries of both polygons. This difference can be eliminated with differential
correction, thereby improving the accuracy of the collected points.
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