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ABSTRACT: Deactivation of selective availability (SA) is expected to provide improvement in Global Positioning System (GPS) accuracy, and recent reports showed such improvements were realized immediately. However, most forestry GPS observations are made in subcanopy environments in which a GPS receiver's antenna can only receive GPS signals from a small region of the sky due to obstructions. Consequently, the satellite geometry will be poor, and position accuracy will suffer. Thus, there is a need to understand the impact of SA removal on positioning accuracy in high-PDOP subcanopy observations. This study found that it is difficult to approach the destination when GPS navigation is adversely influenced by SA accuracy degradation. When GPS positioning is performed after SA deactivation, a GPS observer can walk directly to the intended destination under thick tree canopies. Results also indicated that the horizontal accuracy is 73.3 m at the 95% probability level in the SA-degraded data set and 8.3 m in the SA-free data set. Further data analysis indicated that HDOPs (Langley 1999) recorded in the SA-on and the SA-off periods are not statistically significantly different. This suggests that a major portion of the difference in horizontal accuracies between SA-degraded and SA-free GPS observations can be attributed to difficulties in approaching the destination when SA is active. South. J. Appl. For. 26(3):140-145.
Key Words: Global Positioning System, selective available (SA), positioning accuracy, subcanopy stakeout.
GPS is a fully operational, worldwide, all-weather, satellite-based navigation system, and its Standard Positioning Service (SPS) is available for civilian applications (U.S. DoD/DOT 1982). A code-tracking GPS receiver computes its three-dimensional coordinate and its clock offset from four or more simultaneous pseudorange measurements.
Pseudoranges are measurements of the biased range between the receiver's antenna and the antennas of each of the satellites being tracked. The accuracy of the measured pseudoranges and the fidelity of the model used to process those measurements determine, in part, the overall accuracy of the receiver-derived coordinates.
SPS Position Accuracy
Accuracy is the difference between the GPS-measured position at any given time and the actual or true position. In practice, a position derived from an independent source of higher accuracy can be regarded as the actual position and, if the actual position is not known, the average position is often used...