Thanks to the high performance of the spaceborne GPS receiver and the availability of precise IGS orbit and clock
products, Zero-difference kinematic precise orbit determination (POD) has turned out to be a new method in orbit
determination for the LEO satellites. Zero-difference Kinematic POD, which is based on the GPS measurements only
from the spaceborne GPS receiver, is independent of force models and orbit design. From that point of view, kinematic
POD is well suited for the Earth Observation satellites at very low altitudes, such as CHAMP, GRACE and GOCE et al.
This paper reviews the basic zero-difference GPS model, and the corrections in the model are discussed. A block-wise
least squares algorithm, which firstly separates the parameters in to groups and then solves the parameter by elimination
and back-substitution, is discussed and proposed for the kinematic orbit determination. The orbit solutions for one week
of GRACE observations are calculated Comparisons with the published Rapid Science Orbit (RSO) indicate that the
accuracy in radial, along-track and cross-track direction can achieve 5.5cm, 5.5cm and 6.6cm respectively, and the RMS
in distance is better than 8.6cm.