Bias in pole positions computed from Navy navigation satellites

The accuracy of the computed orbit of a satellite with respect to an earth observer depends on the accuracy of knowledge of the motion of the earth's spin axis with respect to the crust (polar motion). Solutions for the orientation of the spin axis from Doppler observations of Navy navigation satellites 1967-92A and 1970-67A differed by 1 to 2 meters. Despite the fact that the orbital inclinations and heights of these satellites are similar, studies have shown that this bias in pole position can be due to imprecision in knowledge of the non-resonant terms in the gravity model used in the determination of the orbits.