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Measurements of the relative strength of ocean surface wave spectral components have been made from an aircraft using the two-frequency microwave resonance technique. A coherent Ku band radar was used to study the Bragg type resonance matching (at the difference frequency Δf) to the surface wave components. The spatial spectrum of the surface reflectivity modulation was then computed as the value of Δf was varied over a matching range of approximately 15 m to 150 m in surface wavelength. This paper contains experimental results from flights conducted during the 1979 MARSEN project and the 1980 ARSLOE project, plus a theoretical formuation of the relationship between the radar measurement and the directional surface elevation spectrum. The objective here is to evaluate the two-frequency resonance technique in terms of the strength of the radar response and the effects of surface illumination geometry, and to demonstrate the inversion of the radar data to absolute directional surface elevation spectrum. An independent measurement from the aircraft of the modulation transfer function required in this inversion and the resulting estimate of the surface spectrum are presented. Finally, a comparison is drawn between the Langley two-frequency results and an estimate of the spectrum produced by the Wallops Flight Center surface contour radar.