Accuracies of Position Fixes Obtained by GPS―VII The Observation at a Fixed Station on the Land (2)

In recent years, the number of GPS satellites has been increasing, and this system became operational with 15 satellites in October, 1990. In addition, its selective availability has been possible since March 25th, 1990. Thus, we measured the usable time and got position fixes from our fixed station at Nagasaki University in order to evaluate the positioning accuracy for three days in each of September and October, 1990. The usable time was about 19 and a half hours, which was two times longer than that with 6 satellites. Especially, position fixes of 3D (three-dimensional positioning) occupied 12 hours of the usable time. Despite the selective availability, standard deviations of D. lat (difference of latitude) and Dep (departure) were kept less than 20 meters due to increasing high level measurements, and the maximum positioning errors were 60～65 meters. We may conclude that the accuracy of this system has been improved because the standard deviation became so small as to be under 20 meters