Effects of differential single- and dual-frequency GPS and GLONASS observations on point accuracy under forest canopies

A 20-channel, dual-frequency receiver observing dual-frequency pseudorange and carrier phase of both GPS and GLONASS was used to determine the positio nal accuracy of 29 points under tree canopies, The mean positional accuracy based on differential postprocessing of GPS+GLONASS single-frequency obser vations ranged from 0.16 m to 1.16 m for 2.5 min to 20 min of observation a t points with basal area ranging from < 20 m(2)/ ha to greater than or equa l to 30 m(2)/ ha. The mean positional accuracy of differential postprocessi ng of dual-frequency GPS+GLONASS observations ranged from 0.08 m to 1.35 m. Using the dual-frequency carrier phase as main observable and fixing the i nitial integer phase ambiguities, i.e., a fixed solution, gave the best acc uracy. However, searching for fixed solutions increased the risk of large i ndividual positional errors due to "false" fixed solutions. The accuracy increased with decreasing density of forest, increasing length of observation period, and decreasing a priori standard error as reported by the postprocessing software.