Satellite-derived, coincident ocean surface winds and currents are of great importance to enhance our understanding of air-sea interactions, ocean biophysical processes, and multiscale processes. In recent years, several satellite mission concepts have been proposed to measure both parameters. In comparison to other remote sensing systems, the rotating pencil-beam Doppler scatterometer offers the benefit of a broader swath coverage. However, there exists limited azimuth angle diversity within some regions of the swath, posing challenges for inversion procedures. Accurately inverting ocean winds is also crucial for ocean current retrieval, as it allows for the removal of the wind-wave induced artifact surface velocity in velocity measurements. Calibration of the Normalized Radar Cross-Sections (NRCS) or backscatter measurements, which are sensitive to wind speed and direction, is vital for deriving the accurate wind field. This study presents data calibration and wind/current retrievals with measurements from a flight campaign, carrying a Ka-band rotating pencil-beam Doppler scatterometer, i.e. the prototype of the future Ocean Surface Current multiscale Observation Mission (OSCOM). The calibration of the backscatter measurements employs two distinct approaches to compensate for the azimuthal modulation of the backscatter signal, which is larger than anticipated by established Geophysical Model Functions (GMFs) used in Ka-band scatterometry. Both methods are based on what is commonly referred to target or numerical ocean calibration (NOC). The first approach involves an azimuth-dependent calibration, whereas the second adjusts the GMF to align with the observed modulation after calibration. Wind speeds retrieved range from 3 to 6 m/s, with wind directions approximating 155°. When comparing OSCOM and ECMWF winds, the standard deviation (SD) for wind speed and direction differences, is below 1.4 m/s and 26°, respectively. Compared to the modified GMF calibration, winds derived using azimuth-dependent calibration exhibit a lower bias and SD in wind speed and a higher SD in wind direction. In addition, the analysis of the azimuth diversity and the Maximum Likelihood Estimation (MLE) cost function shape suggests that the primary sources of measurement noise stem from sampling issues, which are specifically attributed to the OSCOM viewing geometry and the relatively high measurement noise. To determine whether the observed backscatter modulation stems from instrumental or geophysical sources, a more extensive dataset, accompanied by coincident in situ wind data matching OSCOM spatial resolution (such as buoy measurements), is required. The outcomes of the calibration offer quantitative support for further investigations into retrieving ocean wind and surface currents using both NRCS and phase information from Doppler scatterometry. Moreover, the calibration techniques introduced in this research could be utilized in additional airborne Doppler scatterometer experiments to evaluate the performance of various instrument configurations.

EUMETSAT Meteorological Satellite Conference, 30 September - 4 October 2024, Würzburg, Germany

Peer reviewed