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Title: Evaluation of Application Accuracy and Performance of a Hydraulically Operated Variable-Rate Aerial Application System

Author
item Thomson, Steven
item SMITH, LOWREY - 6402-40-00 RETIRED
item Hanks, James

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2009
Publication Date: 7/2/2009
Citation: Thomson, S.J., Smith, L.A., Hanks, J.E. 2009. Evaluation of Application Accuracy and Performance of a Hydraulically Operated Variable-Rate Aerial Application System. Transactions of the ASABE 52(3):715-722

Interpretive Summary: Variable-rate (or “prescription”) application of field inputs such as pesticide, fertilizer, and fungicides is an established agricultural practice that conserves inputs by targeting only those field areas requiring application. Less material applied to the field has corresponding benefits to the environment as environmental loading is reduced. Variable rate application from agricultural aircraft can be a real challenge as systems must change rates very quickly to place the correct rate of spray material within each management zone. This is because aerial application is typically at ground speeds near 65 to 70 m/s, and automatic flow control and global positioning system receivers must respond rapidly to effect spray rate changes at the desired zone boundaries. Recently, fast-responding hydraulic flow systems for agricultural aircraft have the potential for carrying out rapid flow changes based on input from swath guidance Global Positioning System (GPS) receivers. These GPS-based guidance systems can also read ‘prescription’ files that define management zones and their associated properties (i.e. application rate and boundaries) within fields being sprayed. All system components (the GPS receiver, automatic flow controller, hydraulic actuators, and spray system plumbing) can contribute to application accuracy errors and response delays to a given flow control signal. The first known successful evaluation of an aerial variable-rate application system has been accomplished herein. The system consisting of a DGPS-based guidance system, automatic flow controller, and hydraulically controlled pump/valve was evaluated for response time to rapidly changing flow requirements and accuracy of application. Spray deposition position error was evaluated by direct field observation of water sensitive paper (WSP) cards while traveling across GPS-located field boundaries. Data from the flow controller and a custom-built flow meter monitor were used to evaluate both flow controller and variable rate system error while making applications to a series of four management zones. Observations of WSP showed that average spray deposition position error magnitude was 5.0 m when traveling east to west and was 5.2 m when traveling north to south. Flow controller error and variable rate system error were then evaluated from data collected while making applications to four management zones. Flow controller error ranged from -1.0 to 2.1 percent. Variable rate system error due to rate change timing was evaluated by comparing required rates from the system to required rates from the prescription. Average rate timing error for six application passes ranged from -9.1 to 1.4 percent with an average of -3.04 percent. Considering the speed at which changes have to be made for aircraft flying at 65 m/s, the hydraulically-operated variable-rate system performed well for location accuracy of deposition, response to changing flowrates, and accuracy of application amounts for the prescription.

Technical Abstract: An aerial variable-rate application system consisting of a DGPS-based guidance system, automatic flow controller, and hydraulically controlled pump/valve was evaluated for response time to rapidly changing flow requirements and accuracy of application. Spray deposition position error was evaluated by direct field observation of water sensitive paper (WSP) cards while traveling east to west and north to south across rate change boundaries. Data from the flow controller and a custom-built flow meter monitor were used to evaluate flow controller error and variable rate system error while making applications to a series of four management zones (each 81 m long; 28, 47, 56, and 37 L/ha). Observations of WSP showed that average spray deposition position error magnitude was 5.0 m when traveling east to west and was 5.2 m when traveling north to south. Statistical analysis indicated that direction of travel had a non-significant effect on the magnitude of spray deposition position error. Flow controller error and variable rate system error were evaluated from data collected while making applications to a series of four management zones (each zone required approximately 1.2 s) with application rates of 28, 47, 56, and 37 L/ha. Areas under time plots of required and actual flowrates were compared and indicated flow controller error ranging from -1.0 to 2.1 percent. Variable rate system error due to rate change timing was evaluated by comparing required rates from the system to required rates from the prescription. Area under time plots of these variables showed that average rate timing error for six application passes ranged from -9.1 to 1.4 percent with an average of -3.04 percent. Considering the speed at which changes have to be made for aircraft flying at 65 m s-1 typically, the hydraulically-operated variable-rate system performed well for location accuracy of deposition, response to changing flowrates, and accuracy of application amounts for the prescription.