On May 1, 2000, Selective Availability (SA) was discontinued by the United States Department of Defense for Global Positioning System (GPS) satellites. SA was the government system that created errors in satellite data available to the public by introducing clock errors. These errors were deliberately created because position data was needed for secure military purposes, especially during the Gulf War, and the Department of Defense didn’t want enemies to have accurate data. As a result, a GPS unit without differential correction had errors greater than 30 feet. With SA turned off, the errors became 30 feet or less. Since SA has been discontinued, there have been many reports in the public press describing great improvements in GPS accuracy. What is the impact on precision agriculture?
For those who have handheld GPS units, there is a tremendous improvement in positioning accuracy. But there isn’t a comparable change in GPS systems using differential correction (DGPS). The primary purpose of DGPS was to correct the errors created by SA.
For those using DPS systems on combines, planters, and fertilizer applicators, there will be very little change in the accuracy, which is about three feet. DGPS also corrects some of the errors created by the atmosphere between your receiver and the satellites. But operators will find that they are less likely to lose their signal during operation. And there will be a lesser tendency to see the location of yield data wander or drift. Multi-path errors and satellite signals reflecting from nearby buildings or other structures will still exist.
Eliminating SA is the first step for improving the accuracy of GPS. Other improvements coming include a national differential correction system (National Differential Correction GPS or NDGPS) and other steps to redesign the satellites to insure improved accuracy. More improvements will be added during the next ten years.
GPS was the major development to make precision agriculture a reality. Improved accuracy should have little impact on the future adoption of precision agriculture because the three-foot accuracy available with DGPS is adequate for most applications. The improved accuracy will have a greater impact on guidance system development for tractors and self-propelled farm equipment.
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