Centre county Pennsylvania, USA | Dave, I'll depart from Darin's topic, with apologies to Darin.
Attached scatter plots show scatter as a time series. One shows JD's SF2 DGPS scatter in elevation and one shows Trimble's RTK scatter in all 3 dimensions. Elevation scatter is typically twice as great as horizontal scatter. I don't have on-farm scatter test data for Omnistar's DGPS correction service, sorry.
GPS scatter tests run for any 24 hour period will tend to show cyclic behavior of DGPS fixes caused by GPS satellite 12 hour orbits but they do not show daily, or monthly, or yearly DGPS performance repeatability. DGPS fixes are highly dependent on ionospheric conditions and on changes in GPS satellite orbit geometries that effect precision (DOP). I've included an attachment that shows why folks should check GPS DOP plan before heading to the field. That attachment shows Trimble's predicted GPS Horizontal DOP (HDOP) for next May 12th (2008 corn planting time) here at our farm. The very large spike in HDOP between 9AM and 10 AM on that day will cause very large errors in DGPS here. much larger than those shown in a typical 24 hour static test run.
 (JD-SF2 static elevation scatter.gif)
 (Trimble RTK static 3D scatter.gif)
 (GPS DOP Horizontal, April 11 2008.jpg)
Attachments
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 JD-SF2 static elevation scatter.gif (15KB - 139 downloads)
Trimble RTK static 3D scatter.gif (22KB - 134 downloads)
GPS DOP Horizontal, April 11 2008.jpg (24KB - 143 downloads)
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GPS reciever accuracy with auto section shutoff equipment
