Abstract: A single-epoch ambiguity resolution algorithm without a search process is proposed. Four basic satellites are selected, whose integer ambiguities are estimated by using the INS position, then (1,-1) and (-3,4) ambiguity combinations are formed. The ionospheric error of the (-3,4) combination is polynomially fitted. After systematic influences such as the ionospheric error are compensated, the ambiguity combination can be fixed by simple rounding. The ambiguities of each frequency can be determined by the combination and returned to resolve the receiver position. Based on the receiver position, the ambiguities of all other satellites can be calculated directly and fixed by rounding. The algorithm was validated by field test GNSS/INS data.