Spaceborne Imaging Radar Research Articles

Spaceborne imaging radar system based on gyrotron in terahertz band design and simulation

Spaceborne imaging radar system based on gyrotron in terahertz band design and simulation

The direct use of radar satellites for event-specific flood risk mapping

A near-simultaneous satellite acquisition of a flood event on 12 December 2006 on the Dee River in Wales (UK) is used to illustrate the potential to develop standalone space-borne flood risk mapping techniques. Both the ERS-2 and ENVISAT satellites recorded the event close to flood peak and only 28 min apart. This unique opportunity enables the creation of a very rare but extremely useful observed data set for flood inundation studies. This letter illustrates how this unique set of space-borne radar images can be used for rapid flood risk mapping. An event-specific weighted hazard map was generated based on plausible flood area observations from an aggregation of widely applied image-processing techniques. This map can be further augmented to an event-specific fuzzy flood risk map by fusing the multi-algorithm ensemble map with vulnerability-weighted land cover vector data in a geographic information system (GIS) environment. The technique presented is fairly flexible and leads to a potentially useful data set for direct use in flood management.

Study of Hypersaline Deposits and Analysis of Their Signature in Airborne and Spaceborne SAR Data: Example of Death Valley, California

Field measurements of dielectric properties of hypersaline deposits were realized over an arid site located in Death Valley, CA. The dielectric constant of salt and water mixtures is usually high but can show large variations, depending on the considered salt. We confirmed values observed on the field with laboratory measurements and used these results to model both the amplitude and phase behaviors of the synthetic aperture radar (SAR) signal at C- and L-bands. Our analytical simulations allow reproducing specific copolar signatures observed in both Airborne SAR (AIRSAR) and Spaceborne Imaging Radar (SIR-C) data, corresponding to the saltpan of the Cottonball Basin. More precisely, the main objective of the present paper is to understand the influence of soil salinity as a function of soil moisture on the dielectric constant of soils and then on the backscattering coefficients recorded by airborne and spaceborne SAR systems. We also propose the copolarized backscattering ratio and phase difference as indicators of moistened and salt-affected soils. More precisely, we show that these copolar indicators should allow monitoring of the seasonal variations of the dielectric properties of saline deposits at both C- and L-bands. Because of the frequency dependence of the ionic conductivity, we also show that L-band SAR systems should be efficient tools for detecting both soil moisture and salinity, while C-band SAR systems are more suitable for the monitoring of soil moisture only. Through the study of terrestrial evaporitic environments by means of spaceborne SAR systems, our results could also be of great interest for defining future planetary missions, particularly for the exploration of Mars.

The Utility of Spaceborne Radar to Render Flood Inundation Maps Based on Multialgorithm Ensembles

On December 12, 2006, both the European Remote Sensing Satellite 2 and Environmental Satellite recorded a high-magnitude flood event on the River Dee in Wales (U.K.) only 28 min apart. This unique opportunity enables the creation of a very rare but extremely useful observed data set for flood inundation studies. For flood management purposes, hydrodynamic models are often run after an event but with field data gauged during the event to approximate both flood area and depth. As an adequate a priori definition of model parameters is difficult, they tend to be run with multiple parameter sets to generate a likelihood of inundation map. However, as field observations of events are often very scarce, these output maps cannot be validated with field-observed probabilities. This paper illustrates how this unique set of spaceborne radar images can be used in combination with five widely used image processing techniques to generate an event-specific inundation map that expresses a degree of belief that a given pixel is possibly flooded. It is expected that the value of this multialgorithm ensemble-based map opens up new ways to evaluate the performance of hydrodynamic models, as it contains information which has, to the authors' knowledge, not previously been available.

Lacunarity analysis of raster datasets and 1D, 2D, and 3D point patterns

Spatial scale plays an important role in many fields. As a scale-dependent measure for spatial heterogeneity, lacunarity describes the distribution of gaps within a set at multiple scales. In Earth science, environmental science, and ecology, lacunarity has been increasingly used for multiscale modeling of spatial patterns. This paper presents the development and implementation of a geographic information system (GIS) software extension for lacunarity analysis of raster datasets and 1D, 2D, and 3D point patterns. Depending on the application requirement, lacunarity analysis can be performed in two modes: global mode or local mode. The extension works for: (1) binary (1-bit) and grey-scale datasets in any raster format supported by ArcGIS and (2) 1D, 2D, and 3D point datasets as shapefiles or geodatabase feature classes. For more effective measurement of lacunarity for different patterns or processes in raster datasets, the extension allows users to define an area of interest (AOI) in four different ways, including using a polygon in an existing feature layer. Additionally, directionality can be taken into account when grey-scale datasets are used for local lacunarity analysis. The methodology and graphical user interface (GUI) are described. The application of the extension is demonstrated using both simulated and real datasets, including Brodatz texture images, a Spaceborne Imaging Radar (SIR-C) image, simulated 1D points on a drainage network, and 3D random and clustered point patterns. The options of lacunarity analysis and the effects of polyline arrangement on lacunarity of 1D points are also discussed. Results from sample data suggest that the lacunarity analysis extension can be used for efficient modeling of spatial patterns at multiple scales.

A study on the polarimetric properties of various features using SIR-C data

Spaceborne Imaging Radar (SIR-C) data acquired over Gujarat, India in 1994 were processed and analysed using differnet techniques applicable to polarimetric SAR data such as polarization signatures, polarization index, decomposition of the signal and polarization phase difference and limited groundtruth data. It has been observed that multi-frequency polarimetric data enhances the potential of retrieving geo-physical parameters. The polarization signatures are found to vary with the nature of the target. Target decomposition of the returned signal will be useful for the classification of various features. Polarization Phase Difference (PPD) gives good information about the vegetation parameters.

Retrieval of soil moisture using SIR-C polarimetric data

Spaceborne Imaging Radar (SIR-C) polarimetric data acquired over Gujarat test site, India, during April and October 1994 were processed to retrieve soil moisture and surface roughness using multi-polarization techniques. Synchronous field data were collected and compared with the results obtained using SIR-C data. Indian Remote Sensing Satellite (IRS) images in visible region were used for locating groundtruth points. Multi-polarization inversion techniques are found to be sensitive to retrieval of soil moisture and surface roughness parameters. However, the accuracy is not adequate. There is a need to improve the existing inversion models to suit to the Indian agricultural fields.

Generalized minimum-error thresholding for unsupervised change detection from SAR amplitude imagery

The availability of synthetic aperture radar (SAR) data offers great potential for environmental monitoring due to the insensitiveness of SAR imagery to atmospheric and sunlight-illumination conditions. In addition, the short revisit time provided by future SAR-based missions will allow a huge amount of multitemporal SAR data to become systematically available for monitoring applications. In this paper, the problem of detecting the changes that occurred on the ground by analyzing SAR imagery is addressed by a completely unsupervised approach, i.e., by developing an automatic thresholding technique. The image-ratioing approach to SAR change detection is adopted, and the Kittler and Illingworth minimum-error thresholding algorithm is generalized to take into account the non-Gaussian distribution of the amplitude values of SAR images. In particular, a SAR-specific parametric modeling approach for the ratio image is proposed and integrated into the thresholding process. Experimental results, which confirm the accuracy of the method for real X-band SAR and spaceborne imaging radar C-band images, are presented

Analysis of a Microwave Backscattering Mechanism From a Small Urban Area Imaged With SIR-C

We analyzed the Spaceborne Imaging Radar C (SIR-C) imagery of a small urban area to characterize backscattering mechanisms. The results were compared with the building density so that we could relate the polarimetric backscattering characteristics with urban structures. Relative contributions of odd-bounce, double-bounce, and cross-polarized scatterings were evaluated as a function of the building density for small and large incidence angles. Their behaviors were characterized with respect to urban structures at small and large incidence angles.

Coherence Optimization Using the Polarization State Conformation in PolInSAR

A new coherence optimization algorithm is proposed for polarimetric synthetic aperture radar (SAR) interferometry applications by using the polarization state conformation algorithm based on the polarimetric basis transformation along with the polarization signatures. Through application of this algorithm, the resemblance between the scattering mechanisms of the same target in the repeat-pass polarimetric SAR (POLSAR) images is maximized. Then, coherence maps between the repeat-pass POLSAR images, before and after application of the algorithm, are generated. The coherences obtained by this method represent the best coherences or optimized coherences between the POLSAR images. The effects predicted by the theory are confirmed by the POLSAR data acquired by the Jet Propulsion Laboratory Spaceborne Imaging Radar mission.

Structural control of sinkholes and subsidence hazards along the Jordanian Dead Sea coast

For about four decades, the Dead Sea (DS) level and the surrounding water table has been dropping dramatically. At least from the eighties, the direct vicinity of the Lisan Peninsula (LP), Jordan, has been facing high rates of subsidence and sinkhole hazards. Between 2000 and 2002, the Arab Potash Company (APC) lost two salt evaporation ponds resulting in a loss of $70 million. In the fertile plain of Ghor al Haditha (GAH), three deep and wide bowl-shaped subsidence areas threaten human activities and infrastructures. Over the part of the Lisan Peninsula that emerged before the 1960s, relict fossil sinkholes occurred everywhere, whereas new collapses constantly appear in the southern area only. In this paper, we have integrated 15 years of field observations related to sinkholes and subsidence with interpretation of space borne radar interferometric outputs, aerial photographs and satellite images. This has helped to place hazardous areas in their geological context and to clarify them within the framework of the general tectonic setting of the area.

Produção de carta topográfica por estereoscopia de alta resolução do radarsat-1 integrada com dados tm-landsat 5: uma avaliação para terrenos planos na Floresta Nacional De Tapajós (PA)

The feasibility of production of topographic maps based on orbital remote sensing data was investigated for the Brazilian Amazon Region. The selected test-site was an area with a low topographic relief within the National Tapajos Forest, Para State. Two kinds of radargrammetric DEMs (Digital Elevation Models), relative and absolute, were generated based on a Fine RADARSAT-1 stereo-pair (Fine 2, Fine 5) and evaluated regarding the altimetric information content. The geometric modeling for the DEM extractions was based on the “RADARSAT-1 Specific” model from the OESE (OrthoEngine Satellite Edition, PCI Geomatics) software, an automated matching solution which takes into account slant range distances from sensor/terrain. The planimetric features were extracted from integrated Fine/TM-Landsat products. Precise planialtimetric field information from DGPS (Differential Global Positioning System) was used as GCPs (Ground Control Points) for the modeling of the absolute DEM and for the orthorectification of the radar and optical data, and as ICP (Independent Control Points), for the calculation of planialtimetric accuracies of the topographic map. The investigation has shown that the planialtimetric accuracy of the topographic map met the requirements for 1:100,000-map (class A) as requested by the Brazilian Cartographic Accuracy Pattern (PEC). The spaceborne imaging radar technology represents na alternative to aerial optical and radar data for topographic mapping at semi-detailed scale (1:100,000) in similar environments of the Brazilian Amazon. The investigation was carried out through an INPE-Geoambiente`s project with funds from the PIPE Program of FAPESP.

PAMIR – a wideband phased array SAR∕MTI system

Air- and spaceborne imaging radar systems in forthcoming surveillance and reconnaissance tasks have to meet increasingly severe demands. The next generation of top-level synthetic aperture radar (SAR) systems will comprise, among others, high resolution and long-range imaging capabilities, highly sensitive ground moving target indication and a multitude of sophisticated operational modes. The variety of tasks can be fulfilled only by the use of a reconfigurable phased array antenna together with a comprehensive wideband system design and a multichannel capability. At FGAN a new experimental X-band radar has been conceived, which will possess in its final upgrade an electronically steerable phased array consisting of 16 autonomous and reconfigurable subapertures, five independent receive channels, and a total signal bandwidth of about 1.8 GHz. The sensor is called PAMIR (Phased Array Multifunctional Imaging Radar). It is envisaged to demonstrate SAR imaging at a very high resolution and for a long range. The fine resolution will also be achieved with inverse SAR (ISAR) imaging of ground moving objects. Furthermore, the number of receive channels will allow ground-moving target indication (GMTI) by space–time adaptive processing and single-pass interferometric SAR (IfSAR) with a very high 3-D resolution. In its current stage of extension PAMIR is operable with one receive channel and a mechanically steerable antenna array. The system design and the intended capabilities of PAMIR are described. Ground-based and airborne experimental results concerning high-resolution SAR and ISAR imaging are also presented.

A new scattering mechanism enhancement scheme for polarimetric SAR images

A new scattering mechanism enhancement scheme has been developed for natural (distributed) targets based on the eigenvalues and corresponding eigenvectors of the covariance matrix in order to identify different scattering events. First, three new vectors (v/sub 1/, v/sub 2/, and v/sub 3/) were constructed from the eigenvectors of the covariance matrix with some modifications. Then, those modified vectors were weighted by the eigenvalues of the covariance matrix as a weighting function. Thus, three vertices (A, B, and C) could be obtained in the three-dimensional space. In order to utilize them equally, a triangle (/spl Delta/ABC) was constructed by connecting these three vertices. The shape of the triangle may be changed due to the different scattering mechanisms because the vertices are obtained from the combination of eigenvalues and eigenvectors. The result indicated that different scattering mechanisms can be represented by using an exterior angle derived from the interior angles of the triangle. Results obtained from the newly developed scattering enhancement scheme, when compared with the results derived from existing schemes, were in agreement in terms of the dominant scattering mechanisms, including surface scattering, double-bounce scattering, and volume scattering. The experimental results with the Spaceborne Imaging Radar version C (SIR-C) L-band full polarimetric data demonstrate the effectiveness of the new scattering enhancement scheme.

Wave and wind retrieval from sar images of the ocean

Over the past few years, recognition of the importance of the coastal zone has led to the establishment of international programmes for monitoring the coastal zone environment and its change. The European programme marsais is part of this effort. One important component of such actions aims to better predict the sea surface wind and wave dynamics in these vulnerable regions where most economic marine activity is taking place. Indeed, ocean surface wind and wave observations serves both oceanographic and meteorological communities and have direct applications for driving ocean circulation models, numerical predictions and short term forecasts, but also for advancing in the physical understanding of the complex interactions that take place at the ocean-atmosphere interface. As now well recognised, satellite data and particularly the weather independent radar remote sensing data present potential advantages and applications to achieve these requirements. Nowadays, sea surface remote sensing techniques are rapidly developing throughout the world and need some kind of assessment. Altimetry and scatterometry are well proven techniques, which result in recognised operational applications. Synthetic Aperture Radar (sar) missions have not enjoyed such successes. However numerous space borne radar images of the ocean surface have revealed a wealth of information on different dynamical processes and sar images of the ocean surface very often reveal a remarkable range of signatures on the uppermost layers of the sea. These data have resulted in numerous quantitative scientific findings and theoretical advances in upper-layer and lower atmosphere dynamics. In this review, the main different techniques developed to retrieve surface wave and wind information are recalled. Illustrations are given for envisat wave mode products.

Radar Imaging Mechanism of the Seabed: Results of the C-STAR Experiment in 1996 with Special Emphasis on the Relaxation Rate of Short Waves due to Current Variations

Abstract During the field experiment of the Coastal Sediment Transport Assessment using SAR imagery project of the Marine Science and Technology program of the European Commission an Air–Sea Interaction Drift Buoy (ASIB) system was equipped with special sensors and instruments to measure the position, the water depth, the surface current velocity and direction, the modulation characteristics of short-wave energies, and relevant air–sea interaction parameters due to undulations in the seabed. The ASIB system was operated from on board a research vessel and the data were measured while the buoy drifted in the tidal currents across sand waves of the study area. All buoy measurements were analyzed by computing frequency spectra of low and high frequency waves (scalar spectra between 0.1 and 50 Hz). The whole range of short water waves was recorded by these in situ measurements on board the buoy, which is responsible for the backscattering of commonly used air- and spaceborne imaging radars. A comprehensive da...

Space Imaging Radar in Planetary Exploration and Earth Observation

Spaceborne imaging radars are becoming key tools for mapping Earth and planetary surfaces and near subsurface. Developments in the past decade are leading to capabilities for three-dimensional surface imaging, detection of surface subtle motion due to earthquakes or volcanic activities, color mapping of surface properties, assessment of vegetation cover, etc. In planetary exploration, the radar capability to penetrate through cloud and haze cover and the possibility of sounding below the surface cover are providing a special perspective on the planetary surface and near surface evolution. An overview of the techniques and technology of spaceborne imaging radars and their present and future potential applications is provided.

Parameterization of vegetation backscatter in radar-based, soil moisture estimation

The Integral Equation Model (IEM) was previously used in conjunction with an inversion model to retrieve soil moisture using multifrequency and multipolarization data from Spaceborne Imaging Radar C-band (SIR-C) and X-band Synthetic Aperture Radar (X-SAR). Convergence rates well above 90%, and small RMS errors were attained, for both vegetated and bare soil areas, using radar data collected during Washita 1994. However, the IEM was originally developed to describe the scattering from bare soil surfaces only, and, therefore, vegetation backscatter effects are not explicitly incorporated in the model. In this study, the problem is addressed by introducing a simple, semiempirical, vegetation scattering parameterization to the multifrequency, soil moisture inversion algorithm. The parameterization was formulated in the framework of the water–cloud model and relies on the concept of a land-cover (land-use)-based dimensionless vegetation correlation length to represent the spatial variability of vegetation across the landscape and radar-shadow effects (vegetation layovers). An application of the modified inversion model to the Washita 1994 data lead to a decrease of 32% in the RMSE, while the correlation coefficient between ground-based and SAR-derived soil moisture estimates improved from 0.84 to 0.95.

Sar Interferometric Coherence in Wide Band Systems

This paper presents a study of the interferometric coherence aimed to the understanding of the main sources of phase error of an operative wide band system. This objective is accomplished by finding a theoretical expression of coherence based on a suitable model capable of including the roughness of the surface, which produces decorrelation besides the conventional spectral overlapping approach. The analysis is validated by means of numerical simulations and controlled experiments in anechoic chamber, with the aim of extrapolating the obtained results to airborne and spaceborne imaging radars.

Potential for downscaling soil moisture maps derived from spaceborne imaging radar data

The presence of nonlinear relationships between surface soil moisture and various hydrologic processes suggests that grid‐scale water and energy fluxes cannot be accurately modeled without subgrid‐scale soil moisture information. For land surface and energy balance models run over continental‐ to global‐scale domains, accurate fine‐scale soil moisture observations are nearly impossible to obtain on a consistent basis and will likely remain so through the next generation of soil moisture remote sensors. In the absence of such data sets, an alternative approach is to generalize the statistical behavior of soil moisture fields across the relevant range of spatial scales. Downscaling procedures offer the possibility that the fine‐scale statistical properties of soil moisture fields can be inferred from coarse‐scale data. Such an approach was used for a 29×200 km transect of 25 m active radar data acquired over Oklahoma by NASA's spaceborne imaging radar imaging (SIR‐C) mission on April 12, 1994. Using a soil dielectric inversion model, the radar data were processed to provide estimates of surface soil dielectric values, which can be equated to volumetric soil moisture content. The soil moisture field along each strip was analyzed for evidence of spatial scaling for scales ranging from 100 to 6400 m. Results suggest that a spatial scaling assumption may not always be an appropriate basis for a downscaling approach. Prospects for the development of a more robust downscaling procedure for soil moisture are discussed.