Abstract: GPS/SINStightly-coupled integration generally performs better than loosely coupled integration because they can get a navigation solution even if the number of GPS satellites is less than four. In order to improve the performance of GPS/SINS tightly-coupled integration, the measurement model of tightly-coupled GPS/INS based on double-difference pseudo-range/pseudo-range rate was proposed. The model based on double-difference pseudo-range was compared with the one based on both double-difference pseudo-range and double-difference pseudo-range rate The actual test shows that the two models can deal with the tightly-coupled integrated navigation. The accuracy of estimation and the convergent time of velocity and attitude error are improved by introducing double-difference pseudo-range rate.