|Evett, Steven - Steve|
Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2005
Publication Date: 9/1/2005
Citation: Peters, R.T., Evett, S.R. 2005. Using low-cost GPS receivers for determining field position of mechanized irrigation systems. Applied Engineering in Agriculture. 21(5):841-845. Interpretive Summary: Researchers are working on methods to vary the amount of irrigation water applied to different areas of a field under mechanized irrigation systems such as center pivots or linear move irrigation systems. This may result in improved crop yields with less water. This requires the knowledge of the exact and real-time location of the mechanized irrigation system in the field. Global positioning satellite (GPS) receivers are ideal for this application. The most commonly used center pivot positioning mechanism, a small instrument called a resolver, was tested using a survey grade, sub-meter-accuracy GPS receiver mounted on the end of the pivot. The resolver showed angular position errors as high as five degrees. These errors are too high for center pivot position reporting for variable rate irrigation. A method for correcting these errors using a fitted sine curve is presented. A low-cost GPS receiver was tested in a stationary location over an extended time period. Although 95% of the readings were within a 2.1 m radius, the remaining 5% of points showed errors as large as 6.6 m. Outlying errors this high can present problems for their use with variable rate irrigation work. Strategies for minimizing these GPS errors are suggested.
Technical Abstract: As the accuracy of GPS receivers improves and the costs decrease, more and more applications for GPS become feasible. One such application is reporting center-pivot and lateral move field position. Accurate knowledge of center-pivot or lateral move position in real time is critical for precision irrigation or chemigation. On center pivots, a traditional resolver can only report the location of the first interior tower while a GPS receiver can more precisely show the location of the end of the pivot. This advantage over traditional resolvers becomes more pronounced with longer center pivots. Lateral move systems do not have a readily available mechanism for reporting their position as they travel over the field. GPS is potentially an ideal method for position and alignment reporting on lateral moves. The resolver on a three tower experimental center pivot was tested using a survey grade, sub-meter-accuracy GPS receiver. The resolver-reported angular position had errors as high as five degrees. Fitting a since curve and subtracting the errors from the reported measurements corrected these errors. These errors were corrected by fitting a sine curve and subtracting the errors from the reported measurements. A low-cost GPS receiver was tested in a stationary location on the same center pivot to determine its fitness for reporting field position for mechanized or self-propelled irrigation systems. This low-cost receiver showed an accuracy less than 2.1 m 95% of the time. However, the remaining 5% of points showed errors as large as 6.6 m. Outlying errors this high can present problems for precision or site-specific irrigation work. Suggestions are offered for mitigating these errors.