Joint Wind- and Total Surface Current Vector Retrieval from TanDEM-X Bidirectional Along-Track Interferometric Data

Abstract

In this thesis report the method and quality of Total Surface Current Vector (TSCV) retrieval using an experimental Bidirectional (BiDi) Along-Track Interferometric (ATI) acquisition mode with TanDEM-X are presented. The available data consists of a StripMap acquisition at the coast of Tromsø and a data set acquired over Nova Zemlaya. The measurement concept relies on the ATI phase, which provides an estimate of the first moment of the Doppler spectrum associated to the surface motion. Observing with two beams squinted as far as 15.1 degree apart in azimuth on ground, allows the surface motion to be observed in line of sight of the beams. Projection to the ocean surface gives a velocity field. This surface motion consists of a Normalized Radar Cross Section (NRCS) weighted average of velocities of sea-state dependent biases such as short wind generated waves, long swell waves and underlying currents. Assuming the surface motion is dominated by wind generated waves and underlying currents, the method attempts to solve for Total Surface Current Velocity simultaneously with the surface wind vector by coupling geophysical model functions (GMF) for returned Doppler Centroid (DC) and NRCS from an ocean surface shaped by wind. For NRCS the empirical GMF XMOD2 for X-band radar is used, based on the same regression algorithm as the widely used CMOD5 in scatterometry. For DC a GMF based on statistics of the sea surface and the Kirchhoff Approximation developed by IFREMER is used. A cost function of the wind vector is defined as the squared difference between NRCS observations and values of the GMF in both beams. Minimization of this cost function solves for the wind vector and evaluating the GMF for DC with the estimated wind vector results in a component of surface motion caused by wind generated waves. Wind wave induced surface motion and TSCV can then be separated. Retrieval from simulated data shows that the wind retrieval algorithm gives an ambiguous result for the wind direction, even if noise is as much as the noise equivalent sigma naught for bidirectional TanDEM-X. To constrain the solution ECMWF ERA-5 reanalysis wind data is added to the cost function as an additional term with a low weight factor. Wind retrieval on real data gives a smooth result for the wind vector for both data sets. The average of the retrieved wind vector over the image is highly similar to the lower resolution ECMWF ERA-5 wind vector data. Results of the Total Surface Current Velocity field for Tromsø has a very dominant along-track component that has arisen from the Doppler field. Error analyses demonstrate that it is highly likely the strong along-track componentis a result of erroneous phase calibration. Nova Zemlaya shows a promising TSCV that streams along the coast, therefore indicating a current direction that could be correct. However, with direct interpretation of a current one needs to be cautious, due to model uncertainty and still existing calibration errors.