The aim of our work is to generate Earth's gravity field models from GPS positions of low
Earth orbiters. Our inversion method is based on Newton's second law, which relates the
observed acceleration of the satellite with forces acting on it. The observed acceleration is
obtained as numerical second derivative of kinematic positions. Observation equations are
formulated using the gradient of the spherical harmonic expansion of the geopotential. Other
forces are either modelled (lunisolar perturbations, tides) or provided by onboard …