Crop dusting using GPS

Global Positioning System (GPS) receivers and GPS-based swath guidance systems are used on agricultural aircraft for remote sensing, airplane guidance, and to support variable-rate aerial application of crop inputs such as insecticides, cotton growth regulators, and defoliants. Agricultural aircraft travel at much greater speeds than ground equipment (65 m/s, typically), so longitudinal (along-track) error in GPS-derived position is likely to be pronounced. Due to rapid ground speeds, determination of exact GPS-derived aircraft location requires special methodologies. To meet the need for determining ground position accurately, a ground-based spotting system using two strategically placed mirrors was designed to reflect sunlight to the belly of the aircraft when the plane passed over a ground reference point. The light beam was detected by a photocell on the airplane, causing a solid state relay to close through appropriate circuitry. When the relay closed, a record was placed in the data file for the airplane's Satloc GPS receiver, accurate to 0.01 s allowing exact position to be logged. Two stand-alone GPS receivers were evaluated along with the Satloc system, and their readings were compared with readings from the Satloc. For one day of testing used as an example, the Satloc lagged the ground reference point by an average of -4.53 m (S.D. = 0.68) in the east-west directions and led the reference point in the north-south directions by an average of +7.88 m (S.D. = 0.52). Positioning data from one stand-alone GPS receiver showed a distinct ground speed dependency, making it unsuitable for use in aircraft. Output from the other receiver (designed for aircraft use) was insensitive to ground speed changes but exhibited a look-ahead equivalent to 2-s at the ground speeds evaluated.