Integrated analysis of GPS, GIS, and machine dynamic response in precision liquid fertilizer application

The importance of accuracy in spatially variable rate application (SVRA) has been highlighted in many studies. The cost involved in achieving desired accuracy is high and may render SVRA inappropriate. A map-based spatially variable rate application (MBSVRA) system usually consists of an SVR sprayer navigated by a differential global positioning system (DGPS). The spraying unit, fitted with a DGPS receiver, is triggered automatically by the machine field location indicated in a desired map located in the operator console. The desired map is derived from geographical information system (GIS) analysis done using a yield map. Using a yield monitor fitted with a GPS receiver, yield maps can be developed with collected yield data, followed by data interpolation. It is important to analyze the integrated requirement of spatial parameters an MBSVRA system. The purpose of this study was to develop an integrated MBSVRA system error model for the assessment of accuracy requirements of an MBSVRA system in a tree-based orchard. A 3.6 ha Florida citrus (orange) planting in Manatee county was selected. Spatial parameters used in these studies were two field boundaries (Y), five GPS static horizontal accuracy levels (G), two interpolation methods (P), two levels of navigation error (T) with two levels of DGPS sampling frequencies (F) and two levels of machine delay time (D). A theoretical model was developed to analyze errors among the spatial parameters. Static horizontal positioning accuracy of a citrus yield monitor (Goat) and another DGPS system (Omnistar) were evaluated. Dynamic horizontal positioning accuracy of the two DGPS systems were also measured for real-time machine navigation in MBSVRA. Yield maps were developed using Y, G and P. A desired map was subsequently derived using a yield map based on the recommended fertilizer rate of 4.45 kg nitrogen per ton (t) of harvested fruit. Spraying pattern, T and F were modeled to obtain the applied map based on the appropriate desired map. Absolute error (AE) was used to quantify the difference between the desired and applied maps. An integrated model relating the influence of spatial accuracy requirements among G, T, F, and D using the best boundary and interpolation method was established.