Accuracy improvement of GPS measurement under a bad condition

GPS has been spreading rapidly for its confidence and usefulness, and now it is used in various situations. At present, we can measure a present position within several centimeters from the true position by high performance GPS. But the high performance system requires a lot of cost for its infrastructure. Although the easy system of GPS, so called Standard Positioning Service (SPS), is convenient, the accuracy and precision are not enough to use for application. We have noticed bias error becoming large, when positioning is carried out under the bad condition in SPS. We paid attention to satellite arrangement and pseudo range error. We made an experiment, and reproduced it with simulation. We aim at mitigation of the bias error under the bad condition.