Assessment of the sea surface wind and current retrieval from ALOS-2 and Sentinel-1 SAR data over coastal areas
Authors: Elyouncha Anis , Eriksson Leif E. B. 
Published: March 2021

&lt;p&gt;&lt;span&gt;Synthetic aperture radar (SAR) has become an essential component in ocean remote sensing due it&amp;#8217;s &lt;/span&gt;&lt;span&gt;high&lt;/span&gt; &lt;span&gt;sensitivity&lt;/span&gt;&lt;span&gt; to sea surface dynamics and its high spatial resolution. &lt;/span&gt;&lt;span&gt;The ALOS-&lt;/span&gt;&lt;span&gt;2 SAR&lt;/span&gt;&lt;span&gt; data are &lt;/span&gt; underutilized &lt;span&gt;for&lt;/span&gt;&lt;span&gt; ocean surface wind and current retrieval. Althou&lt;/span&gt;&lt;span&gt;g&lt;/span&gt;&lt;span&gt;h the primary goals of the &lt;/span&gt;&lt;span&gt;ALOS-2&lt;/span&gt;&lt;span&gt; mission are focused on land applications, the extension of the satellite scenes over the coast&lt;/span&gt;&lt;span&gt;al areas&lt;/span&gt;&lt;span&gt; offers an opportunity for ocean applications. Th&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; underutilization &lt;/span&gt;&lt;span&gt;of ALOS-2 data &lt;/span&gt;&lt;span&gt;is mainly due to the fact that at low radar frequencies, e.g. L-band, the sensitivity of the radar scattering coefficient to wind speed and the sensitivity of the Doppler frequency shift to sea surface velocity is lower than at higher frequencies, e.g. C- &lt;/span&gt;&lt;span&gt;and&lt;/span&gt;&lt;span&gt; X-&lt;/span&gt;&lt;span&gt;band&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;This is also due to the fact that most of ALOS-2 images are acquired in HH or HV polarization while the VV polarization is often preferred in ocean applications due the higher signal to noise ratio. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;The wind speed is retrieved from Sentinel-1 and ALOS-2 using the existing empirical C- and L-band geophysical model functions. For Sentinel-1, the Doppler frequency shift provided in the OCN product is used. For ALOS-2, the Doppler frequency shift is estimated from the single look complex data using the pulse-pair processing method. The estimated Doppler shift converted to the surface radial velocity and the velocity is calibrated using land as a reference. The estimated L-band Doppler shift and surface velocity is compared to the C-band Doppler shift provided in the Sentinel-1 OCN product. Due the difference in the local time of ascending node (about 6 hours at the equator) of the two satellites, a direct pixel-by-pixel comparison is not possible, i.e. the wind and surface current can not be assumed to be constant during such a large time difference. Thus, the retrieved wind from each sensor is compared separately to model data and in-situ observations.&lt;/p&gt;&lt;p&gt;In this paper, the quality of the wind speed retrieved from the L-band SAR (ALOS-2) in coastal areas is assessed and compared to the C-band SAR (Sentinel-1). In addition, the feasibility of the surface current retrieval from the L-band Doppler frequency shift is investigated and also compared to Sentinel-1. Examples will be shown and discussed. This opens an opportunity for synergy between L-band and C-band SAR missions to increase the spatial and temporal coverage, which is one of the main limitations of SAR application in ocean remote sensing.&lt;/p&gt;