Airborne Synthetic Aperture Radar Imagery Research Articles

Investigating coincident L- and S-band ASAR imagery over Arctic sea ice

Our investigation into coincident L- and S-band ASAR (airborne synthetic aperture radar) imagery explored Arctic sea ice separability in the Beaufort Sea, particularly in light of the imminent launch of the NASA-ISRO Synthetic Aperture Radar (NISAR) mission. Our research has revealed an improved capability to separate i) multi-year and first-year sea ice at the S-band imagery, as well as ii) a higher separability within first-year sea ice classes at L-band imagery. We have also reported that wind-roughened melt ponds show a distinct signature in the S-band. Importantly, our machine learning algorithm has achieved higher accuracy in sea ice classification at the S-band than the L-band. These findings have significant implications for the future of sea ice research and operations using SAR imagery from the NISAR mission.

Integrating Motion Compensation With Polar Format Interpolation for Enhanced Highly Squinted Airborne SAR Imagery

Spatial-variant motion compensation (MOCO) is critical for high resolution and highly squinted airborne synthetic aperture radar (SAR) imaging. Conventional imaging strategies generally perform systematic imaging algorithm and motion compensation algorithm in a separate way, the residual azimuth-variant motion error usually causes defocusing in the image. It is difficult for existing post-filtering strategies to realize high precision and high efficiency imaging simultaneously. To solve this problem, a novel parametric polar format algorithm (PPFA) is proposed in this paper. The polar format interpolation kernel is redefined and improved by inducing motion error parameter, so the proposed algorithm can realize fast imaging and precise spatial-variant motion compensation at the same time. The proposed algorithm has advantage of high processing efficiency as polar format algorithm (PFA) and effectively improves the focusing precision. Extensive comparisons with conventional algorithms illustrate the superiority of the proposed algorithm in both precision and efficiency.

Adaptive Pulse Compression for Range Focusing in SAR Imagery

Synthetic aperture radar (SAR) images are focused via range and azimuth compression. Typically, a matched filter is used for range compression. However, its inherent operation introduces target masking due to sidelobes, reducing the recognition and interpretability of the underlying targets. To optimize the focusing quality, adaptive pulse compression (APC) can be applied. In this paper, APC is assessed for range processing in SAR image focusing. The performance of the method is evaluated with airborne SAR imagery and compared with those of traditional schemes. We show that by applying APC under single-channel interferometric and stepped frequency scenarios, the quality of the SAR images, the corresponding phase maps, and interferometric coherence images can be significantly improved, and that APC provides both better amplitude and phase estimates of the range profiles. APC is shown to preserve the characteristics of the azimuth signals and can be applied without restricting the azimuth compression scheme.

Waterline Mapping and Change Detection of Tangjiashan Dammed Lake After Wenchuan Earthquake From Multitemporal High-Resolution Airborne SAR Imagery

Several dammed lakes caused by landslides and rock avalanches appeared after Wenchuan earthquake. Tangjiashan dammed lake was the largest one among all of them, which resulted in obvious threats to people around. In this paper, multitemporal high-resolution airborne synthetic aperture radar (SAR) images are used to map the waterline of Tangjiashan dammed lake and its change continuously, which is meaningful for flooding assessment and prediction. An approach combining different techniques is proposed for dammed lake segmentation. In this approach, subpatched intensity thresholding segmentation, morphological operators, and local Gaussian distribution fitting energy (LGDF)-based active contours model (ACM) are used in combination to get the final waterline. Flood-affected area is detected and its size is calculated for each segmental result. In addition, the variations of the flood-affected area are also obtained based on the difference of the above-mentioned segmental results. The proposed approach is tested on real SAR imagery acquired from three different dates in May, 2008 with high-temporal resolution in the same area containing Tangjiashan dammed lake. Experimental results demonstrate the effectiveness of the proposed approach.

Very-High-Resolution Airborne Synthetic Aperture Radar Imaging: Signal Processing and Applications

During the last decade, synthetic aperture radar (SAR) became an indispensable source of information in Earth observation. This has been possible mainly due to the current trend toward higher spatial resolution and novel imaging modes. A major driver for this development has been and still is the airborne SAR technology, which is usually ahead of the capabilities of spaceborne sensors by several years. Today's airborne sensors are capable of delivering high-quality SAR data with decimeter resolution and allow the development of novel approaches in data analysis and information extraction from SAR. In this paper, a review about the abilities and needs of today's very high-resolution airborne SAR sensors is given, based on and summarizing the longtime experience of the German Aerospace Center (DLR) with airborne SAR technology and its applications. A description of the specific requirements of high-resolution airborne data processing is presented, followed by an extensive overview of emerging applications of high-resolution SAR. In many cases, information extraction from high-resolution airborne SAR imagery has achieved a mature level, turning SAR technology more and more into an operational tool. Such abilities, which are today mostly limited to airborne SAR, might become typical in the next generation of spaceborne SAR missions.

Post-earthquake building damage assessment in Yushu using airborne SAR imagery

The synthetic aperture radar (SAR) plays an important role in earthquake emergency response because of its all-time and all-weather imaging capabilities. On April 14, 2010, an MS7.1 earthquake occurred in Yushu county, Qinghai province of China, causing a lot of buildings collapsed. In this paper, the building damage in Yushu city due to the earthquake was assessed quantitatively using high-resolution X-band airborne SAR image. The features of the buildings with different damage levels (collapsed, partial collapsed, non-collapsed) in the SAR image were analyzed first. Based on these building features, we interpreted the individual building damage in Yushu city block by block and got the numbers of the collapsed, partial collapsed and non-collapsed buildings separately for each block, referring to pre-earthquake QuickBird image when necessary. Let the damage index of individual collapsed, partial collapsed, non-collapsed building be 1, 0.5, 0 respectively, the remote sensing damage index of each block was then calculated through remote sensing damage index equation. Finally, the preliminary quantitative relationship between the remote sensing damage index interpreted from the SAR image and that interpreted from the optical image was built up. It can be concluded that a desirable damage assessment result can be derived from high-resolution airborne SAR imagery.

Discrimination of coastal wetland environments in the Amazon region based on multi-polarized L-band airborne Synthetic Aperture Radar imagery

This study assessed the use of multi-polarized L-band images for the identification of coastal wetland environments in the Amazon coast region of northern Brazil. Data were acquired with a SAR R99B sensor from the Amazon Surveillance System (SIVAM) on board a Brazilian Air Force jet. Flights took place in the framework of the 2005 MAPSAR simulation campaign, a German-Brazilian feasibility study focusing on a L-band SAR satellite. Information retrieval was based on the recognition of the interaction between a radar signal and shallow-water morphology in intertidal areas, coastal dunes, mangroves, marshes and the coastal plateau. Regarding the performance of polarizations, VV was superior for recognizing intertidal area morphology under low spring tide conditions; HH for mapping coastal environments covered with forest and scrub vegetation such as mangrove and vegetated dunes, and HV was suitable for distinguishing transition zones between mangroves and coastal plateau. The statistical results for the classification maps expressed by kappa index and general accuracy were 83.3% and 0.734 for the multi-polarized color composition (R-HH, G-HV, B-VV), 80.7% and 0.694% for HH, 79.7% and 0.673% for VV, and 77.9% and 0.645% for HV amplitude image. The results indicate that use of multi-polarized L-band SAR is a valuable source of information aiming at the identification and discrimination of distinct geomorphic targets in tropical wetlands.

Sea-Ice Deformation State From Synthetic Aperture Radar Imagery—Part I: Comparison of C- and L-Band and Different Polarization

In this paper, we present a quantitative comparison of L- and C-band airborne synthetic aperture radar imagery acquired at like- and cross-polarizations over deformed sea ice under winter conditions. The parameters characterizing the deformation state of the ice are determined at both radar bands and at different polarizations. The separation of deformed and level ice is based on a target detection technique. The threshold is set such that image pixels with intensities equal to or larger than the highest 2% of the level-ice intensity distribution are classified as deformed ice, independent of the radar configuration and ice conditions. Optical imagery of sufficient quality for comparison is available only in a very few cases. To characterize the deformation state, the areal fraction of deformation features and the average distance between these features are evaluated. The values obtained for both parameters are very sensitive to the radar frequency. Aeral fractions are larger, and average distances are smaller at L-band than at C-band because of the much higher intensity contrast between the deformed and level ice at L-band. The differences between polarizations at one radar band are smaller but not always negligible. In comparison to optical images, it was observed that deformed-ice areas can be distinguished from level ice over their whole length and extension at L-band, whereas at C-band, often, only prominent parts are visible.

Waterline mapping in flooded vegetation from airborne SAR imagery

Multifrequency, polarimetric airborne synthetic aperture radar (SAR) survey of a salt marsh on the east coast of the UK is used to investigate the radar backscattering properties of emergent salt marsh vegetation. Two characteristics of flooded vegetation are observed: backscatter enhanced by approximately 1.2 dB at C-band, and 180° HH-VV phase differences at L-band. Both are indicative of a double bounce backscattering mechanism between the horizontal water surface and upright emergent vegetation. The mapping of inundated vegetation is demonstrated for both these signatures, using a statistical active contour model for the C-band enhanced backscatter, and median filtering and thresholding for the L-band HH-VV phase difference. The two techniques are validated against the waterline derived from tidal elevation measured at the time of overpass intersected with an intertidal DEM derived from airborne laser altimetry. The inclusion of flooded vegetation is found to reduce errors in waterline location by a factor of approximately 2, equivalent to a reduction in waterline location error from 120 to 70 m. The DEM is also used to derive SAR waterline heights, which are observed to underpredict the tidal elevation due to the effects of vegetation. The underprediction can be corrected for vegetation effects using canopy height maps derived from the laser altimetry. This third technique is found to improve the systematic error in waterline heights from 20 to 4 cm, but little improvement in random error is evident, chiefly due to significant noise in the vegetation height map.

Comparison of airborne synthetic aperture radar imagery with in situ surface‐slope measurements across Gulf Stream slicks and a convergent front

Synthetic aperture radar (SAR) imagery showing two types of features near the inshore edge of the Gulf Stream are compared with nearly simultaneous estimates of the sea surface slope field derived from optical shipboard measurements. One class of feature consists of a set of narrow, dark lines having radar signal modulations of about −10 dB at L band. These modulations are comparable to those observed in the in situ wave‐slope data over wavenumbers corresponding to the radar‐resonant Bragg waves. These modulations are also predicted by a simplified equilibrium wave spectrum model using a surface elasticity of about 25 mN/m (as determined from in situ measurements of surface tension) and a wind friction velocity of about 10 cm/s (from buoy and shipboard measurements). These results support Lyzenga and Marmorino's [1998] inference that the Gulf Stream dark lines represent relatively passive, biogenically derived surfactant slicks that are advected and strained by the large‐scale surface flow field. The second class of feature consists of a pair of bright‐dark signatures over the shallow continental shelf region near Cape Hatteras, North Carolina. These features are characterized by signal modulations of about +4 and −4 dB in both the L band SAR and in situ data. These modulations appear to be due to the interaction of surface waves with the current convergence and divergence regions associated with a rotary circulation in a shallow convergent front. Model calculations of this interaction by Lyzenga [1998] indicate that the positive perturbations associated with current convergence should be relatively independent of the wavenumber, which is in approximate agreement with the in situ measurements of the present paper. The model also indicates that the negative perturbations associated with the region of current divergence should decrease for wavenumbers higher than L band. That trend is observed in the SAR data but not in the in situ data, which show significant perturbations at higher wavenumbers. It is conjectured that this is due to the effects of surfactants and either along‐front or time variability in the frontal dynamics, which are not included in the model. Additional comparisons of this kind should be useful to further test and improve physical models of near‐surface phenomena.

Airborne and spaceborne synthetic aperture radar observations of ocean waves

Abstract The Grand Banks ERS‐1 synthetic aperture radar (SAR) wave spectra validation experiment took place over a study site in which intensive in situ wind and wave measurements were being taken. The unique aspect of the program was the nearly simultaneous acquisition (in space and time) of spaceborne (ESA ERS‐1) and airborne (CCRS CV‐580) SAR imagery of the same ocean wave field. Although both SARs were operating at C‐band with W polarization, the geometry of acquisition was quite different. For example, the range‐to‐velocity ratio parameter was large for ERS‐1 (R/V ∼ 115 s) and relatively small for the CV‐580 (R/V < 50 s). Thus, the SAR image spectra derived from ERS‐1 are significantly more susceptible to velocity bunching non‐linearity and azimuth spectral cut‐off, which are well‐known limitations of SAR in accurately measuring azimuth‐travelling ocean waves. Airborne and spaceborne SAR measurements of ocean waves are presented and compared with directional wave buoy measurements. A technique is developed to estimate the azimuth spectral width based upon a quasi‐linear ocean‐to‐SAR transform, and is applied to all of the SAR spectra. This quasi‐linear transform is qualitatively assessed by forward‐mapping directional wave buoy spectra into SAR image spectra. The width measurements are correlated with observed values for the significant wave height or the azimuth shift and the local wind speed. This allows definition of a quasi‐linear transform that includes both velocity bunching decorrelation effects and wind speed‐dependent coherence time effects. Finally, a new SAR spectral inversion scheme, based upon the quasi‐linear transform, is demonstrated. Wave model spectra are used as the starting point and validation is against directional wave buoy spectra.

Ocean wave spectral distortion in airborne synthetic aperture radar imagery during the Norwegian Continental Shelf Experiment of 1988

C band radar images of ocean gravity waves off the Norwegian coast were processed into one‐dimensional azimuth spectra. These spectra were used to measure the azimuth spectral (width) cutoff on the basis of a least squares fit to a Gaussian spectral shape. The widths were calculated for a range of wave heights (2–5 m) and wind speeds (2–18 m/s) during 3 days in March, 1988. Velocity smearing (συ) estimates were extracted, independent of R/V and incidence angle, based on an imaging model and the measured azimuth cutoffs with συ values varying from 0.4 to 0.7 m/s. Quantitative velocity smearing estimates are important as input to models describing the distortion in wave imagery. We propose a first‐order model which neglects velocity bunching for ocean swell with peak wavelengths longer than about 250 m. This model is offered as a first estimate of when ocean wave swell will be detected by the C band SAR on board the ERS 1 spacecraft. The model predicts that this swell will be imaged under light winds of the order of 2–4 m/s. Higher wind speeds cause larger smearing, which may result in significant distortion of the imaged swell provided that the swell is traveling near the direction of the spacecraft ground track.

On the focusing issue of synthetic aperture radar imaging of ocean waves

It is widely accepted that the imaging of ocean surface waves by synthetic aperture radar (SAR) can be adequately described by velocity bunching theory in conjunction with the two-scale wave model. However, it has been conjectured that this theory is incapable of explaining why, under certain conditions, the image contrast of airborne SAR imagery of ocean waves can be enhanced by defocusing the SAR processor. In the present study the velocity bunching theory is defended. >

Airborne SAR imagery of ocean surface waves obtained during LEWEX: some initial results

The potential and the difficulties of using airborne synthetic-aperture radar (SAR) to study ocean surface waves are presented. Airborne C-band SAR imagery obtained with the real-time SAR processor on the Canada Centre for Remote Sensing (CCRS) Convair 580 during the Labrador Extreme Waves Experiment (LEWEX) shows waves propagating through the marginal ice zone as well as through open water regions. Examples are chosen to illustrate both coherent and noncoherent scene motion effects. Coherent scene motion effects include velocity bunching. acceleration defocus, and coherence time limitations. Noncoherent scene motion effects include scanning distortion and look misregistration. The slow platform velocity of airborne SAR precludes replication of spaceborne observations, largely due to coherent motion effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Prediction of Geometric Distortions in Airborne Synthetic Aperture Radar Imagery from Autofocus Measurements

The synthetic aperture radar (SAR) is an imaging system that achieves high azimuthal resolution by tracking individual point scatterers using their phase histories, with the expected phase history of a particular point scatterer being derived from the assumed motion of the airborne SAR platform. Normally, it is assumed that the platform travels along a straight line path and that the radar pulses are emitted at equal spatial intervals. However, the aircraft carrying the SAR will be susceptible to extraneous across tracl motions and errors in the pulse spacing also may occur. These errors manifest themselves. as two forms of image degradation in the final image. Firstly, the image will be defocused due to an error in the expected quadratic phase history, and secondly, the image will contain geometric distortions due to an error in the linear phase history. An autofocus technique can be used to focus the image and produce a measure of the quadratic phase error that in turn can be used to estimate the geometric distortions that will be present in the final image. These distortions can be independently measured by direct comparison with a map of the imaged area. This paper describes the application of these methods to some real SAR data and discusses the results of the comparison of the measurements of autofocus and geometric distortions in terms of both the likely platform motions present and the viability of predicting geometric distortion using the autofocus measurements.

Preparing for satellite microwave systems for renewable resource management

Abstract Microwave imaging systems hold great promise of meeting one of the most important requirements of data for vegetation monitoring ‐ namely data timeliness. Furthermore, synthetic aperture radar (SAR) imagery will be routinely available within the near future with the launches of the European Space Agency, (1990), Japanese (1991) and Canadian RADARSAT (1994) satellites. Within Canada, there is an extensive program underway (Radar Data Development Program ‐ RDDP) to develop the necessary methodologies to use SAR data for renewable resource management. This program includes data acquisition using both airborne SAR imagery and ground‐based scatterometers. This paper describes this RDDP program in some detail and presents some pertinent results attained to date.

Polarization, land use type and intraurban location as variables in SAR mapping accuracy

Abstract Dual‐polarized X‐band airborne synthetic aperture radar imagery of Los Angeles, California is employed to examine the relationship among urban land use category identification, category location within an urban area, and radar polarization. Results indicate that HV polarized imagery may be preferred for urban land use mapping as the signal response is less sensitive to target orientation and surface scatter. That is, there was more category related variation in tone and texture. For both polarizations there were significant differences in identification accuracy among land use classes and the ability to identify a single land use type varied across the study area, but not equally. A major problem relative to developing applications was the inability to distinguish Commercial‐Services and Industrial activity from other categories.