Abstract: In order to minimize the computational burden of the onboard precise real-time orbit deter-mination system,a Gravity Acceleration Approximation Function(GAAF)is introduced to computethe gravitational acceleration replacing the traditional spherical harmonic representation of the gravityfield.The appropriate order of the polynomial to fit a set of pseudo-centers on various heights at afixed location is discussed in detail,as well as the optimal geographic grid size.Then,GAAF is em-bedded in the SATODS software for real-time orbit determination using space-borne GPS measure-ments.A simulation test of real-time orbit determination was carried out using space-borne GPS dualfrequency pseudo-range data from the CHAMP satellite.Results show that the GAAF can improveorbital accuracy with quadratic polynomial fitting and a global grid where the latitude increment is nomore than 0.75degrees and the longitude increment is less than 1.5degrees,as compared with thetraditional recursive method using a 70× 70gravity field model. The proposed method reduces the computational load of orbit determination significantly.