high-Resolution current measurements by along-track interferometry

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and TerraSAR-X [e.g., Romeiser et al., 2013]).Altogether, there are plenty of scientific and commercial applications that would justify the development of a dedicated spaceborne ATI system for ocean applications.Such a system should have a near-optimal along-track baseline, which is easier to achieve at higher radar frequencies because the optimal baseline range scales with the radar wavelength (Romeiser and Thompson, 2000;Romeiser and Runge, 2007).
Furthermore, it should be a dual-beam   From Romeiser et al. (2010a) of the tandem-X formation of two satellites over europe.Source: DLR (CC-BY 3.0) B y R O l a N d R O m e i S e R Oceanography | Vol. 26, No. 2 Ocean waves become visible in SAR images because of their periodic modulation of the local incidence angle and surface roughness as well as a SARspecific imaging artifact called velocity bunching.This is relatively well understood, and robust techniques for deriving wave information from SAR images have been developed.Similarly, convergent and divergent regions in the surface current field modulate short-scale surface roughness in such a way that pronounced bright and dark signatures, respectively, can become visible in a SAR image.As an example, Figure 1 shows signatures of underwater sand dunes off the coast of China in an ERS-1 SAR image.Other current features whose signatures are frequently observed in SAR images are oceanic internal waves and fronts (see Jackson et al., 2013, in this issue).While many of these signatures are very clear and as nice-looking as those in Figure 1, their quantitative interpretation in terms of current gradients can be very difficult, and absolute current velocity retrievals are not possible at all.Calkoen et al. (2001) proposed a procedure for estimating bathymetric maps by a combined analysis of SAR images and echosoundings.Chapron et al. (2005) pioneered current retrievals from SAR data by Doppler centroid analysis.With this approach, differences between observed mean Doppler frequencies (Doppler centroids) and nominal values corresponding to the relative speed between satellite and rotating Earth in radar look direction are interpreted to be the effect of surface currents and mean contributions of wave motions.Within the last few years, others have demonstrated the soundness of this approach for a variety of test sites and SAR data from different satellites thousands of full-resolution pixels.More than 25 years ago,Goldstein and Zebker (1987) presented the concept of SAR along-track interferometry (ATI), which permits velocity measurements at full SAR resolution.To achieve this, an ATI system uses two antennas that are separated by some distance in flight direction and that receive the same backscattered signals from the ocean or land surface with a small time lag on the order of milliseconds.After processing the data from each antenna into a full-resolution SAR image and keeping track of amplitude and phase of the signals mapped into each pixel, combined interferometric processing of the two images reveals phase differences whose expectation values can be shown to be proportional to Doppler frequencies and thus to line-of-sight scatterer velocities.ati achieVemeNtS aNd State OF the aRt Goldstein and Zebker (1987) explained the basic concept of ATI and presented example results from a first experiment with an airborne system.A more systematic experiment was carried out over ship-generated internal waves at Loch Linnhe (Scotland) in 1989.A comparison between ATI-derived velocities (called "Doppler velocities" if no further corrections have been applied) and surface currents measured by in situ instrumentation revealed significant differences.Thompson and Jensen (1993) explained this discrepancy as the effect of mean contributions of subresolution-scale wave motions to the ATI signatures.Romeiser and Thompson (2000) proposed a simplified numerical model for the computation of the waveinduced contributions to Doppler velocities; Romeiser (2005) demonstrated an automatized iterative correction of data from an airborne ATI experiment for the spatially varying wave contributions that can occur as a result of hydrodynamic wave-current interaction.The numerical Doppler models use parameterizations of the wave spectrum that depend on the wind vector relative to the radar look direction, so the wind speed and direction in the test area must be known in order to do the computations.It is possible to estimate the wind field from the radar image itself.Furthermore, it has to be taken into account that the standard wave parameterizations may be inappropriate where the local wave field is not at equilibrium with the wind, which may be the case, for example, in coastal areas with strong swell waves propagating towards the coast or in rivers.The resulting current field is always supposed to be the surface current field, and it may include contributions of wind drift and Stokes drift.In February 2000, Space Shuttle Endeavour became the platform of the first spaceborne SAR system with ATI capability.For the joint USA-German-Italian Shuttle Radar Topography Mission (SRTM), the Endeavour was equipped with antennas in the cargo bay

Figure 1 .
Figure 1.eRS-1 synthetic aperture radar (SaR) image of the Xinchuan gang Shoals along the east coast of china, north of Shanghai, acquired on July 8, 1995, 02:34 utc.area size = 100 km × 100 km.The signatures in the upper right half of the image result from modulation of tidal currents and surface roughness by underwater topography.Source: European Space Agency (ESA)

Figure 3
Figure 3 shows data acquired at this test site on February 26, 2012, with an effective along-track baseline of 25 m, and a reference current field from the numerical tide computation system POLPRED.The ATI-derived

Figure 3 .
Figure 3. tandem-X data acquired over pentland Firth (Scotland) on February 26, 2012, 06:41 utc.(a) Radar intensity image.(b) interferometric phase image.(c) derived line-of-sight current field.(d) Reference current field from a numerical tide computation system.area size = 30 km × 30 km.Radar look direction = from right to left.positive flow direction = from left to right.Flight direction = from top to bottom.The black frame outlines the area of the subimage presented in Figure 4.

Figure 4 .
Figure 4. a 10 km × 10 km subsection of the Figure 3 data set showing signatures of surface waves in (a) the intensity image, and (b) the full-resolution doppler velocity image derived from the interferogram.
ATI system with vector current measuring capability, and it should have an accessible incidence angle range, swath width (possibly left-and right-looking at the same time), and duty cycle that enable it to access and cover large areas of the ocean with short lead and repeat times.The European Space Agency has sponsored several studies on the design of such a system in recent years (e.g., Márquez et al., 2010).The chances that we will see a dedicated ATI system for ocean applications in space within the next 10-20 years seem to be good.ackNOwledgemeNtS Significant contributions to the work presented in this paper were made by Donald R. Thompson of The Johns Hopkins Universitythe Federal Waterways Engineering and Research Institute, Hamburg, Germany; and Paul Bell of the National Oceanography

Figure 5 .
Figure 5. Bathymetry retrieval example.(a) airborne ati-derived vector current field north of the german island of Sylt, from an experiment in may 2001.area size = 3.5 km × 3.5 km.grid resolution = 25 m × 25 m.(b) depth map from echosoundings with an effective resolution of 200 m.(c) Seventyeight selected reference depth points near the boundaries.(d) depth map derived from the 78 reference depths in combination with the ati-derived current field.FromRomeiser et al. (2010a) Since the Seasat mission in 1978, an impressive amount of information on oceanic and atmospheric features over the ocean has been derived from the thousands of images provided by spaceborne synthetic aperture radars (SARs).
miami.edu) is Associate Professor, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA.aBStR act.