Synthetic aperture radar (SAR) can measure directional wave spectrum based on the closed nonlinear SAR-ocean
mapping mechanism. The according wave spectrum retrieval algorithm has been developing for decades, but some
limitations remain, like high wave number cut off in azimuthal direction and the need for the first guess spectrum. Wave
spectrometer is a kind of new satellite-based real aperture radar (RAR) operating at low incidence, which has a narrow
beam and scans complete 360° by antenna rotation. It derives wave spectrum by the simple linear relation between the
wave spectrum and the modulation spectrum from the wave spectrometer. The linear coefficient can be estimated by the
nadir beam or external wind speed information. This paper proposes a method on the wave spectrum estimation based on
the joint measurement from synchronous SAR and wave spectrometer. Firstly, the modulation spectrum is derived from
the signal spectrum of the wave spectrometer, from which the relative wave spectrum can be constructed. Then the
relative wave spectrum is seen as the first guess spectrum for the wave spectrum retrieval of SAR image. Because the
relative wave spectrum has the same pattern with the real wave spectrum but has different absolute energy, we can
retrieve the directional wave spectrum by the iteration way based on the relative wave spectrum from the SAR image.
This paper makes use of simulation technology to validate the joint measurement. The simulation compares the input
spectrum and the retrieved one in terms of peak direction, peak wavelength and significant wave height, which has a
deviation of 6°, 4m and 0.3m, respectively. Simulation results show the joint measurement has the feasibility for the retrieval of directional ocean wave spectrum.