2005 Annual Report 4d.Progress report. This report serves to document research activity conducted under a Specific Cooperative Agreement between ARS and Houma Avionics, Inc. It has been carried out under the parent research project 6402-22000-038-00D, Developmentof Pesticide Application Technologies for Spray-drift Management and Targeted Spraying. Under this agreement the hardware was purchased and installed on the aircraft to achieve the current state-of-the-art aerial variable-rate application system. In order to test the performance of this system a 32 acre test site has been established by land forming and seeding to Bermuda grass and by georeferencing zones for prescription application rates. A prescription was defined for the area and the physical location of zone corners were identified on the ground using an RTK GPS system with cm level accuracy. Some preliminary testing was conducted to evaluate the ability of the system to consistently identify zone boundaries and apply the proper application rate to them. The manufacturer is developing a faster GPS receiver, therefore, in-depth performance testing will await the faster GPS. Tests were conducted on the dynamics of the automatic flow control component of the variable rate system. Data were collected to describe the relationship between the hydraulic servo position (controls power delivered to the chemical pump) and flowrate for a range of boom setups. These data showed that a linear relationship existed between servo position and pump flowrate, but that the slope of the line became steeper as the total flow area of nozzles became smaller. Controller performance was optimized as a result of this information by modifying the controller algorithms to adjust sensitivity as boom setup was changed. This modification has been implemented and is currently being used by applicators. The significance of working directly with a manufacturer of state-of-the-art equipment is that improvements are immediately available to the aerial applicators who are the primary customers of this work.