Synthetic Aperture Radar Mode Research Articles

A back-projection autofocus algorithm based on flight trajectory optimization for synthetic aperture radar imaging

In this paper, a new autofocus algorithm is presented for back-projection (BP) image formation of synthetic aperture radar (SAR) imaging. The approach is based on maximizing a cost function obtained by prominent points in different sub-apertures of constructed SAR image by varying the flight trajectory parameters. While image-quality-based autofocus approach together with BP algorithm can be computationally intensive, we use approximations that allow optimal corrections to be derived. The approach is applicable for focusing different signal processing algorithms by obtaining modified flight trajectory parameters. Different examples demonstrate the effectiveness of the new autofocus approach applied to the frequency modulated continuous wave mode SAR dataset.

On the Estimation of Melt Pond Fraction on the Arctic Sea Ice With ENVISAT WSM Images

The accuracy of microwave radiometer ice concentration (IC) retrievals in the Arctic is degraded by melt ponds on sea ice during the melting season. For the development of IC retrieval algorithms and for the quantification of their uncertainties, data sets on the area fraction of melt ponds $(f_{mp})$ are needed. $f_{mp}$ retrieval with optical satellite data is limited by clouds. Thus, we have studied $f_{mp}$ retrieval with ENVISAT wide swath mode (WSM) synthetic aperture radar (SAR) images which have large daily coverage over the Arctic Sea ice in 2007–2012. The WSM images used here were acquired north of the Fram Strait in June–August 2009. Data on $f_{mp}$ were available from the Integrated Climate Data Center's daily Moderate Resolution Imaging Spectroradiometer (MODIS) $f_{mp}$ product in a 12.5-km grid. Relationships between SAR $\sigma^{\circ}$ and MODIS $f_{mp}$ were studied visually by comparing daily SAR mosaics and $f_{mp}$ charts and by analyzing $f_{mp}$ and $\sigma^{\circ}$ time series and spatially and temporally coincident $f_{mp}$ and $\sigma^{\circ}$ data. The correspondence between the changes of $f_{mp}$ and the $\sigma^{\circ}$ statistics is too low to suggest $f_{mp}$ estimation from the WSM images. In some cases, there was a 2–3-dB $\sigma^{\circ}$ increase during the ponding period. It is assumed that the variation of snow and sea ice characteristics diminishes $\sigma^{\circ}$ changes due to the melt ponding and drainage. Good correlation between $\sigma^{\circ}$ and $f_{mp}$ has only been observed for smooth landfast first-year ice in previous studies. A very interesting observation was the large temporal $\sigma^{\circ}$ variations during the late melting season, which are likely linked to the atmospherically forced freezing–melting events. These events may also influence radiometer IC retrievals.

Error Propagation Analysis in Three-Dimensional Coseismic Displacement Inversion

Using the Bam earthquake as an example, we apply the multiple-aperture interferometry (MAI) algorithm to measure the azimuth (AZI) displacement with multiple mode synthetic aperture radar (SAR) images (ScanSAR and image modes) and further on produce the 3-D coseismic displacement maps using well-known models: multiple independent interferometric SAR (InSAR) with different viewing angles and combining MAI with conventional InSAR. The 3-D displacement maps show that, besides accuracy of SAR observation, inversion model is another major perturbing factor limiting the accuracy of 3-D component reconstruction. That is, in the former model, the smaller the difference of incidence angles is, the more easily influenced by ill-conditioned systems the estimated parameters are. Based on characteristics of error sources of SAR observations, we classify observation errors into random error, systematic error, and gross error. Moreover, their characteristics of error propagation are analyzed in the two models, respectively. Error propagation analysis indicates that random errors only affect variability of inversion result, while systematic error and gross error cause not only variability but also bias or uncertainty in estimated parameters. The presented error analysis method provides a more comprehensive understanding of bias or uncertainty of 3-D estimated components, which will be a useful tool for evaluating errors in displacement fields obtained from SAR observations. In this letter, we also group multiple InSAR and MAI observations to estimate 3-D components using weighted least squares whose weights are given by the Forstner posterior variance estimation, which may avoid an ill-conditioned system, reducing line-of-sight and AZI random noises and improving accuracies of estimated 3-D components.

Wind Speed Retrieval From VH Dual-Polarization RADARSAT-2 SAR Images

Recent studies show the promising ability of cross-polarization synthetic aperture radar (SAR) for high-wind-speed monitoring. However, no dual-polarization (dual-pol) geophysical model functions (GMFs) have thus far been established. In this paper, we first address the difference between dual-pol and quad-pol VH radar returns with respect to wind speed and show the necessity of having a VH dual-pol GMF for ScanSAR images, for remote sensing of mesoscale wind processes, such as hurricanes. By collocating VH dual-pol RADARSAT-2 SAR measurements with in situ buoy winds, a VH dual-pol GMF was developed after introducing a denoise procedure to increase the signal-to-noise ratio for SAR measurements. Study results suggest that the VH dual-pol radar returns have a linear monotonic piecewise increasing dependence on wind speed, with decreasing scatter in high winds. Successful application of the GMF to high-wind-speed retrieval for hurricane Gustav in 2008 suggests that VH mode SAR images do not exhibit signal saturation or the speed ambiguity problem that affects copolarization SAR images for high-wind-speed retrieval. This result supports the potential future application of VH dual-pol SAR images to hurricane monitoring.

Accurate baseline estimation for synthetic aperture radar‐ground moving target indication systems based on co‐registration and median filtering

In this study, a new method of effective baseline estimation is presented, which can be used for ground moving target indication (GMTI) mode of synthetic aperture radar (SAR).The basic idea is to perform the baseline estimation via exploiting the slope of the along-track interferometric (ATI) phase ramp in two-dimensional frequency domain. An integrated scheme of effective ATI-baseline estimation is proposed, which includes two key procedures: fine co-registration and median filtering. The proposed algorithm is tested with real SAR data. The results show that the incorporation of the fine co-registration and the median filtering procedures in the estimation scheme represents an accurate solution to the effective baseline estimation problem.

A Semi-Analytical Model for Delay/Doppler Altimetry and Its Estimation Algorithm

The concept of delay/Doppler (DD) altimetry (DDA) has been under study since the mid-1990s, aiming at reducing the measurement noise and increasing the along-track resolution in comparison with the conventional pulse-limited altimetry. This paper introduces a new model for the mean backscattered power waveform acquired by a radar altimeter operating in synthetic aperture radar mode, as well as an associated least squares (LS) estimation algorithm. As in conventional altimetry (CA), the mean power can be expressed as the convolution of three terms: the flat surface impulse response (FSIR), the probability density function of the heights of the specular scatterers, and the time/frequency point target response of the radar. An important contribution of this paper is to derive an analytical formula for the FSIR associated with DDA. This analytical formula is obtained for a circular antenna pattern, no mispointing, no vertical speed effect, and a uniform scattering. The double convolution defining the mean echo power can then be computed numerically, resulting in a 2-D semi-analytical model called the DD map (DDM). This DDM depends on three altimetric parameters: the epoch, the sea surface wave height, and the amplitude. A multi-look model is obtained by summing all the reflected echoes from the same along-track surface location of interest after applying appropriate delay compensation (range migration) to align the DDM on the same reference. The second contribution of this paper concerns the estimation of the parameters associated with the multi-look semi-analytical model. An LS approach is investigated by means of the Levenberg-Marquardt algorithm. Simulations conducted on simulated altimetric waveforms allow the performance of the proposed estimation algorithm to be appreciated. The analysis of Cryosat-2 waveforms shows an improvement in parameter estimation when compared to the CA.

COSMO-SkyMed HH/VV PingPong Mode SAR Data to Discriminate Among Sea, Urban, and Vegetated Areas

In this study, the sensitivity of COSMO-SkyMed (CSK) dual-polarimetric HH/VV Synthetic Aperture Radar (SAR) data with respect to sea, vegetated, and urban areas is investigated. A dual polarimetric feature, namely the complex correlation between the copolarized channels, is introduced to exploit the inherent peculiarities of the CSK incoherent dualpolarization PingPong mode for distinguishing the three scenarios. The dual-pol feature is contrasted with the correlation between the HH and VV amplitudes, the copolarized ratio, and with conventional single-pol ones, namely the HH and VV intensities.

Investigating Short-Wavelength Correlated Errors on Low-Resolution Mode Altimetry

Abstract The observation of ocean scales smaller than 100 km with low-resolution mode (LRM) altimetry products is degraded by the existence of a “hump artifact” visible on sea surface height (SSH) spectra. Through an analysis of simulations and actual data from multiple missions, this paper shows that the hump originates in a response to inhomogeneities in backscatter strength. Current retrackers cannot fit their Brown model properly because they were designed for a scene with homogeneous backscatter properties. The error is also smoothed along track because of the size and shape of the LRM disc-shaped footprint. Therefore, the hump is modulated by the altimeter design and altitude and by the retracker used. Because of the random nature of the phenomenon, a large majority of long topography segments (e.g., hundreds to thousands of kilometers) is affected. However, within these segments, a substantial fraction of the corruption is contained in small subsets of data (e.g., less than 10%). This paper shows that oceanography users interested in small-scale SSH signals can mitigate the hump corruption by using better editing and postprocessing algorithms on the 20-Hz rate of current products. Last, the thin stripe-shaped footprint of Cryosat-2’s synthetic aperture radar mode (SARM) is not affected by the hump artifact, thus improving the observation of topography features ranging from 30 to 100 km. The differences between SARM and pseudo-LRM sigma0 can also be used to detect major hump events on pseudo-LRM data, which might be an asset to design/validate a new generation of algorithms aimed at reducing the hump artifact on the existing LRM record.

Wall artifacts removal for target imaging enhancement in UWB through-the-wall radar application

In ultra-wideband through-wall-imaging applications, wall artifacts are always much stronger than the target reflections, and they tend to persist over a long duration of time. Therefore, targets are obscured and not visible in the image. The existing wall artifacts removal methods such as the moving average subtraction method, spatial filtering method and singular-differential decomposition method can efficiently remove wall artifacts in synthetic aperture radar (SAR) mode. However, with multi-input-multi-output (MIMO) radar widely used, the signal model and corresponding characteristics of wall artifacts are significantly different from those in the SAR mode. As a result, the existing methods are seldom adopted because their performances are severely affected. To address this problem, this paper proposes a novel wall artifacts removal method that is based on singular value decomposition and a cost-function ‘varimax norm’. The proposed method is tested with synthetic and experimental data in monostatic SAR mode and MIMO mode. Compared with existing methods using the signal-to-clutter ratio of the processing results, it is demonstrated that the proposed method can efficiently remove wall artifacts in both modes. Particularly in MIMO mode, the proposed method is much more capable than the existing methods in mitigating artifacts and enhancing target imaging.

Categorization of the Glaciated Terrain of Indian Himalaya Using CP and FP Mode SAR

This paper presents the comparison between the potential of synthetic aperture radar (SAR) data in compact polarimetry (CP) versus the fully polarimetric POLSAR mode (FP) for glacier areas. The eigenvalue-based constructed parameters, decompositions and supervised classification schemes were used for this inter-comparison. The comparison is focused on compact polarimetric techniques versus full polarimetric techniques based on both of qualitative and quantitative analysis of the glaciated parameters extraction. Overall performance of CP mode is lower than FP mode for glaciated terrain parameter extraction. Furthermore, it was found over the part of rugged Gangotri glaciated terrain, Indian Himalaya that the reconstruction pseudo polarimetry data is erroneous and highly biased because of data sets do not satisfy the pseudo polarimetric reconstruction conditions in the glaciated terrain.

A New Algorithm to Classify the Homogeneity of ERS-2 Wave Mode SAR Imagette

A new classification parameter is developed using 1535 ERS-2 wave mode synthetic aperture radar (SAR) test imagettes to better differentiate homogeneous and inhomogeneous imagettes. The comparison between the new parameter (Min) and the previous one (Inhomo) (Schulz-Stellenfleth and Lehner, 2004) was done under varied threshold values of Inhomo. It is concluded that the performance of ‘Min’ is much better than ‘Inhomo’ when applying to the 1535 test imagettes. Furthermore, both Min and Inhomo are applied to nearly 1 million imagettes collected for the period from 1 September 1998 to 30 November 2000. The comparisons of the global inhomogeneous distribution between ‘Min’ and ‘Inhomo’ reveal that both the areas and percentage of inhomogeneity calculated by ‘Min’ are larger than that calculated by ‘Inhomo’. By analyzing the low wind speed distribution of HOAPS data, we found that low wind speed over the ocean is one of the key reasons for the inhomogeneity of SAR imagettes.

Maximizing the Intrinsic Precision of Radar Altimetric Measurements

This letter derives the best obtainable sea-surface height measurement precision for the general case of a partially coherent altimeter. Analogous to the Walsh upper bound on pulse repetition frequency for a conventional (noncoherent) altimeter, there is a lower bound on burst period for an advanced synthetic aperture radar (SAR)-mode altimeter that operates coherently on groups of radar returns. The optimal pulse repetition frequency falls between the Walsh upper bound and the Nyquist lower bound. Interleaved (open burst) operation is required. Postprocessing along-track resolution-open to the designer's choice-is the principal performance-determining parameter. At the unfocused limit, the maximum range (height) precision for a Ku-band altimeter is on the order of 5 mm, which is relatively constant with altitude variations. Along-track resolution smaller than the unfocused limit implies better measurement precision but requires a more complex processing algorithm. An optimal design approach is suggested in which the altimeter operates in SAR mode and conventional mode simultaneously, thus capturing the most favorable measurement capabilities of either paradigm.

SUB-APERTURE FOCUSING ALGORITHM OF GEOSYNCHRONOUS SAR

Abstract. Synthetic Aperture Radar (SAR) on the geosynchronous orbit has the advantage in remote sensing, with high resolution to provide a large coverage in a short visit time. the space-variant coverage and range-azimuth couple of GEO SAR are required to compensate, using the electrical beam steering, the irregular data acquisition can be convert to classical SAR modes, such as stripmap, spotlight, sliding spotlight, and terrain observation by progressive scans (TOPS) SAR, the dopper analysis of these work modes of SAR have been studied. Based on deramping processing and improved subaperture chirp scaling algorithm, a novel focusing approach is presented in this paper, whose performance is determined by the work mode, data acquisition and the deramping processing. Data of each mode can be focused by utilizing this presented approach with the information of subapterture centroid time and range rotation center. Simulation results are presented to validate the analysis of the proposed approach.

Improved Motion Compensation Approach for Squint Airborne SAR

Airborne synthetic aperture radar (SAR) raw data are affected by motion errors due to atmospheric turbulence and aircraft properties, and this makes motion compensation (MOCO) an essential step. In order to decrease the influence of variable motion information, a kind of MOCO approach called azimuth-subaperture processing is widely used, which causes the azimuth-signal aliasing in squint SAR mode. Usually, this aliasing can be ignored, since its effect on final results is not worth mentioning. However, when the squint angle grows, it becomes serious and drops the quality of final images severely. To address this problem, an improved MOCO approach is proposed in this paper. Its main idea is to calculate the extended number of subapertures and eliminate the azimuth aliasing by extending the subaperture length. After theoretic analyses, simulations and experimental results are provided to demonstrate our proposed approach.

A New Imaging Approach for Burst Mode Synthetic Aperture Radar

Burst imaging schemes such as ScanSAR and Terrain Observation by Progressive Scans (TOPS) are often adopted to obtain wide swath coverage by synthetic aperture radar (SAR) systems with electronic beam steering in elevation. Long burst trains with zero padding in the inter-burst intervals can be focused by standard stripmap processors for the phase preserving product. However, this approach is less efficient due to the larger azimuth data extension. We discuss an efficient imaging approach for burst mode SAR. The key point of the proposed procedure is a postfiltering step after standard stripmap processors, which employs an azimuth convolution between the focused SAR data and the selected chirp signal. The postfiltering step is generalized by a two-step focusing technique formerly proposed for spotlight/sliding spotlight SAR. This approach extends the focusing capacity of standard stripmap processors to process burst mode raw data. Moreover, it is highly efficient, as it requires a very limited azimuth data extension.

First TerraSAR-X TOPS Mode Antenna Pattern Measurements Using Ground Receivers

The antenna model used for correcting the influence of the antenna pattern on synthetic aperture radar (SAR) images requires on-ground validation and in-flight verification. A methodology for the in-flight verification that is based upon the measurement of azimuth antenna patterns using ground receivers has been successfully demonstrated for the operational SAR modes of the TerraSAR-X (TSX) and TanDEM-X (TDX) missions. Recently, the novel (terrain observation by progressive scans) TOPS mode was for the first time implemented as an experimental mode on TerraSAR-X to demonstrate its feasibility in support of its implementation on ESA's Sentinel-1 mission. In this mode, besides scanning in elevation, the antenna beam is steered in flight direction from aft to the front at a constant rate to achieve an enhanced radiometric image performance. This paper discusses the methodology and presents results of the first in-flight antenna characterization of a SAR instrument operating in TOPS mode, in this case TerraSAR-X, using ground receivers. The results demonstrate that the TOPS one-way azimuth antenna pattern can be accurately modeled by the TSX antenna model indicating the general suitability of this approach for the in-flight antenna model verification during TOPS mode operations.

Estimation of the Degree of Polarization for Hybrid/Compact and Linear Dual-Pol SAR Intensity Images: Principles and Applications

Analysis and comparison of linear and hybrid/compact dual-polarization (dual-pol) synthetic aperture radar (SAR) imagery have gained a wholly new importance in the last few years, in particular, with the advent of new spaceborne SARs such as the Japanese ALOS PALSAR, the Canadian RADARSAT-2, and the German TerraSAR-X. Compact polarimetry, hybrid dual-pol, and quad-pol modes are newly promoted in the literature for future SAR missions. In this paper, we investigate and compare different hybrid/compact and linear dual-pol modes in terms of the estimation of the degree of polarization (DoP). The DoP has long been recognized as one of the most important parameters characterizing a partially polarized electromagnetic wave. It can be effectively used to characterize the information content of SAR data. We study and compare the information content of the intensity data provided by different hybrid/compact and linear dual-pol SAR modes. For this purpose, we derive the joint distribution of multilook SAR intensity images. We use this distribution to derive the maximum likelihood and moment-based estimators of the DoP in hybrid/compact and linear dual-pol modes. We evaluate and compare the performance of these estimators for different modes on both synthetic and real data, which are acquired by RADARSAT-2 spaceborne and NASA/JPL airborne SAR systems, over various terrain types such as urban, vegetation, and ocean.

Multi-Subaperture PGA for SAR Autofocusing

For spotlight mode synthetic aperture radar (SAR) autofocusing, the traditional full-aperture phase gradient autofocus (PGA) algorithm might suffer from performance degradation in the presence of significant high-order phase error and residual range cell migration (RCM), which tend to occur when the coherent processing interval (CPI) is long. Meanwhile, PGA does not perform satisfactorily when applied directly on the stripmap data. To address these shortcomings, we present a multi-subaperture PGA algorithm, which takes advantage of the map drift (MD) technique. It smoothly incorporates the estimation of residual RCM and combines the subaperture phase error (SPE) estimated by PGA in a very precise manner. The methodology and accuracy of PGA-MD are investigated in detail. Experimental results indicate the effectiveness of PGA-MD in both the spotlight and the stripmap modes.

On the COSMO-SkyMed PingPong Mode to Observe Metallic Targets at Sea

A dual-polarimetric model to exploit X-band COSMO-SkyMed (CSK) dual-polarimetric PingPong mode synthetic aperture radar (SAR) data for metallic target observation purposes is first developed. The model relates the correlation between the copolarized channels, in case of sea surface with and without metallic targets, to the time offset between bursts related to the HH and VV polarimetric channels. Following this theoretical rationale, a constant false alarm rate (CFAR) approach is implemented to first exploit CSK PingPong full-resolution SAR data for target observation purposes. Experiments undertaken over a large data set of COSMO-SkyMed single look complex slant (SCS) PingPong HH/VV SAR data demonstrate the physical soundness of the approach and its effectiveness from an operational viewpoint. A whole CSK SAR scene is processed in about 10 s. Furthermore, the dual-polarization approach is also compared with a conventional single-polarization CFAR one and it is shown to perform best.

VALIDATION OF DEMs DERIVED FROM HIGH RESOLUTION SAR DATA: A CASE STUDY ON BARCELONA

Abstract. In recent years, Synthetic Aperture Radar (SAR) data have been widely used for scientific applications and several SAR missions were realized. The active sensor principle and the signal wavelength in the order of centimeters provide all-day and all-weather capabilities, respectively. The modern German TerraSAR-X (TSX) satellite provides high spatial resolution down to one meter. Based on such data SAR Interferometry may yield high quality digital surface models (DSMs), which includes points located on 3d objects such as vegetation, forest, and elevated man-made structures. By removing these points, digital elevation model (DEM) representing the bare ground of Earth is obtained. The primary objective of this paper is the validation of DEMs obtained from TSX SAR data covering Barcelona area, Spain, in the framework of a scientific project conducted by ISPRS Working Group VII/2 "SAR Interferometry" that aims the evaluation of DEM derived from data of modern SAR satellite sensors. Towards this purpose, a DSM was generated with 10 m grid spacing using TSX StripMap mode SAR data and converted to a DEM by filtering. The accuracy results have been presented referring the comparison with a more accurate (10 cm–1 m) digital terrain model (DTM) derived from large scale photogrammetry. The results showed that the TSX DEM is quite coherent with the topography and the accuracy is in between ±8–10 m. As another application, the persistent scatterer interferometry (PSI) was conducted using TSX data and the outcomes were compared with a 3d city model available in Google Earth, which is known to be very precise because it is based on LIDAR data. The results showed that PSI outcomes are quite coherent with reference data and the RMSZ of differences is around 2.5 m.