Roland Romeiser

An improved composite surface model for the radar backscattering cross section of the ocean surface 1. Theory of the model and optimization/validation by scatterometer data

An improved composite surface model for the calculation of the normalized radar backscattering cross section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.

Numerical study on the along-track interferometric radar imaging mechanism of oceanic surface currents

The phase information in along-track interferometric synthetic aperture radar (along-track INSAR, ATI) images is a measure of the Doppler shift of the backscattered signal and thus of the line-of-sight velocity of the scatterers. It can be exploited for oceanic surface current measurements from aircraft or spacecraft. However, as already discussed in previous publications, the mean Doppler frequency of the radar backscatter from the ocean is not exclusively determined by the mean surface current, but it includes contributions associated with surface wave motion. The authors present an efficient new model for the simulation of Doppler spectra and ATI signatures. The model is based on Bragg scattering theory in a composite surface model approach. They show that resulting Doppler spectra are consistent with predictions of an established model based on fundamental electrodynamic expressions, while computation times are reduced by more than one order of magnitude. This can be a key advantage with regard to operational applications of ATI. Based on model calculations for two simple current fields and various wind conditions and radar configurations, they study theoretical possibilities and limitations of oceanic current measurements by ATI. They find that best results can be expected from ATI systems operated at high microwave frequencies like 10 GHz (X band), high incidence angles like 60/spl deg/, low platform altitude/speed ratios, and vertical (VV) polarization. The ATI time lag should be chosen long enough to obtain measurable phase differences, but much shorter than the decorrelation time of the backscattered field.

An improved composite surface model for the radar backscattering cross section of the ocean surface: 2. Model response to surface roughness variations and the radar imaging of underwater bottom topography

In the companion paper we have presented an improved composite surface model for the calculation of normalized radar backscattering cross sections (NRCS) of the ocean surface. The proposed model accounts for the impact of the full two-dimensional ocean wave spectrum on the radar backscatter and was shown to reproduce measured absolute NRCS values for a variety of radar configurations and wind speeds satisfactorily after some reasonable tuning of the input ocean wave spectrum. This paper focuses on the modulation of the NRCS in the presence of spatially varying surface currents. First, the sensitivity of the NRCS to intensity variations of different ocean wave spectral components is investigated. Then the hydrodynamic modulation of the wave spectrum over underwater bottom topography in tidal waters is computed in different ways, and the resulting radar signatures are discussed. The composite surface model yields comparable radar signatures at high (10 GHz, X band) and low (1 GHz, L band) radar frequencies, which is in much better agreement with experimental results than the predictions of a first-order Bragg scattering model. On the other hand, measured variations of the NRCS at high radar frequencies appear to be still underestimated in some cases, which may be due to shortcomings of our description of the wave-current interaction by conventional weak hydrodynamic interaction theory. Possible improvements of the theory are discussed, and requirements for future experiments are formulated.

A three‐scale composite surface model for the ocean wave–radar modulation transfer function

An improved three-scale composite surface model for the modulation of the radar backscatter from the ocean surface by long ocean waves is presented. The model is based on Bragg scattering theory. In the conventional two-scale model, only the geometric modulation of the radar backscatter and the hydrodynamic modulation of the short Bragg waves by the long waves is considered. In the three-scale model, the impact of intermediate-scale waves (wavelengths between the length of the Bragg waves and the length of the long waves which are resolved by the radar) is also taken into account, which leads to a modified theoretical ocean wave-radar modulation transfer function (MTF). For the first time the proposed model includes not only geometric effects associated with the intermediate-scale waves but also the additional hydrodynamic modulation of the Bragg waves. The resulting theoretical expression for the measured “hydrodynamic” MTF depends on the radar polarization as well as on the azimuthal (upwave / downwave or upwind / downwind) radar look direction. Especially for HH polarization, the predicted “hydrodynamic” MTF becomes significantly larger than expected from conventional theory. We compare model results with tower-based scatterometer measurements at L, C, and X band (1.0, 5.3, and 10.0 GHz, respectively), which were obtained during the Synthetic Aperture Radar and X Band Ocean Nonlinearities-Forschungsplattform Nordsee (SAXON-FPN) experiment. The measured magnitudes and phases of the MTF are better reproduced by the proposed three-scale model than by the conventional two-scale model. However, the large measured “hydrodynamic” MTFs for high microwave frequencies (C and X band) are still underestimated. The agreement between model predictions and measurements can be improved if, for example, an additional variation of the wind stress over the long waves is assumed. The required wind stress modulation depends on the long-wave slope and appears to be coupled to the hydrodynamic modulation of the surface roughness by a positive feedback mechanism.

Current measurements by SAR along-track interferometry from a Space Shuttle

We present one of the first studies on ocean current retrievals from interferometric synthetic aperture radar (InSAR) data acquired during the Shuttle Radar Topography Mission (SRTM) in February 2000. The InSAR system of SRTM was designed for high-resolution topographic mapping of the Earths land surfaces, using two SAR antennas on a Space Shuttle with a cross-track separation of 60 m. An additional along-track antenna separation of 7 m resulted in an effective time lag of about 0.5 ms between the two images, which could theoretically be exploited for target velocity retrievals. However, the feasibility of ocean current measurements with SRTM has been questionable, since the time lag was much shorter than the theoretical optimum (about 3 ms at X-band) and the signal-to-noise ratio over water was quite low. Nevertheless, some X-band InSAR images of coastal areas exhibit clear signatures of tidal flow patterns. As an example, we discuss an image of the Dutch Wadden Sea. We convert the InSAR data into a line-of-sight current field, which is then compared with results of the numerical circulation model KUSTWAD. For tidal phases close to the conditions at the time of the SRTM overpass; we obtain correlation coefficients of up to 0.6 and rms differences on the order of 0.2 m/s. Furthermore we find that SRTM resolves current variations down to spatial scales on the order of 1 km. This is consistent with predictions of a numerical InSAR imaging model. Remaining differences between SRTM- and KUSTWAD-derived currents can be attributed mainly to residual motion errors in the SRTM data as well as to a limited representation of the conditions at the time of the SRTM overpass in the available KUSTWAD results.

Theoretical Evaluation of Several Possible Along-Track InSAR Modes of TerraSAR-X for Ocean Current Measurements

The German satellite TerraSAR-X, scheduled for launch in late 2006, will permit high-resolution ocean current measurements by along-track interferometric SAR (along-track InSAR) in various experimental modes of operation, using different sections of its X-band SAR antenna array with a total length of 4.8 m as individual receive antennas. Depending on antenna and receive-chain settings, effective InSAR time lags of about 0.17 to 0.29 ms can be realized in combination with different noise levels, single-look resolutions, swath widths, and incidence angles. We give an overview of the characteristics of the possible InSAR modes and evaluate their suitability for current measurements on the basis of simulated data products. Our results indicate that the quality of interferometric stripmap data from TerraSAR-X will be clearly superior to the quality of the existing data acquired over the Dutch coast during the Shuttle Radar Topography Mission; accurate current retrievals can be expected at effective spatial resolutions on the order of 500 m. However, in modes using a multiplexed single receive chain, the effective swath width of stripmap data will be limited to only 15 km, while dual receive-chain operation offers a swath width of 30 km for stripmap data and promises a reasonable data quality even for ScanSAR data with a maximum swath width of 100 km. Finally, we consider fundamental relations between along-track baseline, instrument noise, and resulting InSAR phase noise to discuss the potential for current measuring performance improvements of TerraSAR-X follow-on satellites

Wind Retrieval From Shipborne Nautical X-Band Radar Data

In this paper, we study the retrieval of wind information from nautical X-band radar data. In contrast to previous studies, where data from stationary research platforms were used, this study focuses on data from a moving platform, encountering a larger variety of conditions than a platform at a fixed location. Compared to traditional in situ sensors, wind data derived from nautical radar images are much less susceptible to air flow distortion by the platform, since the images cover a large area around the ship. Images collected with a standard nautical HH-polarized X-band radar operating at grazing incidence exhibit a single intensity peak in upwind direction. The wind retrieval method developed here uses a harmonic function that is least-squares fitted to the radar backscatter intensity as a function of antenna look direction. The upwind direction is given by the direction that corresponds to the peak of the fitted function. An empirical model function is derived to retrieve the wind speed from the average radar backscatter intensity. Contrary to wind retrieval methods that have been proposed before, this approach is well suited for data acquired from a moving platform, as it functions well even if the radar field of view is partially shadowed and does not require ship motion correction. Here, we focus on data that were collected during two storms, using the first storm to derive and the second to test the empirical model functions. The method is validated using measurements from two ship-based anemometers.

Current Measurements in Rivers by Spaceborne Along-Track InSAR

The global monitoring of river discharges is a technologically challenging problem, with important applications in a variety of disciplines. Due to the limited availability and/or quality of river runoff data from many regions, an increasing use of remote sensing techniques is highly desirable. Altimeter specialists have already demonstrated water level retrievals in rivers from available data. The along-track interferometric synthetic aperture radar [along-track InSAR (ATI)] capabilities of state-of-the-art imaging radars on satellites such as the German TerraSAR-X, which was launched on June 15, 2007, also permit high-resolution line-of-sight surface current measurements. In this paper, we evaluate the potential of current measurements in rivers by spaceborne ATI on the basis of fundamental theoretical considerations, existing spaceborne InSAR data from the shuttle radar topography mission (SRTM), and simulated TerraSAR-X data. We show that an SRTM-derived line-of-sight surface current field in the Elbe river, Germany, agrees well with numerical hydrodynamic model results. The data quality is sufficient to resolve characteristic lateral variations of the currents in the river around a pronounced main flow channel. Assuming that the flow direction is usually aligned with the river bed, even a quasi-2D total surface current field can be derived. Simulations indicate that TerraSAR-X was even better suited for current measurements in rivers. Depending on width, surface roughness, and relative flow direction of a river, current estimates with an accuracy better than 0.1 m/s were possible with an effective spatial resolution of a few hundred meters to kilometers.

Global validation of the wave model WAM over a one-year period using Geosat wave height data

The high quality of wave fields simulated by the third-generation wave model WAM has already been demonstrated in various validation studies using in situ measurements as well as data from satellites as reference. However, owing to limitations of the reference data sets, the previous studies concentrated on relatively small regions or short time periods only, for which adequate measurements were available. In this paper the first global verification of the WAM model over a full 1-year period is presented. The significant wave heights hindcast for 1988 by the WAM model as implemented at the European Centre for Medium Range Weather Forecasts are compared with measurements obtained by the Geosat radar altimeter. The wave heights from WAM and Geosat show good agreement in general. However, significant regional and seasonal differences are found. The underestimation of WAM wave heights in the southern hemisphere, which was already known from a validation study for the Seasat period, shows significant seasonal variations. The hindcast wave heights are underestimated by about 20% in large parts of the southern hemisphere and the tropical region during May-;September. For the rest of the time, the agreement with Geosat data is fairly good. Together with the fact that also the rms variability of wave heights in the tropical region is clearly underestimated by WAM, this can possibly be attributed to simplifications like the neglect of atmospheric stratification effects when converting wind speeds to the wind stress fields driving WAM. Furthermore, the intercomparison indicates that low wave heights below ∼ 1.5 m are generally overestimated by WAM. As it is planned to use altimeter wave heights for updating wave models in future data assimilation systems, it is quite important to have efficient quality control criteria for these data. We show that some additional Geosat parameters, e.g., the off-nadir angle of the altimeter, can be useful quality parameters. The difference between the Geosat and WAM wave heights shows a clear dependence on the additional parameters in some cases, which must be related to quality problems of the Geosat data. Some new criteria for the rejection of incorrect Geosat data points are obtained.

Current measurements by airborne along-track InSAR: measuring technique and experimental results

Since the first demonstration of high-resolution mapping of surface currents by airborne along-track interferometric synthetic aperture radar (along-track InSAR) in the late 1980s, theoretical models of the along-track InSAR imaging mechanism and recommendations for ideal instrument parameters, measuring strategies, and data processing and interpretation techniques have been discussed in a number of publications. However, due to the experimental nature of existing instruments and algorithms and a very limited reference database from actual experiments, potential users have not recognized the along-track InSAR as a readily available tool for current measurements until now. In order to promote the use of InSAR and to validate and demonstrate current measurements on the basis of instrument parameters and data processing techniques proposed earlier, the authors have carried out experiments with an airborne X-band along-track InSAR over spatially varying current fields at two test sites in the German Bight of the North Sea. In this paper, an overview of the experimental scenarios and the acquired data is given, and the newly implemented algorithms for the retrieval of two-dimensional (2-D) surface current fields from the InSAR raw data are described. Using acoustic Doppler current profiler (ADCP) data and predictions of a numerical circulation model as reference, a root mean square (rms) error of spatial variations in the InSAR-derived current fields on the order of 0.1 m/s at an effective resolution of about 100 m is obtained, which is consistent with theoretical expectations. Furthermore, it is shown that the proposed iterative correction scheme for nonlinearities of the InSAR imaging mechanism on the basis of numerical simulations works well and leads to a significant improvement. Altogether, it is concluded that the proposed technique for current measurements by along-track InSAR is efficient, robust, and sufficiently mature for applications that require high-resolution snapshots of surface current fields within areas of some square kilometers, such as the monitoring of bathymetric changes in coastal waters.