Susanne Lehner

Mesoscale wind measurements using recalibrated ERS SAR images

The precision images (PRI) of the synthetic aperture radars (SAR) on board the European Remote Sensing Satellites ERS-1 and ERS-2 are used to derive mesoscale wind fields over the ocean. For calculation of the wind speed the C-band model (CMOD4) is used, which was originally developed by Stoffelen and Anderson [1993] for the European Space Agency (ESA) to derive wind fields from measurements of the wind scatterometer (SCAT). In the case of the ERS-1/2 SAR the CMOD4 is used to compute the wind speed from the normalized radar backscatter cross section (NRCS) and the incidence angle of the radar beam, both computed from the SAR.PRI data. The third input variable is the wind direction, which is estimated from the wind streaks in the images or from ground truth measurements. The SAR data are affected by a power loss, caused by saturation of the analog to digital converter (ADC) of the SAR. Therefore the images have to be recalibrated. Errors in the derived wind speed are mainly due to ADC saturation and uncertainties of the input wind direction. These errors are estimated for various wind conditions. Mesoscale wind fields computed from ERS-1/2 SAR.PRI images taken between the Shetland Islands and the west coast of Norway are compared to ground truth measurements and modeled wind fields from the German weather service (DWD). Wind fields of the nonhydrostatic mesoscale model Geesthacht simulation model of the atmosphere (GESIMA) are compared to the derived wind field of the ERS-1 SAR.PRI image at the island Rugen in the Baltic Sea.

Ship Surveillance With TerraSAR-X

Ship detection is an important application of global monitoring of environment and security. In order to overcome the limitations by other systems, surveillance with satellite synthetic aperture radar (SAR) is used because of its possibility to provide ship detection at high resolution over wide swaths and in all weather conditions. A new X-band radar onboard the TerraSAR-X (TS-X) satellite gives access to spatial resolution as fine as 1 m. In this paper, first results on the combined use of TS-X ship detection, automatic identification system (AIS), and satellite AIS (SatAIS) is presented. The AIS system is an effective terrestrial method for tracking vessels in real time typically up to 40 km off the coast. SatAIS, as a space-based system, allows almost global coverage for monitoring of ships since not all ships operate their AIS and smaller ships are not equipped with AIS. The system is considered to be of cooperative nature. In this paper, the quality of TS-X images with respect to ship detection is evaluated, and a first assessment of its performance for ship detection is given. The velocity of a moving ship is estimated using complex TS-X data. As test cases, images were acquired over the North Sea, Baltic Sea, Atlantic Ocean, and Pacific Ocean in Stripmap mode with a resolution of 3 m at a coverage of 30 km 100 km. Simultaneous information on ship positions was available from TS-X and terrestrial as well as SatAIS. First results on the simultaneous superposition of SatAIS and high-resolution radar images are presented.

Global wind speed retrieval from SAR

The global availability of synthetic aperture radar (SAR) wave mode data from the European Remote Sensing (ERS) satellites ERS-1 and ERS-2, as well as ENVISAT, allows for the investigation of the wind field over the ocean on a global and continuous basis. For this purpose, 27 days of ERS-2 SAR wave mode data were processed, representing a total of 34310 imagettes of size 10 km /spl times/5 km, available every 200 km along the satellite track. In this paper, two methods for retrieving wind speeds from SAR imagettes are presented and validated, showing the applicability of ENVISAT alike SAR wave mode data for global ocean wind retrieval. The first method is based on the well-tested empirical C-band scatterometer (SCAT) models, which describe the dependency of the normalized radar cross section (NRCS) on wind speed and direction. To apply C-band models to SAR data, the NRCS needs to be accurately calibrated. This is performed by a new efficient method utilizing a subset of colocated measurements from ERS-2 SCAT and model winds from the European Centre for Medium-Range Weather Forecast (ECMWF). SAR wind speeds are computed from the calibrated imagettes and compared to the entire set of colocated ERS-2 SCAT and ECMWF model data. Comparison to ERS-2 SCAT winds result in a correlation of 0.95 with a bias of -0.01 m s/sup -1/ and an rms error of 1.0 m s/sup -1/. The second approach is based on neural networks (NNs), which allow the retrieval of wind speeds from uncalibrated SAR imagettes. NNs are trained using the mean intensity of ERS-2 SAR imagettes and colocated wind data from the ERS-2 SCAT and ECMWF model data. Validation of the NN-retrieved SAR wind speeds to ERS-2 SCAT and ECMWF model wind data result in a correlation of 0.96 with a bias of -0.04 m s/sup -1/ and an rms error of 0.93 m s/sup -1/.

Ocean Winds from RADARSAT-1 ScanSAR

This paper discusses an algorithm designed to retrieve high-resolution wind fields from scanning synthetic aperture radar (ScanSAR) data acquired on board the Canadian satellite RADARSAT-1. The ScanSAR operates at C-band with horizontal polarization. The wind directions are extracted from wind-induced streaks, e.g., from atmospheric boundary layer rolls or wind shadowing, which are approximately in line with the mean wind direction near the ocean surface. The wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated ScanSAR images, together with the local wind direction retrieved from the image. Therefore the semi-empirical C-band model CMOD4, which describes the dependency of the NRCS on wind and image geometry, is used. The CMOD4 was originally developed for the scatterometer of the European remote sensing satellites ERS-l and ERS-2 operating at C-band with vertical polarization. Consequently, the CMOD4 required modification for horizontal polarization, which is performed by considering the polarization ratio. To demonstrate the applicability of the algorithm, wind fields were computed from 20 RADARSAT-1 ScanSAR wide-swath images and compared to co-located results from the Danish high-resolution limited-area model (HIRLAM). In addition, the error sources in ScanSAR wind retrieval are discussed and sensitivity studies were carried out to estimate wind speed errors due to uncertainties in the NRCS, wind direction, and incidence angles.

Algorithm for Sea Surface Wind Retrieval From TerraSAR-X and TanDEM-X Data

A geophysical model function (GMF), which is denoted by XMOD2, is developed to retrieve sea surface wind field from X-band TerraSAR-X/TanDEM-X (TS-X/TD-X) data. In contrast to the previously developed XMOD1, XMOD2 consists of a nonlinear GMF, and thus, it depicts the difference between upwind and downwind of the sea surface backscatter in X-band synthetic aperture radar (SAR) imagery. By exploiting 371 collocations with in situ buoy measurements that are used as the tuning data set together with analysis wind model results, the retrieved TS-X/TD-X sea surface wind speed using XMOD2 shows a close agreement with buoy measurements with a bias of -0.32 m/s, a root-mean-square error (RMSE) of 1.44 m/s, and a scatter index (SI) of 16.0%. Further validation using an independent data set of 52 cases shows a bias of -0.17 m/s, an RMSE of 1.48 m/s, and an SI of 17.0% comparing with buoy measurements. To apply XMOD2 to TS-X/TD-X data acquired at HH polarization, we validate three X-band SAR polarization ratio models that were tuned using TS-X dual-polarization data by comparing the retrieved sea surface wind speed with buoy measurements.

Dual-Polarized TerraSAR-X Data for Oil-Spill Observation

The paper presents a novel method to estimate the speed of a moving ship and the range velocity component of the current sea surface. The estimate of the ship speed is obtained by multilooking techniques [1]. The generation of a sequence of images from one single complex SAR image corresponds to an image time series with reduced resolution. This allows evaluating the velocity components in range and azimuth of the target by applying change detection techniques of the time series. The estimation of the displacement vector of a moving target in consecutive images of the sequence allows the estimation of the azimuth velocity component. The range velocity component is estimated by the variation of the signal amplitude in time. The results are applied on TerraSAR-X StripMap and High-Resolution SpotLight data and validated by Automatic Identification System (AIS). Furthermore the method is applied in order to estimate the range component of the current speed. The measurements are compared to model results of Federal Maritime and Hydrographic Agency of Germany (BSH) and Federal Waterways Engineering and Research Institute (BAW).In this study to investigate the capability of high resolutionSAR data to detect small quantities of oil near offshore platforms, a data set acquired both in single and dual-polarization mode over the Ekofisk oil platform is analyzed. Instantaneous wind information retrieved directly from the SAR image using the XMOD Geophysical Model Function (GMF) and wind history from the DWD forecast global sea wave model (GSM) are provided to discriminate a probable wind shadowing effect near the offshore installation and estimate the age of the spills. Foremost examples from the data set acquired between January, 2008 and February, 2010 are shown here and discussed in detail.A study exploiting dual-polarimetric X-band synthetic aperture radar (SAR) data to observe oil at sea is undertaken for the first time. The polarimetric model exploits the interchannel correlation between the like polarized channels. Accordingly, two parameters related to the interchannel correlation, namely, the amplitude coherence and the copolarized phase difference (CPD) standard deviation, are accounted for, and their performances, with respect to sea oil slick observation, are carefully discussed. Single-look Slant range Complex dual-polarized TerraSAR-X SAR data, in which both certified oil slicks and weak-damping look-alikes are present, are used to verify the efficiency of the proposed approaches. Results show the advantage of the CPD approach and the effectiveness of TerraSAR-X dual-polarized products for such application.

Ocean Wave Integral Parameter Measurements Using Envisat ASAR Wave Mode Data

An empirical algorithm to retrieve integral ocean wave parameters such as significant wave height (SWH), mean wave period, and wave height of waves with period larger than 12 s (H12) from synthetic aperture radar (SAR) images over sea surface is presented. The algorithm is an extension to the Envisat Advanced SAR (ASAR) wave mode data based on the CWAVE approach developed for ERS-2 SAR wave mode data and is thus called CWAVE_ENV (CWAVE for Envisat). Calibrated ASAR images are used as the only source of input without needing prior information from an ocean wave model (WAM) as the standard algorithms used in weather centers. This algorithm makes SAR an independent instrument measuring integrated wave parameters like SWH and mean wave period to altimeter quality. A global data set of 25 000 pairs of ASAR wave mode images and collocated reanalysis WAM results from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to tune CWAVE_ENV model coefficients. Validation conducted by comparing the retrieved SWH to in situ buoy measurements shows a scatter index of 0.24 and 0.16 when compared to the ECMWF reanalysis WAM. Two case studies are presented to evaluate the performance of the CWAVE_ENV algorithm for high sea state. A North Atlantic storm during which SWH is above 18 m as observed by SAR and Radar Altimeter simultaneously is analyzed. For an extreme swell case that occurred in the Indian Ocean, the potential of using SWH measurements from ASAR wave mode data derived by the CWAVE_ENV algorithm is demonstrated.

Investigation of Ocean Surface Wave Refraction Using TerraSAR-X Data

As a scientific and technological continuation of the X-band Synthetic Aperture Radar (X-SAR) and Shuttle Radar Topography Mission (SRTM) missions, the new X-SAR, namely, TerraSAR-X (TSX), was launched on June 15, 2007. Since then, it has provided numerous high-quality data over land and ocean operationally. In this paper, surface wave refraction and diffraction are investigated using TSX imagery acquired over the coast of Terceira island situated in the North Atlantic. Peak wavelength and wave direction are determined by SAR 2-D image spectra. They are compared to measurements of X-band marine radar and results of the WAve prediction Model (WAM). Significant wave height in the near-shore shallow water region is estimated from TSX Spotlight mode data following the wave refraction laws and using the developed XWAVE empirical algorithm. Image spectra of the TSX subscenes in the full-coverage region are given to investigate significant changes of wave direction and length. By analyzing another TSX image acquired in StripMap mode, a shadow zone in the lee side of Terceira island is identified. It is influenced jointly by wave refraction and diffraction. Furthermore, a cross-sea pattern revealed in the image spectra is investigated. The cross sea is generated by the diffracted wave rays from the northern and southern coasts of the island. Less wave directional spreading for the cross-sea situation is observed as well when compared to the image spectra at the origin of diffraction.

First demonstration of surface currents imaged by hybrid along- and cross-track interferometric SAR

This paper is concerned with the simultaneous measurement of terrain heights and currents using an airborne interferometric synthetic aperture radar (INSAR). For the first time, a hybrid two-antenna INSAR system with both along- and across-track baseline components is used to measure high-resolution digital elevation maps (DEMs) and current fields in a Wadden sea area. Coastal applications like the monitoring of sediment transport or the numerical modeling of morphodynamic processes require measurements of topography and currents. Classical in situ measurements of these parameters are both expensive and time consuming, or even impossible if higher spatial resolution is required. Pure along-track interferometers (ATIs) have demonstrated their ability to provide information on ocean currents, while across-track interferometric systems (XTIs) have been successfully used to measure DEMs. In this paper, a hybrid system with both ATI and XTI components is used to acquire synoptic measurement for the first time. An experiment with an airborne system taking data over a Wadden sea area is presented demonstrating the potential of the technique. A geometrical model is developed for the interferometric phase of hybrid INSAR systems. This model combines ATI and XTI techniques using a baseline that spans between two antennas consisting of along-track and across-track components. In this model, both the effect of topography and the radial velocity of the water surface enter into the resulting interferometric phase. To separate both components, the system takes data of the respective scene flying two or more tracks with different flight directions, e.g., antiparallel tracks. This approach leads to a set of linear equations that has a unique solution for the along- and cross-track phase. Finally, an additional phase bias has to be considered due to the radial velocity and the influence of squint. This is caused by a misregistration effect between both antennas, which is related to cross-track imaging of surface motion. The new INSAR technique is tested with data acquired during a campaign in February 1997 over the Weser Estuary at the German coast. The airborne INSAR system AeS-1 was used. The interferometer configuration consists of two SAR antennas separated by a mixed along-track and cross-track baseline. Two datasets acquired on antiparallel tracks are used. The calculated velocities were compared with a hydrodynamic model operated by the Federal Waterway Engineering and Research Establishment. The experimental results agree well with the numerical model. In particular, the mean velocity of 0.7 ms/sup -1/ matches in both datasets. Deviations in the fine-scale structure of the current field are discussed. Topographic analysis and validation are performed in a separate investigation. The impact of surface gravity waves and wind drift, which are known to cause significant artifacts in the ATI phase under certain circumstances, is discussed.

Detection of wave groups in SAR images and radar image sequences

The properties of individual wave groups in space and time utilizing synthetic aperture radar (SAR) images and nautical radar image sequences are studied. This is possible by the quantitative measurement and analysis of wave groups both spatially and spatio-temporally. The SAR, with its high spatial resolution and large coverage, offers a unique opportunity to study and derive wave groups. In addition to SAR images, nautical radar image sequences allow the investigation of wave groups in space and time and, therefore, the measurement of parameters such as the group velocity. The detection of wave groups is based on the determination of the envelope function, which was first adopted for one-dimensional (1-D) time series by Longuet-Higgins. The method is extended from 1-D to spatial and spatio-temporal dimensions to derive wave groups in images and image sequences. To test the algorithm, wave groups are derived from SAR images and two radar image sequences, recorded at locations in deep and shallow water. It is demonstrated that the algorithm can be employed for the determination of both location and size of wave groups from radar images. Investigating the detected wave groups in radar image sequences additionally allows the measurement of the spatial and temporal development of wave groups and their extension and phase velocities. Comparison of measured wave group velocities in shallow and deep water gives a deviation of the average value from the group velocities resulting from linear wave theory and shows a clear oscillation of the group velocities in two dimensions.