## 1 Introduction  
The dynamics of the atmosphere and ocean are significantly influenced by ocean surface wind and currents. Ocean surface wind not only acts as a primary force driving the creation of surface waves but also plays a pivotal role in shaping the broader patterns of global ocean circulation. Concurrently, ocean currents serve as vital conditions, transporting heat, salinity, and nutrients, thereby exerting a profound impact on both climate and marine biology. Observations of sea surface current and winds are essential for understanding the air-sea interaction [1, 2]. Despite their high accuracy, in situ measurements are limited by fixed spatial locations and by the great expenses and risks associated with the use of the instruments [3]. Over the past decades, the great advancements in spaceborne remote sensing have made it an invaluable source of measurements for the observation of the geophysical variables across the global ocean. Among these, microwave active radars have gained particular attention given their capability in collecting data under all kinds of weather conditions during day and night. Wind field is the most basic atmospheric variable over the open ocean surface. The spaceborne scatterometer is to date the most widely used instrument for observing the sea surface wind. Such a radar measures the backscattering, often termed as normalized radar cross section (NRCS and \(\sigma^{0}\) used interchangeably throughout the text), that is not only related to radar configurations (radar wavelength, polarization, incidence angle), but also to the sea surface state (wind, waves, temperature and current et al.) [4]. For an inversion scheme of wind vector from the radar measurements, the empirical geophysical model function (GMF) associating NRCS and the radar parameter as well as the surface states is required. Note that each GMF is dedicated to a particular polarization and particular radar wavelengths [5]. The typical configuration of a scatterometer with multiple fixed beams or a rotating beam allows the sensor to independently resolve a unique wind vector based on a Bayesian scheme. To date, scatterometer-retrieved wind fields have been reported with an accuracy of 1.5 m/s and 20\({}^{\circ}\) for the wind speed and direction, respectively. These wind products are now routinely distributed to help monitor the spatial pattern of ocean winds, wave fields, weather forecasting, and ocean and atmospheric research, to name a few. Yet, the relatively coarse resolution of the scatterometer wind data at around 12.5 to 25 km has significantly limited its application in fine-resolution scenarios, like the rapidly changing winds across an atmospheric front or over coastal regions [6]. The other variable, ocean surface current, is as yet unresolved from the point view of remote sensing [7]. Radar altimeters that measure the sea surface height are able to obtain current fields in combination with geostrophic equilibrium relationships. This kind of technique is better than the use of in situ instruments in terms of the extended coverage, high accuracy, and shorter revisit period. It should be noted that the altimeter products are available at a spatial scale larger than 100 km; they are incapable of resolving fine-scale geophysical processes. In addition to that, the challenges of radar altimeters are the long revisit temporal period and extended gap distance between adjacent tracks. However, such situations have been alleviated by merging multiple altimeter satellites to achieve a relatively shorter revisit time and greater coverage [8]. Another popular remote-sensing instrument to observe the sea surface wind and current is synthetic aperture radar (SAR), which has a higher spatial resolution. In addition to the traditional NRCS magnitude, it adds another dimension of Doppler information [9]. As with to the scatterometer, sea surface wind data can also be obtained from SAR observations. However, SAR has only one incident radar beam, making it difficult to attain near-surface wind vectors without external inputs.