Forecast and improvement of shift error with GPS Standard Positioning Service

Recently, GPS (Global Positioning System) has been spreading rapidly. Especially, SPS (Standard Positioning Service) was improved in 2000 by releasing SA (selective availability), and now attention is being paid to it due to its reliability and usefulness. SPS is expected to be used in various fields. We have discovered that the position obtained by SPS shifts from true position. We propose a shift error reduction method using only satellites information (angle of elevation, angle of direction) from a GPS receiver. We compare the result of simulation with that of positioning experiments using the index of angle and distance. As a result, we are able to forecast the direction of the shift and the amount of the shift and also are able to improve SPS precision.