Ocean Currents 2

The Wavemill is a new radar instrument concept that offers the possibility of generating two-dimensional wide swath, high resolution, high precision maps of surface current vectors. Based on a single spacecraft, it avoids the difficulties of synchronisation and baseline estimation associated with other interferometric SAR systems based on two or more satellites.

The WaPA project, completed in 2015, carried out an investigation into the validity and utility of products from a Wavemill ocean surface current mission, including a consideration of the impact of different potential instrument configurations. This presentation summarises the findings:

-        It was demonstrated that in the absence of a current it would be possible to retrieve the unambiguous wind vector without need for ancillary information, but that when there is a surface current, dual polarisation or a third antenna would be needed to separate wind and current contributions.

-        The Wavemill End-To-End Simulator was shown to accurately represent different test situations and allowed the calculation of a preliminary geophysical error budget.

-        Analysis of the airborne Proof of Concept Campaign data confirmed that the data characteristics could be explained by relatively simple geophysical models, so indicating a strategy for inverting the measurements into the required geophysical products.

-        An investigation of instrument and satellite performance considered different potential technical configurations for the antenna and processing approach, including the possibility of dual polarisation and wide swath, and the requirements this placed on available power and data downlink capability. An approach which involved switching between different imaging modes in different locations was suggested, ranging from low spatial resolution with global coverage to high spatial resolution for only selected regions.

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In addition to the primary mission to measure surface currents, it was also shown that Wavemill measurements could be used to generate useful, potentially unique, secondary products in the marginal ice zone (ice motion and snow water equivalent) and in the realm of ocean atmosphere interactions.

There has been much progress in recent years in the development of new measurements of ocean surface currents (OSC) from space, exemplified by pioneering spaceborne demonstrations of the Doppler Centroid Anomaly and along-track interferometry methods using Envisat and TanDEM-X/TerraSAR-X data. These address, to a greater or lesser extent, a critical observational gap in spaceborne measurements of total ocean surface currents that impacts important science questions and operational needs. 

The ESA Earth Explorer 9 (EE9) call for mission proposals could provide an opportunity to put forward an Ocean Surface Current mission as a future Earth Explorer. As for all Earth Explorers, EE9 will call for missions that demonstrate scientific excellence underpinned by technological innovation and, in line with the 2015 ESA Science Strategy for Earth Observation, deliver societal benefits.

This paper considers the opportunities and challenges associated with an Ocean Surface Current mission as a possible EE9 mission. We review present-day sources of surface current data, consider to what extent they address scientific and operational needs and review possible satellite ocean surface current measurement concepts in terms of innovation, and scientific and technical readiness, in the context of the constraints of the EE9 programme.

Over the last three decades, the application of SAR systems to physical oceanography has
become a very successful scientific and technological achievement. It is also true,
however, that the construction of effective models describing real phenomena has always
been a challenging task that still requires major advances in both theoretical and empirical
understanding of the ocean dynamics. Of particular interest is to know how the
improvement of parameters estimation —such as wind and surface currents— can
contribute to a better understanding of ocean wave behaviour in the presence of breaking
waves. Having this general goal in mind, we are specifically concentrated on defining and
testing novel statistical-computing methods that have the capacity for removing an
undesirable visual effect in SAR images known as defocusing, which is an inherent
problem in every SAR image of the moving ocean surface. Generally speaking, our
research work is based on the hypothesis that four major elements can be joined together
to estimate motion parameters that cause defocusing: 1) sea-wave models, 2) SAR-ATI
theory, 3) emerging composite-surface models describing ocean-radar interactions, and 4)
computational methods that are both effective and efficient. Methodologically speaking, a
first task comprises a detailed analysis of the degree of defocusing; a second task involves
knowing the state of re-focusing when the orbital velocities of illuminated points of the
ocean surface are known in advance; a third task consists of estimating motion
parameters through autofocusing techniques and ATI information; finally, the last step is
intended to reconstruct the SAR image via the motion parameters that have been
estimated previously. From a computer science perspective, this strategy employs
numerical optimisation algorithms capable of producing an autofocused image, where
Bayesian estimation theory and machine learning techniques play an important role in the
way the defocusing problem is approached and solved. Some experiments using real data
—collected from satellites TanDEM-X and TerraSAR-X owned by the DLR— are performed.
The obtained results demonstrate that generated values such as intensity, interferometric
coherence, interferometric phase, and Doppler velocity vector field, produce SAR images
that are in geophysical accordance with real wind-driven waves.

The GSAR synthetic aperture radar (SAR) processor developed by Norut has been used in retrieving the geophysical range Doppler shift from Sentinel-1 and Envisat ASAR scansar acquisitions. Improvements in spatial resolution and uncertainty of the geophysical Doppler shift and the inverted surface velocity are addressed through examples from the Agulhas Current and the Norwegian Atlantic Current. The surface velocity retrievals are then assessed and compared to other direct and indirect estimates of the upper ocean current, in particular those obtained in the ESA GlobCurrent project.

Nowadays, sea surface remote sensing techniques are rapidly developing. Altimetry, scatterometry and passive radiometry are all well proven techniques, which result in recognised operational and numerous scientific applications. However, considering Synthetic Aperture Radar (sar) missions, and the potential to retrieve high resolution surface wave fields, important difficulties remain to preclude major use and scientific advances.

The question we will address is : can we go beyond present day retrieval methodologies to propose new algorithm strategy to interpret SAR high resolution information ?

With the improved quality of the Sentinel-1 measurements, as well as the increasing wealth of available ancillary information, we intend to present a suite of novel-methodologies to help tackling the long-standing issue to retrieve ocean wave information from SAR data.