Wavemill: a new mission for high-resolution mapping of total ocean surface current vectors

Synoptic maps of total ocean surface currents from space are needed to improve parameterisations of oceanic submesoscale dynamics and represent their impact on global ocean circulation, air-sea exchanges and the marine ecosystem. Wavemill is a hybrid interferometric SAR instrument that seeks to deliver high-resolution high-accuracy maps of ocean surface current vectors. It measures the total ocean surface current, including ageostrophic components, in one single-pass. Other secondary products include ocean wind vectors, swell, and some estimate of ocean surface topography. Proposed as an ESA Earth Explorer, the mission’s prime objective is to deliver ocean surface current vector maps over two 100 km swaths with a resolution of 1 km or finer. 1 Science Drivers for HighResolution Total ocean Surface Current Vectors High-resolution satellite images of sea surface temperature and ocean colour reveal a multitude of highly dynamic small oceanic features that dominate the ocean variability at the mesoscale (10-100km) and submesoscale (1-10km). Features such as eddies, fronts and filaments are ubiquitous and have been associated with energetic upper ocean dynamics and mixing processes. There is growing scientific evidence that these small oceanic scales play a major role for horizontal and vertical mixing, large-scale oceanic transport and ocean biology. For example, 50% of the vertical transport of ocean biogeochemical properties is thought to take place at scales smaller than 100km [1], while ageostrophic secondary circulation associated with eddies produce very large upwelling velocities of the order of 10 m/day [2]. Improved understanding of these processes is needed to develop improved parameterizations to represent these sub-mesoscale processes in models used for long-term climate predictions. Thus, with the response of the ocean biosphere to climate change remaining one of the greatest uncertainties in climate projections, there is a strong demand for synoptic observations of the ocean surface total current fields at these small scales. Today, none of the available satellite techniques are able to provide direct measurements of these quantities with sufficient accuracy and resolution. Sequences of satellite sea surface temperature or ocean colour images can provide estimates of advection by tracking features from image to image using the Maximum Cross-Correlation method [3] but the method is affected by cloud and relies on the presence of strong trackable features in successive images. Satellite altimeters give all-weather dayand-night estimates of currents globally, based on differences in sea surface height from which the geostrophic component of the currents can be estimated. But the altimeters narrow swaths, large track-to-track separation and reliance on coarse geoid data mean that altimeters cannot resolve features below 70-100 km scales. This well-known limitation of altimetry is the prime motivation for the NASA/CNES Surface Water and Ocean Topography (SWOT) mission, which will rely on cross-track interferometry to deliver twodimensional maps of sea surface height at 1km resolution [4]. Direct estimates of total surface currents can be obtained with satellite Synthetic Aperture Radar (SAR) by measuring the shift of the Doppler spectrum centroid in a method pioneered by [5]. More recently, significant advances in spaceborne measurements of currents were made possible thanks to along-track interferometric SAR experiments onboard TerraSAR-X [6]. However, even though these techniques do deliver total currents with useful spatial resolution and accuracy, they only provide one component of the current in the direction of the instrument line-of-sight. 2 Wavemill Instrument & Mission Concept 2.1 Instrument Concept The Wavemill instrument concept was originally conceived by [7] to provide both alongand across-track interferometric baselines, thus enabling simultaneous Along Track (ATI) and Across Track (XTI) interferometry. Since then, the concept has undergone several iterations to result in the present configuration, which is optimised for ATI and offers certain advantages of implementation. In this configuration, the phase centres are aligned in the along-track direction, and there is no physical across-track baseline. XTI is achieved with squinted beams. This ‘Javelin’ in-line concept and the swath positions are shown in Figure 1. Figure 1: Artist impression of the Wavemill concept for the “Javelin” configuration based on squinted SAR showing the swaths and acquisition sequence The measurement acquisition consists of four regions illuminated in sequence, two forward left and right and two backward left and right, for each antenna, making a total of eight beams. Simultaneous returns from two antennas are combined to form interferograms for which the two phase centres are separated in the look direction (XTI), for which height differences will result in interferometric phase shifts which provide information on ocean surface topography. Since both antennas see the surface at the same time, no surface motion is registered in the phase. Combining returns from two looks taken at slightly different times forms interferograms, which register surface motion-induced phase shifts (ATI) that provide measurements of ocean surface displacement in the line of sight direction. By repeating this process in the fore and aft directions, it is possible to retrieve two components of surface displacement, from which current direction is derived. The present configuration provides baselines between phase centres, which are pure along track (ATI), pure across track (XTI) and also hybrid baselines (HTI), which contain both co-time (XTI) and co-located (ATI) information. The present configuration leads to two swaths, one left and one right of the ground track, each 100 km wide, separated by about 100km. Theoretical modelling of microwave backscatter indicate that higher incidence angles would ensure better current retrieval performance, thus trade-offs are necessary between incidence angle, swath width and PRF. The present antenna design is based on planar leaky waveguide slotted arrays, approximately 4.5 meters in length. The orientation of the antenna face towards nadir ensures that all antennas experience the same solar flux, thus minimising thermoelastic distortion.