  An innovative orbit determination method which makes use of gravity gradients
for Low-Earth-Orbiting satellites is proposed. The measurement principle of
gravity gradiometry is briefly reviewed and the sources of measurement error
are analyzed. An adaptive hybrid least squares batch filter based on
linearization of the orbital equation and unscented transformation of the
measurement equation is developed to estimate the orbital states and the
measurement biases. The algorithm is tested with the actual flight data from
the European Space Agency Gravity field and steady-state Ocean Circulation
Explorer. The orbit determination results are compared with the GPS-derived
orbits. The radial and cross-track position errors are on the order of tens of
meters, whereas the along-track position error is over one order of magnitude
larger. The gravity gradient based orbit determination method is promising for
potential use in GPS-denied spacecraft navigation.
