THE IMPACT OF POST PROCESSING GPS CODE-BASED FILES VOID OF SELECTIVE AVAILABILITY

Abstract

The primary objective of the project was to determine the impact of post processing (relative positioning) vs. the Navigationsolution (single-point positioning) for files collected in an absence of selective availability (S/A). A secondary objective wasto factor in the overall affect of a network adjustment on the final position values. The GPS observations were collected with12 channel Trimble XRS receivers using the course/acquisition code. Single-point positioning void of S/A with a PDOP ofless than 6 and at least 125 positions over 10 minutes yields 3D positions good to 3-5 meters. Relative positioning undersimilar conditions yields 3D positions good to 0.5 to 1.0 meters. The adjustment of single-point positions relative to a fixedpoint will yield improved relative locations but there still remains a 4-5 meter uncertainty in true position. The randomnature of GPS data rewards procedures, which average and adjust position locations. It is recommended that relativepositioning be used where possible followed by a network adjustment. This will provide good positions, identify bad dataand produce reliability data for the entire project. If a known base station is not available one can be established with theaverage data from of an unmasked single-point position that has been occupied long enough to reduce file position standarddeviations.