Abstract: With the development of space-borne GPS technologies and IGS precise orbit and clock products,precise orbit determination(POD) of Low Earth Orbiters(LEOs) based on PPP technology has become a hot issue.At present,an accuracy of 3～5 cm in Radial(R),Along-track(T),Cross-track(N) components can be achieved with conventional PPP.However,the results are less precise or reliable than the double-differenced fixed solution due to the ambiguity-float solution in traditional PPP.In order to further improve the precision and reliability of POD with PPP,daily observations from all IGS tracking stations through day 2 to 7 in 2012 were used to estimate the wide and narrow-lane fractional cycle bias(FCB) of all satellites for PPP integer ambiguity resolution.The POD of the two GRACE satellites were respectively compared with GFZ’s post precise orbit and K-band measurements for the analysis of the inner and external conformities.As compared to GFZ,the daily orbital accuracy of the ambiguity-fixed solution in this work is about 2～3 cm in R direction,better than 2 cm in T direction and better than 2 cm in N direction for GRACE-A,an improvement of 19%,30% and 50% respectively when compared with that of the float solution.The POD precision for GRACE-B are 2-3 cm,2 cm and 1-2 cm in R,N,T directions for the fixed solution,respectively.The accuracy improvement in the fixed solution for GARCE-B is nearly equivalent with that for GRACE-A.For K-band cases,the mean STD error of KBR residual is about 16.4 mm for the fixed solution,and 22.6 mm for the float solution.These results show that PPP with ambiguity-fixed can greatly improve POD precision of LEO satellites and provide more reliable orbital service.