Using Low-Cost GPS Receivers for Determining Field Position of Mechanized Irrigation Systems

As the accuracy of GPS receivers improves and the costs decrease, more applications for GPS become feasible.One such application is reporting center-pivot and lateral-move field position. Accurate knowledge of center-pivot orlateral-move position in real time is critical for site-specific irrigation. On center pivots, a traditional resolver can only reportthe location of the first interior tower while a GPS receiver can more precisely show the location of the end of the pivot. Thisadvantage over traditional resolvers becomes more pronounced with longer center pivots. Lateral-move systems do not havea readily available mechanism for reporting their position as they travel over the field. GPS is potentially an ideal methodfor position and alignment reporting on lateral-moves. The resolver on a three tower center pivot was tested using a surveygrade, sub-meter-accuracy GPS receiver. The resolver-reported angular position had errors up to 5 degrees. Fitting a sinecurve and subtracting the modeled errors from the reported measurements corrected these errors to plus or minus one degree.A low-cost GPS receiver was tested in a stationary location on the same center pivot to determine its fitness for reporting fieldposition for mechanized or self-propelled irrigation systems. This low-cost receiver was accurate to within 2.1 m 95% of thetime. However, the remaining 5% of points showed errors up to 6.6 m. Outlying errors this large can present problems forprecision or site-specific irrigation. Suggestions are offered for mitigating these errors.