SAR Missions Research Articles

From Previous C-Band to New X-Band SAR Systems: Assessment of the DInSAR Mapping Improvement for Deformation Time-Series Retrieval in Urban Areas

We investigate the capability improvement of the advanced differential interferometric synthetic aperture radar (DInSAR) techniques to map deformation phenomena affecting urban areas by exploiting multitemporal SAR data acquired by the new X-band sensors with respect to those of the previous C-band systems. In particular, we perform a comparative analysis of the deformation time-series retrieved by applying the full-resolution Small BAseline Subset DInSAR technique to selected sequences of SAR data acquired by the ENVISAT and RADARSAT-1 sensors (both operating at C-band) and by the X-band radar systems onboard the SAR sensors of the COSMO-SkyMed (CSK) constellation. This study, focused on the city of Napoli (Italy), allows us to quantify the dramatic increase of the DInSAR coherent pixel density achieved by exploiting the high-resolution X-band CSK SAR images (a few meters), resulting in an improvement factor of about 320% and 550%, with respect to the RADARSAT-1 and ENVISAT products, respectively. This improvement permits us to analyze nearly all the structures located within the investigated urbanized area and, in many cases, also portions of the same building. The improved coherent pixel spatial densities, combined with the reduced revisit times of the new X-band SAR missions, allow us to significantly increase the effectiveness of the advanced DInSAR methodologies, further extending the role of those Earth Observation data in the development of monitoring scenarios.

Toward Fully Automatic Detection of Changes in Suburban Areas From VHR SAR Images by Combining Multiple Neural-Network Models

Recent X-band SAR missions, such as COSMO-SkyMed (CSK), which is able to provide very high spatial resolution images of an area of interest with a short revisit time, are expected to be quite useful sources of information for monitoring the terrestrial environment and its changes. On the other hand, the huge amount of data involved, as well as the need to promptly act in case of emergency, requires the development of automatic change detection tools. This paper reports on a novel automatic change detection algorithm combining multilayer perceptron neural networks (NNs) and pulse coupled NNs, which has been implemented and tested on pairs of Stripmap and Spotlight CSK images acquired on the Tor Vergata University area in the southeast outskirts of Rome, Italy, where a significant and continuous urbanization process is occurring.

Phase Pattern Calibration for Interferometric Applications in Spaceborne SAR Systems

SAR is a widely used technique to acquire images for geoscience and earth observation applications. Active phased array antennas are commonly used in spaceborne SAR systems. For certain modes and applications, it is necessary to know the phase behavior of these phased array antennas. For applications utilizing the different polarization channels for interferometry, the phase difference between the polarizations needs to be calibrated very accurately as it is the main evaluation parameter. Also for single-pass interferometric missions, the difference between the two antennas in terms of phase gradients is of major importance. This paper demonstrates for the first time the usage of phase patterns in an operational interferometric SAR mission. It describes why these phase patterns are required and how they are used to fulfill the different goals of the missions. Then, the mathematical model to derive the phase of the antenna patterns is shown. Finally, the paper explains how the antenna patterns are calibrated in order to minimize their residual errors and describes in detail the measurements performed for this calibration and verification.

TanDEM-X: DEM acquisition in the third year era

TerraSAR-X add-on for digital elevation measurement (TanDEM-X) is a space-borne X-band SAR mission designed to derive a digital elevation model (DEM) of the Earth’s land surface with an unprecedented relative vertical accuracy of 2 m at a 12 m posting. To achieve this goal, the two satellites fly in close controlled formation with the opportunity to adjust flexible along- and across-track baselines. A combination of multiple acquisitions with different baseline lengths is required to allow stable phase unwrapping and to achieve the high height accuracy. This paper provides an overview of the TanDEM-X acquisition plan for the third year over Antarctica, mountainous areas and deserts. Critical aspects of the areas to be acquired are presented and the acquisition strategy is illustrated. Moreover, the formation flight and the baselines are compared to the ones of the previous years in terms of their impact on the height of ambiguity and on the performance with respect to the interferometric quality.

Multichannel synthetic aperture radar systems with a planar antenna for future spaceborne microwave remote sensing

Future spaceborne SAR missions will require complete and frequent coverage of the Earth with a reasonably high geometric resolution. To achieve this imaging capacity, multichannel SAR systems with a planar antenna will be widely adopted in the next generation spaceborne microwave remote sensing missions. However, the high data rate, the increased system complexity, the difficulty of multichannel signal processing, the heavy load, and the wide bandwidth downlink may hamper the demonstration of these imaging modes. With the rapid development of electronic engineering and digital signal processing, these problems will be well resolved. Furthermore, regarding the implementation complexity, multichannel SAR systems with a deployable reflector antenna seem to be a better alternative for HRWS imaging [17, 18]. However, the major drawback of the reflector antenna is the limited beam-steering capacity compared with a planar antenna, which will affect some of the remote sensing applications. Thus, multichannel SAR systems with a planar antenna rather than a reflector are more suitable for the higher geometric resolution, especially for the higher-carrier frequency (such as X-band).

Opposite-side radargrammetry of Magellan synthetic aperture radar on Venus

In the early 1990s, synthetic aperture radar onboard Magellan spacecraft has imaged 54% of the Venusian surface in the opposite-side stereo configuration while only 21% in the same-side. Much of the data for the opposite-side radargrammetry (OSR) on Venus were underutilized due to higher topographic distortion than the same-side radargrammetry (SSR). In this paper, the performance and limitations of the OSR are demonstrated based on an example over Lullin impact crater, Venus. The initial stereo match points are classified into three categories; GOOD, BAD, and TOPO, and different methods are suggested for each category to improve the quality of digital elevation model. Large areas of gentle slope on Venusian volcanic plain can be successfully matched using conventional stereo-matching algorithm. Image contrast of featureless surface can be improved by enlarging the master image patch at a cost of spatial resolution, while high to moderate relief regions need further treatments such as radarclinometry. Utilization of OSR is expected to provide additional DEM dataset of Venus and to give flexibility for the design of future planetary SAR missions.

Investigation on Electronic Azimuth Beam Steering in the Spaceborne Sar Imaging Modes

This paper reviews different spaceborne SAR imaging modes with electronic azimuth beam steering adopted for greater azimuth resolution in spotlight/sliding spotlight mode and wider swath coverage in TOPS (Terrain Observation by Progressive Scans) mode. However, the use of electronic beam steering via a planar phased antenna, especially in the spaceborne SAR system, leads to a quantization of the beam steering law, grating lobes and main lobe gain reduction. The distorted synthetic azimuth antenna pattern (AAP) due to angle quantization and squinted angle will affect system performances. These effects in different imaging modes are analyzed in detail for the design of future spaceborne SAR missions.

A Feasibility Assessment for Low-Cost InSAR Formation-Flying Microsatellites

Multistatic interferometric synthetic aperture radar (InSAR) is a promising potential payload for a small satellite constellation. CanX-4 and CanX-5 are a pair of formation-flying nanosatellites launching in 2009; once formation flight has been demonstrated, a future multistatic InSAR constellation of low-cost microsatellites can exploit subcentimeter intersatellite baseline knowledge, with digital elevation map height errors on the order of 1 m in the flat-terrain case. This paper evaluates the feasibility of such a mission, using case studies of commonly proposed configurations: the Interferometric Cartwheel, the Cross-Track Pendulum, and the Cartwheel-Pendulum (Car-Pe) configuration. In each case, several SAR transmitters are considered: L-, C-, and X-band transmitters with parameters mirroring existing ldquolargerdquo satellite SAR missions, and a theoretical X-band microsatellite transmitter. The available interferometric baselines, ground coverage, and image resolutions are evaluated in each scenario. The X-band ldquomicrordquo transmitter is feasible, but the low transmit power severely limits the ground coverage. The X-band ldquolargerdquo transmitter provides the largest ground coverage and the highest resolution along with the X-band ldquomicrordquo option. The resolutions are wavelength dependent and remain relatively constant among the configurations. The operating areas of the pendulum demonstrate the largest degree of overlap, while the longer along-track baselines of the cartwheel result in a smaller overlap. Both two-receiver (pendulum and cartwheel) configurations demonstrate baseline characteristics that may be optimal for different applications, while the three-receiver Car-Pe demonstrates the advantages of both the pendulum and cartwheel.

Improving Search and Rescue Using Contextual Information

Search and rescue (SAR) is a challenging application for autonomous robotics research. The requirements of this kind of application are very demanding and are still far from being met. One of the most compelling requirements is the capability of robots to adapt their functionalities to harsh and heterogeneous environments. In order to meet this requirement, it is common to embed contextual knowledge into robotic modules. We have previously developed a context-based architecture that decouples contextual knowledge, and its use, from typical robotic functionalities. In this paper, we show how it is possible to use this approach to enhance the performance of a robotic system involved in SAR missions. In particular, we provide a case study on exploration and victim detection tasks, specifically tailored to a given SAR mission. Moreover, we extend our contextual knowledge formalism in order to manage complex rules that deal with spatial and temporal aspects that are needed to model mission requirements. The approach has been validated through several experiments that show the effectiveness of the presented methodology for SAR.

Permanent Scatterers for landslide investigations: outcomes from the ESA-SLAM project

Within the SLAM project (Service for Landslide Monitoring), launched in 2003 by the European Space Agency (ESA) the Permanent Scatterers (PS) technique, a multi-image interferometric approach, coupled with the interpretation of aerial-photos and optical satellite images, was carried out for landslide investigations. The PS analysis was applied at a regional scale as support for landslide inventory mapping and at local scale for the monitoring of single well-known slope movements. For the integration of the PS measurements within a landslide inventory the Arno river basin (Italy) was chosen as test site for the presence of a high number of mass movements (to date about 300 areas at high landslide risk and more than 27,000 individual landslides mapped by the institutional authorities). About 350 SAR images have been interferometrically processed by means of the PS technique, with the detection of about 600,000 PS. The use of optical images contributed spatial meaning to the point-wise information provided by the PS, making it easier to identify terrain features related to slope instability and the landslide boundaries. Here we describe the employed methodology and its impact in the updating of a preexisting landslide inventory. 6.8% of the total number of landslides were characterized by ground displacement measurements from the PS: 6.1% of already mapped landslides and 0.8% of new unstable areas detected through the PS analysis. Moreover, most of the PS are located in urban areas, showing that the proposed methodology is suitable for landslide mapping in areas with a quite high density of urbanization, but that over vegetated areas it still suffers from the limitations induced by the current space-borne SAR missions (e.g. temporal de-correlation). On the other hand, the use of InSAR for the monitoring of single slow landslides threatening built-up areas has provided satisfactory results, allowing the measurement of superficial deformations with high accuracy on the landslide sectors characterized by a good radar reflectivity and coherence.

ONERA-DLR bistatic SAR campaign: planning, data acquisition, and first analysis of bistatic scattering behaviour of natural and urban targets

Bistatic configuration is an attractive concept for spaceborne and airborne SAR missions when distributed radars are necessary as for example in the case of interferometric applications. The first reason is the important cost reduction achieved over the multiple radar elements, by having only one transmitter (expensive part) and multiple receivers. The most promising applications are single-pass interferometry with a large baseline and target or surface characterisation from bistatic scattering signature analysis. In a defence context, the improved stealth associated with the receive-only component can provide a wider operational capability. In order to explore the potentials and technical challenges associated with bistatic radar, DLR and ONERA have conducted a joint bistatic airborne radar experiment involving both their SAR systems E-SAR and RAMSES between October 2002 and February 2003. Two main geometrical configurations were flown to explore different scientific and technical objectives. In the first geometrical configuration, the quasi-monostatic mode, the two planes were flying very close to each other to acquire interferometric data in a single-pass cross-platform configuration with large interferometric baselines. The second geometrical configuration was designed to acquire images with a large bistatic angle. The two planes were flying on parallel tracks around 2 km apart, at about the same altitude, with the antennas pointing at the same area on the ground. The authors describe this research programme, including the preparation phase, the analysis of the technological challenges that had to be solved before the acquisition, the strategy adopted for bistatic image processing, the first results and a preliminary analysis of the acquired images.

Survey and monitoring of landslide displacements by means of L-band satellite SAR interferometry

This paper illustrates the capabilities of L-band satellite SAR interferometry for the investigation of landslide displacements. SAR data acquired by the L-band JERS satellite over the Italian and Swiss Alps have been analyzed together with C-band ERS-1/2 SAR data and in situ information. The use of L-band SAR data with a wavelength larger than the usual C-band, generally considered for ground motion measurements, reduces some of the limitations of differential SAR interferometry, in particular, signal decorrelation induced by vegetation cover and rapid displacements. The sites of the Alta Val Badia region in South Tyrol (Italy), Ruinon in Lombardia (Italy), Saas Grund in Valais (Switzerland) and Campo Vallemaggia in Ticino (Switzerland), representing a comprehensive set of different mass wasting phenomena in various environments, are considered. The landslides in the Alta Val Badia region are good examples for presenting the improved performance of L-band in comparison to C-band for vegetated areas, in particular concerning open forest. The landslides of Ruinon, Saas Grund, and Campo Vallemaggia demonstrate the strength of L-band in observing moderately fast displacements in comparison to C-band. This work, performed with historical SAR data from a satellite which operated until 1998, demonstrates the capabilities of future planned L-band SAR missions, like ALOS and TerraSAR-L, for landslide studies.

The SAR train concept: An along-track formation of sar satellites for diluting the antenna area over [formula omitted] smaller satellites, while increasing performance by [formula omitted

This concept implements the coherent combination of N separate SAR flying along the same orbital arc as seen from ground. A particular case is when the N SAR are in visibility with a single one that transmits. A “signal cleaning” class of trains keeps the antenna area requirement of each individual SAR unchanged and brings a factor N advantage that applies on SNR and ambiguity protection. The main formation flying constraint is the width of the tube containing the satellite trajectories. The multiplication by N of the total antenna area is the other counterpart to these advantages. An “antenna dilution” class of train enables the distribution of an unchanged total antenna area into N smaller elementary antennas, together with multiplication by N of the SAR merit factor (swath over resolution ratio). With respect to the first class, the tube width constraint is increased and the space–time separation along the track has to be very accurate. Use of appropriate spread spectrum waveforms instead of conventional pulse waveforms removes the major part of the extra orbit constraints introduced by the “antenna dilution” class. A train of N SAR in visibility makes the concept more robust against loss of coherence and eases the metrology of the formation (DGPS). Moreover, the global energy efficiency is increased by N since with only a single transmit SAR the same performance is achieved. However, the along-track separation constraints for antenna dilution are made more stringent because of being restrained to the space domain, which reinforces the spread spectrum interest. As part of its applications, the concept can circumvent the matter of huge antenna size for SAR mission in very low frequency (P band) or at high altitude (surveillance).

Integration of 3D data in SAR mission planning and image interpretation in urban areas

Abstract State-of-the-art airborne SAR sensors provide high-resolution ground mapping data. This offers the opportunity of using this technology for the analysis of built-up areas. However, especially at building locations different SAR specific phenomena like layover, shadow, and multipath-propagation burden the interpretation of the SAR imagery even for experts. In order to consider such effects in the analysis and the geocoding of SAR data, high-resolution information about the 3D structure of the urban scene is required. Three dimensional elevation data from a GIS can be provided in different representations, like DEM (raster data) or city models (vector data). In this paper, the benefit of GIS data for the SAR mission planning and the analysis of acquired SAR data are discussed. For the first task, the SAR acquisition parameters are optimized a priori with respect to the best mapping of a local area or a certain object class. For this optimization, a large number of simulations with systematically varying aspect and viewing angles are carried out. For the second task, the 2D and 3D context information are fused with the acquired SAR imagery to support the interpretation for a change detection task. Additionally, simulated features of man-made objects are offered to the image interpreter for comparison with the data. The feasibility of different kinds of GIS data for these purposes is discussed.

The Model of Surface Currents Field as an Element of Determining Search Domain

A number of units of different tonnage and purpose may participate in SAR missions together with aircrafts. An mission to be effective must be properly co-ordinated and carried out in accordance with the specified scenario. The first and the main point of this scenario is determination of the most probable Datum (Po) of the search object. To find this position (Po), the initial position (Pp), initial position error, time of the distress, time difference between the distress and commencement of the search mission, (ΔT), speed and direction of wind leeway of the search object must be known. Rescue missions to a considerable degree are dependent on the state of basic meteorological and hydrological factors. Taking into consideration these features of areas should be the basis of planning any rescue action.

Performance and simulation of an on-board guided-wave optical processor for airborne SAR applications

This paper describes the performance and simulation of a compact integrated optical processor for the real-time reconstruction of two-dimensional images in airborne stripmap synthetic aperture radar applications. The functional behavior of the processor is explained in some detail. The design criteria are briefly given. The simulation step allowed the main processor characteristics and properties to be identified. A number of comparisons were obtained in airborne SAR mission scenarios between the predictions of the optical device and those achieved by the modern electronic digital approach, based on the wavefront reconstruction method by matched filtering.

Spirals on the sea

Spiral eddies were first seen in the sun glitter on the Apollo Mission 30 years ago; they have since been recorded on SAR missions and in the infrared. The spirals are globally distributed, 10-25 km in size and overwhelmingly cyclonic. They have not been explained. Under light winds favorable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. We have proposed that frontal formations concentrate the ambient shear and prevailing surfactants. Horizontal shear instabilities ensue when the shear becomes comparable to the coriolis frequency. The resulting vortices wind the liner features into spirals. The hypothesis needs to be tested by prolonged measurements and surface truth. Spiral eddies are a manifestation of a sub-mesoscale oceanography associated with upper ocean stirring; dimensional considerations suggest a horizontal diffusivity of order 103 m2 s-1.

Amazon floodplain water level changes measured with interferometric SIR-C radar

The authors find that interferometric processing of repeat-pass L-HH-band shuttle imaging radar (SIR-C) data reveals centimeter-scale changes in the elevations of water surfaces within flooded vegetation. Because radar pulses reflect specularly from the water surface, interferometric observations of open water are incoherent. However, within flooded forests and inundated shrubs, the L-band radar pulse penetrates the vegetation canopy and follows a double-bounce travel path that includes the water and vegetation-trunk surfaces. In these environments, the returned radar energy and associated phase coherence are both stronger than the surrounding nonflooded terrain, permitting determination of the interferometric phase. Phase errors related to atmospheric water vapor are usually longer in wavelength and spatially distinct from phase signatures related to stage changes in tributaries and floodplain lakes. The interferometrically measured stage decreases match gauge data, providing further verification. Water level changes across 150 m to 2.75 km-wide water bodies containing inundated vegetation can be reliably measured. The authors' results suggest that if future interferometric L-HH-band SAR missions are implemented with short temporal baselines, it is possible to measure the hydrologic response of wetlands and inundated floodplains to changes in mainstem water level.

ABSTRACT: Instrument Characteristics and Data Products for NASA's Next SAR Mission

ABSTRACT: Instrument Characteristics and Data Products for NASA's Next SAR Mission

Backscattering behavior and simulation comparison over bare soils using SIR-C/X-SAR and ERASME 1994 data over Orgeval

During April 1994, the three-frequency radar system flew on the Space Shuttle Endeavour, known as SIR-C/X-SAR mission (Shuttle Imaging Radar C/X-Synthetic Aperture Radar). Over the Orgeval watershed (France), the ground condition stayed very wet throughout the 5-day SAR mission. The SAR imagery allows a data collection over a range of roughness conditions on bare soils. Three classes were identified: very smooth sowings with crusted top layer, cloddy surfaces, and different ploughed fields for future crops. To complement the Shuttle Radar data (three frequencies L, C, X, incidence range from 44° to 57°), the helicopter-borne scatterometer ERASME (C- and X-bands, copolorized configurations) was used. Merging of the two databases was possible. As a result, incidence angles ranging from 25° to 50° are available in C- and X-bands for the copolarized cross sections. Then the major objective of the article is, over this available data collection, to begin the validation of current surface backscattering models to natural surfaces, the theoretical integral equation model (IEM) of Fung et al. (1992) and the empirical model of Oh et al. (1994). It shows adequacies and limits. The IEM model reproduces well radar scatter over smooth surfaces, but fails over rough surfaces, predicting a flatter response with incidence angle than the observed signals in C- and X-bands. Difference in backscatter response due to direction angles (perpendicular and parallel to the row direction) is difficult to reproduce over smooth surfaces by this model integrating anisotropic surface but may be due to the unadequacy of the surface representation. The Oh algorithm agrees well with the backscatter response over rough surfaces at medium incidence angle, but fails with a systematic underestimation over smooth conditions. As a conclusion, further developments are necessary on derivation of theoritical solutions over rough surfaces and on validation of semiempirical algorithms over data sets of various training sources (radar and natural conditions).