Complex demodulation of the sea surface heights computed from the altimeter range measurements provides an original way to characterize and empirically remove the radial orbit error in the spectral domain. The complex demodulation method is described and applied to a simulated series of altimeter data and to Geosat altimeter data. The results are interpreted in terms of gravity field error contributions using Kaula's linear theory of satellite dynamics. The mean sea surface and the oceanic tides contributions to the observed modulations are also discussed. About 90% of the energy around 1 cycles/rev is attributable to the time-varying component of the radial orbit error, whereas the energy at 2 and 3 cycles/rev is mainly due to the sea surface topography signal. The estimated full radial orbit error in Geosat altimeter data using GEM-T1 orbits has a rms of 69 cm. ¿ American Geophysical Union 1993