Abstract: In GPS interferometric attitude determination, the size of phase integer ambiguity searching volume afifects directly the speed of determining the correct integer ambiguity. This paper provides two approaches:one uses multiply antennas, and the other uses the phase smoothed pseudoranges to reduce searching volume and to uncorrelate with the length of the baseline. Both approaches can satisfy the requirements of high precision and small searching volume.The derived data show that they are superior to the conventional way.