Search

Marine surface films, be they of biogenic or anthropogenic origin, dampen the small-scale waves that are responsible for the radar backscattering, hence they can be seen on synthetic aperture radar (SAR) imagery as areas of reduced image brightness. Examples of different kinds of marine surface films are presented, that were observed on spaceborne SAR imagery since the launch of SEASAT in 1978. In addition, some results from SAR data analyses are used to demonstrate the key aspects of the visibility of marine surface films on spaceborne SAR imagery.

We used 26 Radarsat-2 and TerraSAR-X single- and dual-co-polarization Synthetic Aperture Radar (SAR) images of a test site on the German North Sea coast to investigate the influence of imaging geometry and environmental conditions (wind speed, water level, and vegetation period) on the radar return from exposed intertidal flats. Multi-temporal analyses of single- (VV-) polarization SAR imagery indicate that the surface roughness is more variable at shorter scales (responsible for the X-band backscatter) than at longer scales (C-band). Less variation at both radar bands was found in sea-grass meadows. TerraSAR-X dual-co-polarization data were used for polarimetric analyses based on a decomposition of the Kennaugh matrix, whose elements provide information on the total intensity at both co-polarizations and on the relative strength of even- and odd-bounce backscattering. At steep incidence angles (around 30°) the radar backscatter from bare sand flats is similarly strong at both co-polarizations, while the vertically polarized radar return dominates at higher incidence angles (above 40°). At low water levels, resulting in lower moisture of the sandy sediments, strong single-bounce radar backscattering was observed, while higher water levels (and moisture) caused weak mixed (single- and double-bounce) backscattering. Apart from the absolute water level its history, e.g. the time and level of the closest low tide, must be considered. During the vegetation period, sea-grass meadows cause a stronger increase in horizontally (HH-) polarized radar backscatter, along with an increased double-bounce backscattering. We conclude that the Kennaugh element framework has potential to be used for classification purposes in intertidal areas, but also that, for a full interpretation of the SAR imagery, the exact topography and the surface roughness have to be known.

Quad-polarization Synthetic Aperture Radar (SAR) imagery can be well used to identify bivalve beds on dry-fallen intertidal flats and to investigate radar backscattering mechanisms on exposed sediments. In this study, a general classification approach to use spaceborne SAR data to distinguish sediment types, especially mud and sand, and to detect mussel beds, is presented. We propose a new feature set including Double-bounce Eigenvalue Relative Difference (DERD) and the Kennaugh elements, and combine those indicators with the unsupervised Wishart classifier. The main classification results of sediments and habitats agree with data from regular monitoring campaigns, which are frequently performed to fulfill the requirements of coastal environmental monitoring.

We analyzed a large amount of high-resolution Synthetic Aperture Radar (SAR) data of dry-fallen intertidal flats on the German North Sea coast with respect to the imaging of sediments, macrophytes, and mussels. TerraSAR-X and Radarsat-2 images of four test areas acquired from 2008 to 2013 form the basis for the present investigation and are used to demonstrate that pairs of SAR images, if combined through basic algebraic operations, can already provide indicators for morphological changes and for bivalve (oyster and mussel) beds. Multi-temporal analyses of series of SAR images allow detecting bivalve beds, since the radar backscattering from those beds is generally high, whereas that from sediments may vary with imaging geometry and environmental conditions. Our results further show evidence that also single-acquisition, dual-polarization SAR imagery can be used in this respect. The polarization coefficient (i.e., the ratio of the difference and the sum of both co-polarizations) can be used to infer indicators for oyster and blue-mussel beds.

In this chapter passive and active microwave sensors are introduced, their basic measurement principles are described, and few examples of microwave (MW) remote sensing of the Asian Seas are given. The space-borne sensors under consideration are MW radiometer, altimeter, scatterometer, and synthetic aperture radar (SAR). It is demonstrated that changes in the sea surface roughness can be used to infer ocean surface vector wind fields, but also that this roughness may depend on other parameters such as SST, waves and currents.

We demonstrate that dual-co-polarization Synthetic Aperture Radar (SAR) imagery can be used to derive new indicators for bivalve beds on dry-fallen intertidal flats and to investigate radar backscattering mechanisms on exposed sediments and seagrass meadows. The Kennaugh element framework forms the basis for our studies, and particular Kennaugh elements are used to generate maps indicating the location of bivalve beds and to demonstrate the seasonal change of the radar backscattering from exposed intertidal during low tide.

We analyzed a great deal of high-resolution Synthetic Aperture Radar (SAR) data of dry-fallen intertidal flats in the German Wadden Sea with respect to the imaging of sediments, macrophytes, and mussels. TerraSAR-X and Radarsat-2 images of five test areas along the German North Sea coast acquired between 2008 and 2013 form the basis for the present investigation and are used to demonstrate that pairs of SAR images, if combined through basic algebraic operations, can already provide useful indicators for morphological changes and for bivalve (oyster and mussel) beds. Depending on the type of sediment, but also on the water level and on environmental conditions (wind speed) exposed sediments may show up on SAR imagery as areas of enhanced, or reduced, radar backscattering. The (multi-temporal) analysis of series of such images allows for the detection of mussel beds, and our results show evidence that also single-acquisition, multi-polarization SAR imagery can be used for that purpose.

The Wadden Sea along the North Sea coasts of Denmark, Germany, and the Netherlands is the largest unbroken system of intertidal sand and mud flats in the world. Its habitats are highly productive and harbour high standing stocks and densities of benthic species, well adapted to the demanding environmental conditions. Therefore, the Wadden Sea is one of the most important areas for migratory birds in the world and thus protected by national and international legislation, which amongst others requires extensive monitoring. Due to the inaccessibility of major areas of the Wadden Sea, a classification approach based on optical and radar remote sensing has been developed to support environmental monitoring programmes. In this study, the general classification framework as well as two specific monitoring cases, mussel beds and seagrass meadows, are presented. The classification of mussel beds profits highly from inclusion of radar data due to their rough surface and achieves agreements of up to 79 % with areal data from the regular monitoring programme. Classification of seagrass meadows reaches even higher agreements with monitoring data (up to 100 %) and furthermore captures seagrass densities as low as 10 %. The main classification results are information on area and location of individual habitats. These are needed to fulfil environmental legislation requirements. One of the major advantages of this approach is the large areal coverage with individual satellite images, allowing simultaneous assessment of both accessible and inaccessible areas and thus providing a more complete overall picture.

We analyzed a large amount of high-resolution Synthetic Aperture Radar (SAR) data of exposed intertidal flats on the German North Sea coast with respect to the imaging of sediments, macrophytes, and mussels. TerraSAR-X and Radarsat-2 images of four test areas acquired from 2008 to 2013 form the basis for the present investigation and are used to demonstrate that pairs of SAR images, if combined through basic algebraic operations, can already provide indicators for bivalve (oyster and mussel) beds. Our results show evidence that single-acquisition, dual-polarization SAR imagery can be used in this respect. The polarization coefficient (i.e., the normalized difference polarization ratio) can be used to infer indicators for oyster and blue-mussel beds.

A helicopter-based Doppler scatterometer (Multi3Scat) is described. It allows simultaneous measurements of the surface radar backscatter at five different frequencies at co- and cross-polarization at incidence angles of 20deg -65deg from an altitude of 30-300 m. Video and infrared (IR) cameras simultaneously sense the surface in the scatterometers' footprint. The Multi 3Scat is calibrated using measurements carried out over corner reflectors. The stability of the Multi3Scat's signal is found to be, on average, better than 0.5 dB. Typical signal-to-noise-ratio values for sigma-0 range between 10 and 20 dB for cross-polarization and between 15 and 25 dB for copolarization over snow and ice surfaces. The potential of the Multi3Scat to acquire multifrequency multipolarization radar backscatter data and coincident video and IR temperature observations at different incidence angles over remote terrain such as the Arctic Ocean or the Alps is demonstrated.

We present a new method for the extraction of roughness parameters of sand ripples on exposed tidal flats from multi-frequency synthetic aperture radar (SAR) data. The method is based on the Integral Equation Model (IEM) which predicts the normalized radar cross-section (NRCS) of randomly rough dielectric surfaces. The data used for this analysis were acquired in the German Bight of the North Sea by the Spaceborne Imaging Radar-C/X-Band SAR (SIR-C/X-SAR) in 1994. In-situ measurements of the root-mean-squared (rms) height and the correlation length of the sand ripples clearly demonstrate a relationship between these roughness parameters and the C-band NRCS determined from an ERS SAR image. Using the IEM we have calculated NRCS isolines for the three frequency bands deployed by SIR-C/X-SAR (L, C, and X band), as a function of the rms height and the correlation length of the sand ripples. For each SIR-C/X-SAR image pixel these two roughness parameters were determined from the intersections of the NRCS isolines at different radar bands, and they were used for a crude sediment classification for a small test area at the German North Sea coast. Comparing our results with available sediment maps, we conclude that the presented method is very promising for tidal flat classification by using data from presently existing airborne and future spaceborne multi-frequency SAR systems.

Images of rain events over the ocean acquired by a multi‐frequency/multi‐polarization Synthetic Aperture Radar (SAR) show different radar contrasts at different frequencies and polarizations. In order to better understand these effects, field and laboratory experiments were performed at different rain rates and wind speeds with scatterometers working at different radar frequencies, polarizations, and incidence angles. Our results show that the dominant scattering mechanism on a rain‐roughened water surface, observed at VV polarization, at all incidence angles is Bragg scattering from ring waves. At HH polarization the radar backscatter is caused by both ring waves and non‐propagating splash products, with the dominating effect depending on incidence angle. The reduction and enhancement of the surface roughness by ring waves and sub‐surface phenomena, respectively, result in a transition wavenumber between reduction of the radar backscattering and its enhancement of about 100 rad m−1. We assume that this t...

We have carried out wind-wave tank measurements using wave-height and wave-slope gauges and a coherent 9.8 GHz (X band) scatterometer, when the water surface was agitated by heavy rain (160 mm h- 1 to 300 mm h -1 ) and by wind (2 ms- 1 to 12 ms -1 ). The upwind-looking scatterometer was operating at co- (VV- and HH-) and cross- (HV-) polarisation at a steep incidence angle of 28°. In the presence of rain, the power spectral density of the wind-wave spectra is enhanced at frequencies above about 5 Hz and it is reduced at lower frequencies. This is the net effect of surface roughness production by the rain-induced splash products and of wave damping by the rain-induced turbulence. We measured isotropic (rain-dominated) wave spectra at low wind speeds and anisotropic (wind-dominated) wave spectra at high wind speeds, with a transition wind speed that increases with rain rate. The radar backscattering at co-polarisation at low wind speeds is mainly caused by rain-induced ring waves, whereas at cross-polarisat...

We propose new indicators for bivalve (oyster and mussel) beds on exposed intertidal flats, derived from dual-copolarization (HH + VV) TerraSAR-X, Radarsat-2, and ALOS-2 images of the German North Sea coast. Our analyses are based upon the Kennaugh element framework, and we show that different targets on exposed intertidal flats exhibit different radar backscattering characteristics, which manifest in different magnitudes of the Kennaugh elements. Namely, the inter-channel correlation’s real (K3) and imaginary (K7) part can be used to distinguish bivalve beds from surrounding sandy sediments, and together with the polarimetric coefficient (i.e., the normalized differential polarization ratio, K0/K4) they can be used as indicators for bivalve beds using multi-frequency dual-copolarization SAR data. Our results show that continuous bivalve bed monitoring is possible using dual-copolarimetric SAR acquisitions at all radar wavelengths.

The possibility of detecting breaking long surface gravity waves in S-band radar backscatter measurements is being investigated using multi-polarization scatterometer measurements. To detect breaking waves the change in the spectral shape of the radar Doppler spectra was analyzed with respect to possible indicators for wave breaking events. They are identified here using the ratio of the second and fourth spectral moments after normalization by the squared radar frequency. A comparison with simultaneous estimates of the fractional area of whitecaps suggests that a normalized spectral moment ratio might be used as one indicator for events of breaking surface gravity waves.

Radar images acquired over the ocean by the C band synthetic aperture radar (SAR) aboard the European Remote Sensing satellites ERS 1 and ERS 2 often show sea surface manifestations of rain cells. We have searched the archives of several weather stations for weather radar data acquired concurrently with ERS SAR data and have found four concurrent data pairs: one in the South China Sea, two in the Baltic Sea and one in the North Sea. The comparison of ERS 1/2 SAR images showing radar signatures of rain cells with weather radar images reveals that the radar signatures of rain cells on ERS SAR images vary considerably, which makes it often difficult to distinguish them from radar signatures of other mesoscale or submesoscale atmospheric and oceanic phenomena. The present analysis, together with results obtained from previous analyses of spaceborne multifrequency SAR data and laboratory data as well as results obtained from theoretical models on radar backscattering at the sea surface suggest the following: C band radar signatures of rain cells with rain rates below 50 mm/h are mainly caused by a modification of the sea surface roughness induced by (1) the raindrops impinging on the sea surface and thus modifying the sea surface roughness and by (2) local wind field variations associated with rain cells (spreading downdrafts). Raindrops impinging on the sea surface generate ring waves as well as turbulence in the upper water layer. Depending on rain rate, drop size distribution, wind speed, and temporal evolution of the rain event, the net effect can be an increase or a reduction of the amplitude of the C band Bragg waves and thus of the backscattered radar power. Thus ocean areas struck by rain can show up on ERS SAR images as areas with higher or lower image brightness than the surroundings.

This investigation addresses the difficult problem of using a multi data set of water levels to study seich oscillations in the Baltic Sea. The Baltic Sea is a semi-enclosed sea area and is connected with the North Atlantic Ocean through the North Sea. The applied method is based on the analysis of environmentally corrected radar altimeter (RA) data in conjunction with simulated water levels obtained by an operational circulation model of the Federal Maritime and Hydrographic Agency (Bundesamt fur Seeschiffahrt und Hydrographie, BSH) and tide gauge data. RA data of the First European Remote Sensing Satellite (ERS-1) have been analysed. The RA data were collected during the three-day repeat cycle from August 12 to December 9, 1991. An advantage of the analysis is the coverage and the geographical location of one and the same frequently repeated descending sub-satellite pass over the Bothnian Bay and the central Baltic Sea. Due to the assumed linear dynamics of this sea area the observation of sea level changes in the sense of a standing wave between the Bothnian Bay and the Baltic Proper is investigated. Amplitudes of water level changes up to 1 m have been recorded at tide gauge stations in the northern and southern parts of the Baltic Sea, respectively, during August and December 1991. Water level changes of less than 20 cm were calculated from the corrected altimeter data during the overflights of ERS-1. Tide gauge data and water level differences of tide gauge data measured at Kemi (Bothnian Bay), Finland, and Kolobrzeg at the Polish coast were used for frequency analyses. The analysed time series have been limited to five months due to the duration of the three-day repeat cycle of ERS-1. For frequency analysis the given sampling interval of 4 h is not appropriate, but a period of 12.4 h was clearly identified. This period corresponds to the fourth mode of theoretical estimated ones. The differences of water levels of the simulated data between Kemi and Kolobrzeg of about 25 cm agree fairly well with the results obtained by the RA data. Due to these results and the synoptic view of the along-track subsatellite passes it turned out that seiches can be observed by the ERS-1 radar altimeter.

More than 660 synthetic aperture radar (SAR) images acquired over the southern Baltic Sea, the North Sea, and the Gulf of Lion in the Mediterranean Sea by the Second European Remote Sensing Satellite (ERS-2) have been analyzed since December 1996 with respect to radar signatures of marine pollution and other phenomena causing similar signatures. First results of our analysis reveal that the seas are most polluted along the main shipping routes. The sizes of the detected oil spills vary between0.1 km2 and56 km2. SAR images acquired during descending (morning) and ascending (evening) satellite passes show different percentages of oil pollution, because most of this pollution occurs during night time and is still visible on the SAR images acquired in the morning time. Moreover, we found a higher amount of oil spills on SAR images acquired during summer (April-September) than on SAR images acquired during winter (October-March). We attribute this finding to the higher mean wind speed encountered in all three test areas during winter. By using an ERS-2 SAR image of the North Sea test area we show how the reduction of the normalized radar backscattering cross section (NRCS) by an oil spill depends on wind speed.

Radar signatures of mineral oil spills consisting of heavy and light fuel were measured by an airborne five-frequency ( L - S - C - X - and K -band) multi u polarization microwave scatterometer flown on a helicopter during a controlled oil spill experiment in the North Sea. The damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, was measured at different radar frequencies and incidence angles such that the Bragg wavenumbers, k, between 20 radm 1 and 500 radm 1 were covered. The B following results were obtained: for the five oil spills deployed in the experiment the damping ratio, in general, increases monotonically from k 20 radm 1 to B k 500 radm 1 . At S - C - X - and K -band, the damping ratio is larger for heavy B u fuel than for light fuel spills, while at L- band it is almost the same. For heavy fuel, the damping ratio increases with increasing thickness of the oil layer. Furthermore, for wind speeds between 6 m s 1 and 10 m s 1 the ...

Radar signatures of rain cells are investigated using multifrequency/multipolarization synthetic aperture radar (SAR) images acquired from the space shuttle Endeavour during the spaceborne imaging radar-C/X-band SAR (SIR-C/X-SAR) missions in April and October 1994. In SIR-C/X-SAR images, radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization. The radar signatures of rain cells observed in SIR-C/X-SAR imagery of the ocean originate from (1) the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and (2) the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells. Raindrops impinging on the sea surface generate ring waves, which enhance the sea surface roughness, but they also generate turbulence in the upper water layer, which reduces the sea surface roughness. Depending on the radar wavelength, ocean areas struck by rain can have higher or lower normalized radar cross section (NRCS) than the surrounding rain-free area; in ocean areas where heavy rain is impinging on the sea surface, the X- and C-band NRCS is usually enhanced, and the L-band NRCS is reduced. From the phase difference between the horizontally and vertically copolarized signals, estimates of the rain rate are obtained. The present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell.

Radar signatures of sea surface films of different origin are investigated, which have been acquired by airborne and spaceborne multi-frequency/multi-polarisation microwave sensors during the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR), missions in 1994, as well as by the ERS SAR in 1996–98. During SIR-C/XSAR, controlled surface film experiments were performed in the German Bight by deploying various quasi-biogenic substances and mineral oil on the sea surface, in order to study the radar signatures caused by surface films of different visco-elastic properties. In general, our results show that multi-frequency capabilities, rather than multi-polarisation capabilities, are needed for a radar-based system for the discrimination of marine surface films. We show that, under high wind conditions (> 10 m s−1), a discrimination between the different kinds of surface films is very difficult, whereas at low to moderate wind speeds (≤ 5 m s−1) a discrimination seems to be possible. This finding is supported analytically by means of a new model for the wave number-dependent radar contrast at high wind speeds (> 10 m s−1) and statistically through the analyses of more than 700 ERS SAR images. In addition, results of laboratory experiments are presented that were carried out in the wind-wave tank of the University of Hamburg. At certain wind-speed ranges a different damping of bound and freely propagating surface waves by monomolecular surface films is observed, which may explain the high radar contrasts measured by the microwave sensors.

Laboratory measurements with artificial rain were carried out in the wind-wave tank of the University of Hamburg, in order to gain better understanding of the radar backscattering from a slick-free and slick-covered water surface, particularly when it is agitated by strong rain. We used a coherent 9.8 GHz (X band) scatterometer at different polarisations and at an incidence angle of 28 degrees, a resistance-type wire gauge, and a two-dimensional laser slope gauge. A water surface area of 2.3 m2 was agitated by strong artificial rain with a rain rate of 160 mm h−1 and rain drops of 2.9 mm diameter. The wind-speed range used in the present investigation was between 2 and 10 m s−1. A monomolecular surface film was produced by deploying oleyl alcohol on the water surface. The results of the analyses of the measured radar Doppler spectra and wave amplitude and slope spectra are presented. We show that while the wind-induced surface roughness is strongly reduced in the presence of the slick, at a high rain rate (of 160 mm h−1) the surface slick less strongly affects the rain-induced increase of the surface roughness (i.e., the generation of crowns, cavities, stalks, ring waves, and secondary drops).

We have carried out wind-wave tank measurements to investigate the small-scale roughness of the water surface at the edges of monomolecular surface films. Small amounts of oleyl alcohol, triolein, and palmitic acid methyl ester were carefully deployed on the water surface at wind speeds between 3 m/s and 9 m/s, thus allowing for wave measurements at either end (i.e., downwind and upwind) of the surface films. Data were acquired using a 10 GHz (X-band) scatterometer, and a two-dimensional laser slope gauge, and a video camera. We found that the effect of slick-edge asymmetry (i.e., a sharp downwind edge and a diffuse upwind edge) depends on both the deployed substance and the wind speed. Due to different states of film compression at the downwind or upwind edges of the surface film, the damping capability of the slick material can vary in these areas. This in turn may cause a varying spectral density of small-scale waves, which are bound to the dominant wind waves, while the slick is drifting by. This effect is shown and interpreted qualitatively using the video data, and quantitatively using the scatterometer and laser gauge data.

At the wind-wave tank of the University of Hamburg the authors carried out laboratory measurements with artificial rain and wind. The radar backscatter from the rain- and wind-roughened water surface was measured with a coherent 9.8 GHz (X-band) scatterometer at different polarizations (VV, HH, and HV) and incidence angles (28/spl deg/, 40/spl deg/, and 55/spl deg/). At VV-polarization the radar Doppler spectra show two Doppler-shifted maxima at all incidence angles. At HH-polarization a third maximum is visible at zero Doppler shift. With increasing incidence angle the two Doppler-shifted maxima are weakened, whereas the third (non Doppler-shifted) maximum increases. The authors explain this as being an effect of the different backscatter mechanisms caused by the different splash products, like Bragg scattering at ring waves and scattering at the static splash products like stalks. They measured that the relative radar cross sections at HV-polarization increase with increasing high rain rate (50 mm/h and 100 mm/h), which they attribute to enhanced edge and multiple scattering.

Includes remote sensing data from the same algae bloom in the northern Baltic (north of Gotland) acquired by four different sensors, including one from a radar. All satellite scenes were acquired on July 15, 1997, over the same area within less than two hours. Using this extensive data set, for the first time, an investigation of the usefulness of the fusion of different remote sensing data (particularly by incorporating radar data) for monitoring of an ongoing algae bloom has been performed. The results of the analysis show that the fusion of optical and microwave remote sensing data (even with different spatial resolutions) can yield more information about the biological and oceanic characteristics of sea surface areas.

Wind and current variations at the ocean surface can give rise to a modulation of the sea surface roughness and thus become visible in radar images. The discrimination between radar signatures of oceanic and atmospheric phenomena can be difficult, since signatures of different origin can have very similar shapes and magnitudes and are often superimposed upon each other. From an oceanographer's point of view it is therefore important to identify characteristics of radar signatures of atmospheric phenomena to distinguish them from those of oceanic phenomena as well as to profit from the information they contain on parameters like, e.g., wind speed. In this work the authors employ a numerical radar imaging model for an investigation of typical properties of radar signatures of atmospheric convective cells. They show that main characteristics of observed multifrequency/multipolarization radar signatures of atmospheric convective cells over the Gulf Stream are reproduced quite well by the proposed model. This encourages them to vary wind and radar parameters systematically in order to get an overview of the dependency of atmospherically induced radar signatures on these parameters.

The authors present data from four different satellite sensors, which have been acquired over the same sea area during an algae bloom, within less than two hours. The data includes: the Thematic Mapper (TM) on Landsat-5, the synthetic aperture radar (SAR) aboard the Second European Remote Sensing Satellite (ERS-2), the Wide Field Scanner (WiFS) aboard the Indian IRS-1C satellite, and the Advanced Very High Resolution Radiometer (AVHRR) flown on the NOAA-14 satellite. Using this extensive data set, for the first time, an investigation of the usefulness of the fusion of different remote sensing data (particularly by incorporating radar data) for monitoring of an ongoing algae bloom has been performed. The results of their analysis show that the fusion of optical and microwave remote sensing data (even with different spatial resolutions) can yield more information about the biological and oceanic characteristics of sea surface areas.

Radar signatures of different mineral oil spills were measured by an airborne five-frequency, (L-, S-, C-, X-, and K/sub u/-band) four-polarization (HH, HV, VV and VH), microwave scatterometer during a controlled oil spill experiment in the North Sea. The damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, increases monotonically for Bragg wavenumbers from k/sub B/=20 m/sup -1/ to k/sub B/=500 m/sup -1/. The damping ratio depends on the oil type and the thickness of the oil layer, but was found to be independent of the polarization and the look direction of the radar relative to the wind direction for wind speeds between 6 m/s and 10 m/s. These experimental results are interpreted in terms of a theoretical model on the damping of short gravity-capillary waves by surface films. >

Rain showers (rain cells) over the sea can be monitored by active microwave sensors because of two processes: (1) the scattering and attenuation of the microwaves by raindrops in the atmosphere and (2) the modification of the sea surface roughness and thus its radar backscatter cross section by the impact of raindrops and by wind gusts associated with rain cells. The first process is used by ground-based weather radars which measure the power backscattered by raindrops in the atmosphere, from which the rain rate can be derived. A combination of both processes causes signatures of rain cells on synthetic aperture radar (SAR) images of the sea. Such signatures consist of irregularly-shaped bright and dark patches with a diameter of several kilometers, often embedded in almost circular structures. In this study, for the first time SAR images of European marginal seas acquired by the C band SAR aboard the European remote sensing satellites ERS-1 and ERS-2 are compared with concurrent C band weather radar images, which allows to unambiguously attribute signatures on the SAR images to rain cells. The study shows even rain cells with relatively low rain rates (

The authors have carried out wind-wave tank measurements of wave slopes, radar backscattering, and video image intensity, while the water surface was covered with small patches of surface-active material. Their results show that, at low wind speed, the slick patches consist of regions of different damping behavior caused by the freshly spread slick. By analyzing the radar Doppler shifts they could show that the distribution of bound and freely propagating waves along a slick patch strongly varies. Since the video camera is less sensitive to the observed small-scale features they support the use of such devices for automated slick detection.

The authors introduce a new method for the remote sensing of coastal underwater bathymetry by radar. In contrast to existing methods on the basis of backscatter intensity signatures, the proposed method is based on the along-track interferometric synthetic aperture radar technique, which allows direct measurements of scatterer motions and, thus, of spatially varying surface current fields over bathymetric features. Promising results are shown by the first field experiment.

The authors have performed radar backscattering measurements with a scatterometer in a wind-wave tank under different wind and rain conditions. The aim of these experiments was to gain further insight into those mechanisms which are responsible for the combined rain- and wind-induced modifications of the water-surface roughness and, thus, of the radar backscatter. The results from their measurements imply that the ratios of the co-polarization and cross-polarization backscatter may help to estimate rain rates, and therefore enable the authors to interpret radar images of rain cells more quantitatively.

Laboratory measurements in the wind-wave tank of the University of Hamburg with artificial rain have been carried out by using a coherent X-band scatterometer, a resistance-type wire gauge and a two-dimensional laser slope gauge. The authors used raindrops with a diameter of 2.9 mm, rain rates between 160 mm/h and 300 mm/h, and wind speeds between 2 m/s and 12 m/s. They show that, by comparing the along-wind and the crosswind slope spectra, a wind dependent orientation of the entire wave field (consisting of wind- and rain-generated waves) can be found. From the X-band Doppler spectra measured at low wind speeds they infer that the backscattered radar power (BRP) at co-polarization is caused by the rain-generated ring waves, whereas at cross-polarization it is associated with the cavities and stalks generated by the impinging rain drops. Moreover, they measured a strong increase of the relative BRP, /spl sigma//sub rel/, at co-polarizations (VV, HH) in the presence of rain at wind speeds below approximately 8 m/s. However, at cross-polarization (HV) /spl sigma//sub rel/ was enhanced more strongly in the presence of rain than at co-polarization for all wind speeds (i.e. up to 12 m/s) and rain rates.

More than 400 synthetic aperture radar (SAR) images acquired over the southern Baltic Sea, the North Sea, and the Gulf of Lions in the Mediterranean Sea by the Second European Remote Sensing Satellite (ERS-2) have been analyzed since December 1996 with respect to radar signatures of (natural and anthropogenic) oil pollution and other phenomena causing similar signatures. First results of this analysis reveal that the seas are most polluted along the main shipping routes. SAR images acquired during descending (morning) and ascending (evening) satellite passes show different percentages of oil pollution, because most of this pollution occurs during night time and is still visible on the SAR images acquired in the morning time. By using an ERS-2 SAR image of the North Sea test area we show how the reduction of the normalized radar backscattering cross section (NRCS) by an oil spill depends on wind speed. In order to point out the difficulties in detecting oil pollution undoubtedly, we finally present signatures of atmospheric and oceanic phenomena which are very similar to those of marine pollution.

The authors compare multi-channel radar images of the Gulf Stream front off the east coast of the US with results of numerical simulations based on available in-situ data. The radar images used for this study were acquired during the SIR-C/X-SAR campaign in 1994. According to model predictions, spatial variations in the surface current field and in the near-surface wind field at the Gulf Stream front can cause radar signatures of comparable shape and modulation depth. On the other hand, the individual dependencies of radar signatures of oceanic and atmospheric origin on radar frequency and polarization can be quite different. This enables the authors to identify radar signatures of oceanic and atmospheric origin in multi-channel data and to estimate the corresponding current and wind variations independently. By varying current and wind parameters at the Gulf Stream front within reasonable ranges, the authors arrive at a most likely scenario of oceanic and atmospheric conditions during the Shuttle overflight, for which good agreement between observed and simulated radar signatures is obtained at all radar channels.

In order to gain further insight into the radar backscattering from rain-and wind-roughened water surfaces, we performed field experiments at the mouth of the river Elbe in the German Bight. The backscattered radar power from the water surface was measured at different wind speeds, wind directions, and rain rates, as well as at different radar frequencies, polarizations, and incidence angles. Our results show that the dominant scattering mechanism at VV-polarization is Bragg scattering which shows a strong dependence on the Bragg wavelength. The enhancement and reduction of the surface roughness by splash products and sub-surface phenomena, respectively, result in a transition wavenumber between enhancement of the radar backscattering (due to additional surface roughness) and its reduction (due to wave damping) of about 100 rad/m. We hypothesize that this transition wavenumber depends on the drop-size distribution of the rain. Taking into consideration the different dependencies on rain rate of the radar backscatter at different frequencies and polarizations, we suggest a method to estimate rain rates by calculating the ratio of the radar cross-sections at L band, VV-polarization and at C band, HV-polarization. Provided an availability of synthetic aperture radar (SAR) data at the respective frequency/polarization combinations, this method will allow for investigating the nature of small-scale (convective) rain events over the ocean by analyzing multi-frequency/multi-polarization SAR images.

In the wind-wave tank of the University of Hamburg laboratory measurements have been carried out in the presence of artificial rain and wind. By using a resistance type wire gauge the surface wave elevation, and by using a laser slope gauge the surface slope was measured. The radar backscattering from the water surface was measured with a coherent 9.8 GHz (X-band) scatterometer, First results of this study show that the wave spectra are affected differently by the rain in different spectral domains. The wave spectra are enhanced in the frequency range of about 5 Hz to 100 Hz, while they are reduced at frequencies smaller than about 5 Hz. By comparing wave slope spectra measured in downwind and crosswind direction the authors found that the rain affects the orientation of the wave spectra: the transition region from an isotropic to an anisotropic wave spectrum increases with wind speed. By operating the scatterometer at different polarizations (VV, HH, and HV) they measured a strong increase of the relative backscattered radar power (/spl sigma//sub rel/) in the presence of rain at wind speeds below approximately 8 m/s at all polarizations. At wind speeds above 8 m/s, only /spl sigma//sub rel/ at cross-polarization increases in the presence of rain. The strongest increase, however, was measured at cross-polarization, which is in accordance with observations which the authors obtained from the analysis of multifrequency-SAR data.

The oil pollution of the southern Baltic Sea, the North Sea, and the northwestern Mediterranean Sea has been studied within a two-year period from December 1996 until November 1998. In total, the authors have analyzed more than 700 synthetic aperture radar (SAR) images, which have been acquired over the three test areas by the Second European Remote Sensing Satellite (ERS-2). The authors present the results of their statistical analyses. Moreover, the authors introduce advanced image processing techniques for the classification of the observed radar signatures, namely the calculation of the fractal dimension, and discuss the possibility of identifying different types of oceanic phenomena.

The authors present three pairs of concurrent images of rain cells over the sea which have been acquired by the C-band synthetic aperture radar (SAR) aboard the European Remote Sensing Satellites ERS-1 and ERS-2 and by local weather radars. By comparing these data sets the authors find that the part of the radar signature caused by the wind gusts associated with the rain cell usually covers a much larger area than the part of the radar signature caused by the impinging raindrops. Furthermore, in the center of the quasi-circular patterns visible on SAR images of the sea surface, often areas are encountered that have an enhanced or a reduced roughness relative to its surroundings. They conjecture that this is caused by the spatial and temporal variability of the rainfall within the rain cell.

The possibilities to use different types of satellite data to detect and monitor algae blooms is investigated in this study. Two cases are presented, both during extensive algae blooms in the Baltic Sea at times when data from several satellite sensors are available. In the first case on July 15 1997, data from four different sensors, including radar data, were acquired within a time frame of two hours. In the second study from the end-of-July 1999, the acquired data set stretches over 5 consecutive days. The total (possible) number of satellite scenes from the second period is more than 30. In the second case-study ocean colour data were available, a data type unfortunately lacking in the first study. The results of the two studies, preliminary for the second case, show that the combination of various satellite sensors not only provides a better coverage in space and time of an algae bloom, but also yields more information about the biological and oceanic characteristics of the sea surface.

Results of the analyses of SIR-C/X-SAR images from the test site North Sea as well as from other sea areas showing radar signatures of natural surface films and of oil spills are presented. Damping ratios (i.e., the ratios of the normalized radar cross section from a slick-free and a slick-covered water surface) and polarization signatures have been calculated and compared. A strong wind dependence of the damping ratio for the same substance is shown and explained by means of the source terms for the spectral energy in the ocean wave spectrum. It is therefore concluded that surface films of different visco-elastic properties can only be discriminated at low to moderate wind speeds. Furthermore, polarization signatures from film-covered as well as from film-free water surfaces have been calculated and compared. Differences between them are found only at low wind speeds and small incidence angles which is in accordance to Bragg scattering theory.

During the two SIR-C/X-SAR missions in April and October 1994, a large number of multifrequency/multipolarization synthetic aperture radar (SAR) images were acquired from the Space Shuttle "Endeavour" which show radar signatures of rain cells over land as well as over the ocean. The authors discuss the radar signatures of tropical rain cells that were acquired over the Brazilian rain forest and over the Gulf of Mexico. It is shown that multifrequency/multipolarization radar imagery has the potential to yield valuable information on tropical rain cells, especially on the altitude of the melting layer and the rain rate.

W-band backscatter experiments have been performed at the wind-wave tank of the University of Hamburg. The aim of the experiments is to study the backscatter mechanisms under shallow incidence angles and to compare the results with previous ones obtained with an X-band scatterometer. The measurements were performed at three grazing angles, 7.5/spl deg/, 10/spl deg/, and 20/spl deg/, and at VV-, HH-, and VH-polarization. The wind speed ranged from 2 to 10 ms/sup -1/ and was increased in steps of 1 ms/sup -1/. The analysis of the acquired radar Doppler spectra shows that Bragg scattering from both bound and freely propagating Bragg waves is the dominant backscattering mechanism at all deployed wind speeds. In particular, at low wind speeds (up to 4 ms/sup -1/), when the overall backscatter is small, bound waves axe the dominating scatterers, whereas at higher wind speeds (5 ms/sup -1/ and above) the acquired signal is mainly caused by freely propagating Bragg waves. This is in qualitative agreement with similar findings, which have been made earlier with an X-band scatterometer working at moderate incidence angles.