Forestry and Agrifoods
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 Will users need to differentially correct GPS data now that SA is no longer in effect?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 Although the shapes of the polygons are similar now that SA is no longer in effect, the clipped section in the top corner clearly shows a difference between the boundaries of both polygons.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.