Accuracies of Position Fixes Obtained by GPS―I The Oservation at a Fixed Station on the Land

The United States has been developing a GPS (Global Positioning System), and now We can make use of it too in Japan but the usable time is limited due to the incompleteness of the system. We measured the usable time and took position fixes from our fixed station at Nagasaki in order to evaluate the positioning accuracy for about one month in Dec. 1986. We received positioning signals from 6 satellites. The usable time at this station was about 6.5 hours of which high level positioning accounted for 45% in the first half of the month but only 17% in the latter half. The positioning errors of 3H (three-dimensional high level positioning) ,3M (three-dimensional middle level positioning) and 2H (two-dimensional high level positioning) were smaller than 100 meters. Even the positioning error of 2L (two-dimensional low level positioning), which was largest, was only about 181 meters (0.1 nautical mile). From the practical standpoint, 2L is sufficiently accurate to be used for the positioning of an ordinary ship sailing normally. Until the system is completed,if we use not only high level but also middle and low level, we will be able to prolong the usable time. We measured the positioning signal at ten-minute intervals, so data were not as full as possible, and the evaluation of the positioning accuracy by combination of satellites was limited to only partial combinations.The positioning accuracy by combination of satellites (3・6・9・11) in the case of 3H and combination of satellites (3・11・13) on 2H and 2M was good by comparison with other combinations and these combinations had long usable times