Tests of reception of the combination of GPS and GLONASS signals under and above forest canopy in the Black Forest, Germany, using choke ring antennas

Mobilizing timber resources from the smallest private forest ownership should be possible by ensuring straight information management along the logistics chain, including precise localization of trees and their ownership. Precise navigation during harvest operations by heavy forest machines is necessary to keep them on established skid trails to avoid additional soil compaction. Furthermore, the construction of a skid trail network itself as a contribution to soil protection requires precise positional information. Therefore, the main purpose of this study was to find an easy-to-apply and high-precision solution for real-time navigation within forest stands. Static measurements with two choke ring antennas, which are touted to highly reduce multipath effects, were conducted in two heights on 19 test sites in the Black Forest Mountain region, South Western Germany, to investigate Global Navigation Satellite System (GNSS) signal reception below and above forest canopy. All sites showed a significantly lower precision under the canopy, far away from sub-meter level. Since our aim for dynamic navigation is a trueness of 0.35 m, the conclusion is that the application of high precision GNSS-antennas, as a single means of optimization, does not meet the requirements for real-time navigation within forest stands and further research and improvement is needed. The combination of GPS and GLONASS satellite systems was investigated, which delivers a slightly higher precision than GPS only and is therefore recommended. The precision of GLONASS only is much lower and therefore unsuitable. The causes for low precision under canopy were investigated. Our data clearly shows a correlation between canopy density and decreasing precision. Under small opened canopy, we identified a dominating influence of scattering effects on the precision. Leaf type of trees, slopes and direction have an influence on the precision, but much smaller than the sky openness.