Effects of moisture in wood materials on the SNR of GPS signals

This study was performed to investigate the factors responsible for the reduction in the signal-to-noise ratio (SNR) of a global positioning system (GPS) when used in a forest. We investigated the relationship between the SNR values of GPS signals and the moisture in wood materials and clarified this relationship through experimentation and simulation. Although the wood material itself had a minimal effect on SNR reduction, moisture in the wood had an obvious effect. It is noteworthy that wood material with a moisture resistance quantity (Mw) of 2 g/cm2 caused a reduction of more than ~10 dB in the SNR. This Mw value corresponds to Japanese cedar with a green stem thickness of ~7 cm. Under unobstructed sky conditions, SNR is less than ~20 dB, so a reduction of 10 dB has a marked effect on GPS reception. The reduced SNR under canopies resulted from moisture in stems and the canopy. Furthermore, a simulation performed in a Japanese cedar forest revealed that 77.8% of the SNR reduction was caused by stems, 8.1% by branches, and 14.1% by needles.