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This study optimizes the GPS static positioning algorithm based on only four observable satellites. An improved differential algorithm is proposed after analyzing the disadvantages of the classical algorithm and the limitation of traditional differential algorithm. The linear equations are built and solved directly using pseudorange measurement from four observable satellites. The data conclude two neighboring epoch data from a best-position satellite and the data from other three satellites in current epoch. The best-position satellite is chosen based on improved satellite-selecting algorithm. The experimental result indicates that our new algorithm requires no derivative and iterative calculation procedures. Comparing to the traditional algorithm, the problem for GPS worse positioning situation with only four observable satellites is solved without lowering the positioning accuracy. This new algorithm is significantly important to the study of GPS static positioning algorithm based on pseudocode measurement.