Contributions of differential GPS and GLONASS observations to point accuracy under forest canopies

A 24-channel, single-frequency receiver observing the C/A code and carrier phase of both GPS and GLONASS was used to determine the positional accuracy of 27 points under tree canopies. The mean positional accuracy based on combined differential postprocessing of GPS C/A code and carrier phase observations ranged from 0.20 m to 5.72 m for 2.5 min to 30 min of observation at points with basal area ranging from 2/ha to >=25 m2/ha. The mean positional accuracy of combined differential postprocessing of GPS+GLONASS C/A code and carrier phase observations ranged from 0.09 m to 2.85 m. The accuracy increased with decreasing density of forest and improving geometric satellite distribution as observed at the base station. Static differential postprocessing of 30-min observations of GPS-GLONASS carrier phase gave fixed solutions for 13 of the 27 points. The accuracy of these solutions ranged from 0.01 m to 0.09 m. The probability of obtaining a fixed solution under tree canopies was modeled by logistic regression. The regression model classified correctly 81.5 percent of the processed solutions.