POLARIMETRIC RESPONSE OF THE OCEAN SURFACE OVER THE FULL RANGE OF INCIDENCE ANGLE FROM 18-35 GHZ: RESULTS FROM THE COWVR PITCH EXPERIMENT

S. Brown¹, F. Polverari¹, M. Morris¹, A. Akins¹

¹Jet Propulsion Laboratory

The Compact Ocean Wind Vector Radiometer (COWVR) is an 18-35 GHz fully polarimetric, conically imaging microwave radiometer with a 75cm aperture designed to provide measurements of ocean vector winds COWVR has been operating on the International Space Station (ISS) since January 2022 as a part of the Space Force Space Test Program (STP-H8) mission for demonstrating this new radiometer design. The sensor performed extremely well over its 3.7-year mission, meeting all objectives for sensor calibration and wind vector retrieval performance.

In August 2025, as the mission was in its final month, two special maneuvers were conducted to sample the ocean surface over the full range of incidence angle. The first maneuver pitched COWVR by 47o pointing the antenna boresight to nadir in the back portion of the scan and covering 0-60o incidence angle over the remainder of the scan. The second maneuver pitched COWVR to 101o to view incidence angles between 60-90o. In all, 5 weeks of data were acquired in these two configurations, yielding the most extensive database ever acquired of fully polarimetric data (V,H,3rd,4th stokes) at 18-35 GHz over 0-90o incidence angle.

Previously, only very limited data from airborne campaigns was able to test ocean emissivity models.

The COWVR top-of-atmosphere brightness temperature (TBs) are converted to surface emissivity using a one-layer radiative transfer algorithm and ancillary sea surface temperature. The surface emissivity is combined with ancillary wind vector data to derive harmonic emissivity model functions of the four stokes vectors. We find the harmonic coefficients over incidence angle generally agree with predictions from two-scale emissivity models, but important differences are observed, particularly in the frequency dependence and shape of the harmonic coefficients over incidence angle.

We will summarize the COWVR pitch experiment and highlight the new insights gained of the polarimetric response of the ocean surface over incidence angle. We will compare COWVR derived emissivity as a function of wind vector to two-scale models. We will show how adjusting free parameters in the two-scale models (such as wave spectrum) can produce better fits to the data. These results will be important for physical model development and improved fast empirical models that are used in modern data assimilation systems (e.g. FASTEM).