This paper constructs combined orbit determination (COD) models from the angle of range-sum observation data by analyzing Low Earth Orbiter (LEO) combined orbit determination ranging mode based on Bi-satellite Positioning System (BPS).and designs a ranging system error parameters estimation method and a numerical fusion method based COD algorithm.Simulation computation results show that COD strategy can restrain influence of geostationary orbit satellites ephemeris errors

and LEO orbit determination precision can achieve 11.26 meters which is estimated by two days range sum simulation data

which proves the validity of the system error estimation method and numerical fusion algorithm.