The geometric separation among the navigation satellite antenna phase centers at different frequencies affects the accuracy of TGD measurement.According to the measurement and utilization method of TGD parameter,this paper establishes the relational model between the parameter and the satellite inter‐frequency phase center separation.Whereas the relative motion of the satellite,the monitor station and the sun has a performance of complex diversity,this paper obtains an approximate expression of TGD estimating error about phase center separation,along with its effect on user ranging accuracy.Afterwards in the condition of single‐station monitoring and double‐station monitoring,the maximum TGD error is respectively deduced based on the worst geometry between the satellite and the station(s).Simulations result in numerical values and variation trends of the maximum TGD error with phase center separation.The work could be used in theoretical analysis and estimation of navigation accuracy,and also in design and manufacture of navigation satellites and monitor stations.