Testing of GPS accuracy for precision forestry applications

The use of existing geospatial technologies such as GIS and global positioning systems (GPS) to precision forestry is booming. The objective of this study was to examine the applicability of GPS and radio beacon differential global positioning system (RBN DGPS) for forestry machinery positioning. This would be an important step toward the realization of site-specific chemical application in China. A Trimble AG132 GPS receiver was mounted on a moveable platform and real-time differential GPS signals were obtained from the Yangtze River Beacon Station. The experiments were conducted on campus of Nanjing Forestry University, China. GPS data were collected and downloaded into a laptop computer via a RS-232 serial port and customer-developed data-acquisition software. Two tests were conducted: (1) a stationary test (where the mobile platform was maintained at a fixed location) in an open sky and forest condition to examine the precision (repeatability) of GPS signals, and (2) a mobile test (where the platform was moving) to examine the dynamic conformity degree. Analysis of variance was used to clarify the effects of positioning mode and stand area on the precision of GPS. In the stationary test, the variability of X and Y coordinates is smaller in RBN DGPS mode than that in the non-differential mode; and the performance of GPS in terms of position dilution of precision, horizontal dilution of precision, vertical dilution of precision, time dilution of precision values, effective positioning rate and the number of visible satellites was superior when used in RBN DGPS mode. In the mobile test, the desired traces of the moveable platform (circles) were much better followed by the GPS locations recorded in the RBN DGPS mode. Compared with non-differential mode, RBN DGPS is more suitable for precision forestry positioning requirements. RBN DGPS improves accuracy both in open sky and forest condition, and the service is available to end users in China with no additional costs