A simulation study was carried out to find explanations for the following observational facts found in the GPS interferometric positioning by the Geographical Survey Institute: 1) The positioning results are sensitive to the unmodeled systematic errors remaining in the observables. 2) The spatial error distribution of position is independent of baseline orientation. The both phenomena are confirmed in this study to be natural for GPS observations in Japan where satellite constellation is

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... ct. Conclusions obtained are consistent with the results from a geometric interpretation proposed in a different paper by the authors. It is also confirmed that positioning results are more stable under a better satellite configuration currently available in U.S.A.