Abstract
GPS has an ability to fix a position with the accuracy of meter order. It is also possible to provide an accurate time. The authors tried to evaluate the accuracy of the time of 1PPS signals by outputting from 2 general GPS receivers of which antennas are set at the positions separated 20m from each other. The data were acquired for 24hours on the roof of a building at Tokyo University of Fisheries. The result shows that the time difference between two 1 PPS signals is 150ns in average, and 50ns in standard deviation, except for the instance when the enough number of the satellites is not available due to the blocking by surrounding tall buildings. Another experiment is tried that one antenna is set close to the wall to see the influence of the reduction of the number of satellites. The result proves that the time difference and its standard deviation are comparable to those for 24hours data as long as the number of satellites is more than three. Next, we estimated the influence of measurements in underwater to be only 0.075mm on the range difference and less than 0.001m/s on the speed by the measured standard deviation of time difference of 50ns and the underwater speed of sound of 1, 500m/s.
