Doppler Centroid Estimation for Ocean Surface Current Retrieval from Sentinel-1 SAR Data

Abstract

Synthetic aperture radar (SAR) is the most efficient tool to provide high-resolution data for Earth Observation (EO). Doppler centroid (DC) estimation is indispensable for high precision SAR data analysis such as extracting the ocean surface current, which is important for scientific pursuits. Correlation doppler estimation (CDE), and energy balancing (EB) based DC methods are implemented in this paper. A 2-D sliding window is deployed to estimate DC on small blocks of data while covering the whole scene so that all parts of the scene are potentially represented. We analyzed Sentinel-1 single look complex (SLC) data from the coastline of a non-homogeneous scene. The CDE method utilizes the azimuth shift in the time domain which is associated with the DC, and this fDc history is used to extract ocean surface current. We find the results of DC estimates are confined to primitive baseband (±PRF⁄2). Moreover, the corresponding retrieved ocean surface current component is reasonable, particularly values vary within the limit of error bounds. Finally, the parameters of ocean surface current are compared with ocean wave models reported in the literature. Efficacy and simulation of implemented methods are good fit for Sentinel-1 SAR data.

Coastline Extraction From SAR Data Using Doppler Centroid Images

“…To study this footprint, we estimate Doppler velocity (V D ) [18], which varies up to 2.25 m/s and significant wave height (SWH) [19], that goes up to 1.45 m. The distributions of Doppler parameters of three data sets are presented in red, green, and blue plots, where data-3 (in blue) has the majority of its distribution around zero, which states the sea state condition is very calm. This study helps monitoring of coastline and delineates natural hazards to protect the shoreline.…”

“…Subsequently, as given in Fig. 2, to estimate f DC we make use of correlation Doppler estimation (CDE) method which involves an autocorrelation of the SLC image with its shifted version in the azimuth direction '∆η' [18]. By using the sliding window 'w' of size 9 × 9 we locally compute the correlation in the azimuth direction and perform an averaging operation in the range direction, (⟨•⟩, known as ensemble average).…”

Synthetic Aperture Radar (SAR) for Ocean: A Review

“…The Doppler shift of the backscattered SAR signal can be measured through the difference between the Doppler centroid of the SAR data and the nominal Doppler centroid calculated using the trajectory parameters of the satellite orbit [83]. Furthermore, the wave component should be extracted from the Doppler shift to improve accuracy in ocean surface current estimation [84], [85]. It is worth noting that there is a two-way relationship between ocean surface wind and current [10] and, thus, considering them together in studies of ocean surface parameters would result in a better accuracy [10].…”