Accuracies of Position Fixes Obtained by GPS―VIII The Observation at Foreign Fixed Stations (2)

We have measured the usable time and got position fixes from overseas fixed stations for the past three years in order to evaluate the positioning accuracy of GPS. We were able to receive positioning signals from only 6 satellites until 1989, but in October 1990, 15 GPS satellites became available. And the orbits of satellites have been corrected. The usable time was 7～9 hours until 1989, but it came to be more than 18 and a half hours in 1990. Then we were generally able to get position fixes by 3D (three-dimensional positioning) and the high level of 2D3 (two-dimensional positioning with three satellites). Therefore, position fixes of 2D2 (two-dimensional positioning with two satellites) became hardly necessary. The accuracy of position fixes went down once in 1989. In 1990, the standard deviations of D. lat (difference of latitude) and Dep (departure) were kept less than 45 meters. We may conclude that in spite of selective availability, the accuracy has been improved owing to the increasing number of GPS satellites and the correction of thier orbits