Performance of GPS Precise Point Positioning Under Conifer Forest Canopies

A 20-channel, dual-frequency GPS receiver collecting pseudorange and carrier phase observations was used as a stand-alone receiver to determine positional accuracy of 19 points under conifer tree canopies. The positions were determined utilizing precise satellite orbit and clock products from the International GNSS Service. The mean positional accuracy ranged from 0.27 to 0.88 m for an observation period of 120 minutes, and 0.95 to 3.48 m for 15 minutes. For the 15 minute observation period computed positions could not be found for 8 to 44 percent of the locations. Accuracy increased with decreasing forest stand density. Stand basal area (R 2 � 0.11, p � 0.001) and number of tree stems (R 2 � 0.07, p � 0.001) were significantly correlated with accuracy. The probability of determining a position increased with longer observation periods and decreasing number of tree stems. For natural resource applications where the costs associated with the length of the observation period on each site in field is a critical factor, differential GPS seems to be a more robust alternative than precise point positioning with GPS.