Title: Ka-band Doppler Scatterometer for Measurements of Ocean Surface Vector Winds and Currents
Presenting Author: Ernesto Rodriguez
Organization: Jet Propulsion Laboratory

Abstract:
The Ka-band Doppler scatterometer (DopplerScatt) will be designed and built for the first simultaneous measurements of ocean vector currents and winds from an airborne platform, using a single spinning antenna. The instrument and the novel technique of estimating currents and winds from the same radar scanning platform will form a basis for next-generation spaceborne Earth Science Missions aimed at better understanding climate and weather and ocean circulation. Ocean surface currents impact heat transport, surface momentum and gas fluxes, ocean productivity and marine biological communities. Ocean currents also have social impacts on shipping and disaster management. There is an intrinsic two-way coupling between ocean currents and surface winds and concurrent measurements of the two enable the understanding of the relevant air-sea interaction. The ability to simultaneously measure winds and currents improves the accuracy of both individual measurements. To date, measurements of ocean surface currents and winds have not been made simultaneously. Currently there are no planned global direct measurements of ocean surface currents and winds. The novel approach combines the well-established spinning pencil-beam scatterometry concept (e.g., QuikSCAT) with integrated Doppler capabilities that will allow simultaneous measurement of ocean vector winds and currents. Based on the strength of the echo reflected from the surface, the normalized radar cross-section (NRCS) of the beam footprint can be measured. The NRCS varies as a function of wind speed and angle between the host platform track and wind direction. Based on the phase difference between pairs of transmitted pulses Doppler shift can be measured. The Doppler shift varies as a function of platform and surface motion. The continuous scan of the pencil beam along the satellite track produces a scan pattern such that the scanning beam views each point within the swath twice, once in the forward and once in the aft direction. These two viewing angles correspond to two different azimuth angles with respect to the host platform. The estimation of the wind speed and direction from measured radar backscatter acquired at several azimuth look angles is performed by searching the possible response curves that fit the measured data. To infer surface current from the radar measured Doppler shift a removal of the Doppler shift contributed by the platform motion is performed as well as the correction for the surface current contribution from the wind. Traditionally wind vectors have been derived from precisely calibrated Ku- and C-band scatterometers, and while well understood for wind retrievals their main drawback for current estimation is the accuracy of the measurements of radial velocity (proportional to the wavelength), worse by at least a factor of 3 with respect to Ka-band measurements. Ka-band also lends itself to more compact instrument architecture, enabled by its smaller wavelength. The advantage that the DopplerScatt will have compared to the traditional surface currents retrievals (ATI), is a large swath coverage produced by a scanning beam (~ 1000 km vs. ~100 km) enabling faster revisit times. While having a better accuracy of current retrievals DopplerScatt will have similar wind speed dependence compared to Ku-band measurements.