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

Abstract

An aerial variable-rate application system consisting of DGPS (Differential Global Positioning System)-based guidance, an automatic flow controller, and a hydraulically controlled pump was evaluated for response 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 flowmeter monitor were used to evaluate flow controller error and variable-rate system error while making applications to a series of four management zones (28, 47, 56, and 37 L ha-1; each 81 m long). Observations of WSP showed that average spray deposition position error magnitude was 5.0 m when traveling east to west and 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-1. Areas under time plots of required and actual flow rates were compared and indicated flow controller error ranging from -1.0% to 2.1%. 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% with an average of -3.04%. Considering the speed at which changes have to be made for aircraft typically flying at 65 m s-1, the hydraulically operated variable-rate system performed well for location accuracy of deposition, response to changing flow rates, and accuracy of application amounts for the prescription.