Spaceborne SAR Systems Research Articles

Forest Height Inversion Using Dual-pol Polarimetric SAR Interferometry

Polarimetric Synthetic Aperture Radar Interferometry (PolInSAR) has been extensively applied for forest parameter inversion over different frequencies and polarimetric conditions. So far, most research was based on full-pol SAR images with relatively small coverage. A spaceborne SAR system will have the potential for PolInSAR applications used for global forest monitoring. Spaceborne dual-pol SAR images usually have higher resolution and larger swath than full-pol mode. In this paper, forest height retrieval was attempted by PolInSAR from a L-band spaceborne dual-pol SAR pairs using HH and HV channels. The random volume over ground (RVoG) model was used to retrieve the height and the coherence optimization method was extended to the dual-pol PolInSAR, which makes use of polarimetry to enhance the quality of SAR interferograms. The three-stage process is also used in the dual-pol PolInSAR technique. Finally, the experimental test was performed for forest height estimation on the dual-pol L-band SAR data of the Saihanba forest acquired by the ALOS PALSAR sensor in 2009.

HRWS Imaging for Space-borne Multi-static SAR System with Along-track Multi-channel

结合数字波束形成技术，星载方位多通道合成孔径雷达(SAR)系统可以克服最小天线面积限制，实现高分辨宽测绘带(High Resolution and Wide Swath, HRWS)SAR成像。结合多站SAR系统即可实现高分辨干涉合成孔径雷达(Interferometric SAR, InSAR)地形测绘。该文通过分析星载多站模式下各接收通道接收回波的信号模型，推导得到了其相对于参考接收通道接收回波的通用相位补偿公式，该公式同时补偿了由沿航向和垂直航向基线引起的相位误差，然后对基于最优Capon法的多相位中心解模糊成像的保相性进行了分析证明，由此得到了解模糊后每个方位时刻回波所对应的卫星轨道信息，为后续干涉处理及目标定位等奠定基础，最后利用地球椭球模型仿真的星载双通道多普勒模糊回波数据验证了该文方法的有效性。

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.

A universal adaptive vector quantization algorithm for space-borne SAR raw data

Codebook of conventional VQ cannot be generally used and needs real time onboard updating, which is hard to implement in spaceborne SAR system. In order to solve this problem, this paper analyses the characteristic of space-borne SAR raw data firstly, and then utilizes the distortion function of multidimensional space as criterion, and finally the adaptive code book design algorithm is proposed according to the joint probability density function of the input data. Besides, the feasibility of the new algorithm in cascade with entropy coding and the robustness of the algorithm when error occurs during transmission are analysed based on the encoding and decoding scheme. Experimental results of real data show that codebook deriving from the new algorithm can be generally used and designed off-line, which makes VQ a practical algorithm for space-borne SAR raw data compression.

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.

Very High Resolution Spaceborne SAR Tomography in Urban Environment

Synthetic aperture radar tomography (TomoSAR) extends the synthetic aperture principle into the elevation direction for 3-D imaging. It uses stacks of several acquisitions from slightly different viewing angles (the elevation aperture) to reconstruct the reflectivity function along the elevation direction by means of spectral analysis for every azimuth-range pixel. The new class of meter-resolution spaceborne SAR systems (TerraSAR-X and COSMO-Skymed) offers a tremendous improvement in tomographic reconstruction of urban areas and man-made infrastructure. The high resolution fits well to the inherent scale of buildings (floor height, distance of windows, etc.). This paper demonstrates the tomographic potential of these SARs and the achievable quality on the basis of TerraSAR-X spotlight data of urban environment. A new Wiener-type regularization to the singular-value decomposition method-equivalent to a maximum a posteriori estimator-for TomoSAR is introduced and is extended to the differential case (4-D, i.e., space-time). Different model selection schemes for the estimation of the number of scatterers in a resolution cell are compared and proven to be applicable in practice. Two parametric estimation algorithms of the scatterers' elevation and their velocities are evaluated. First 3-D and 4-D reconstructions of an entire building complex (including its radar reflectivity) with very high level of detail from spaceborne SAR data by pixelwise TomoSAR are presented.

A novel circularly polarized synthetic aperture radar (CP-SAR) system onboard a spaceborne platform

For decades, linearly polarized synthetic aperture radar (LP-SAR) systems have been used in spaceborne platforms. In the transmitting and receiving processes, Faraday rotation in the ionosphere often affects the LP signals, leading to degradation in the backscatter signature in SAR images as well as loss of available power. Although the use of circular polarization can provide a solution to the Faraday rotation problem in many microwave systems, it has not previously been applied to SAR systems. In this paper, we describe the novel concept of a spaceborne (especially small-satellite) SAR system based on circular polarization, and show that a smaller antenna size, lower transmitted peak power and high precision signal reconstruction can be achieved by the proposed circularly polarized (CP)-SAR system.

Interferometric Potential of High Resolution Spaceborne SAR

The new class of high resolution spaceborne SAR systems, like TerraSAR-X and COSMO-Skymed opens new possibilities for SAR interferometry. The 1m resolution is particularly helpful when 2D, 2.5D, 3D, or 4D (space-time) imaging of buildings and urban infrastructure is required, where the non-interferometric interpretation of SAR imagery is difficult. Structure and defor-mation of individual buildings can be mapped, rather than only coarse deformation patterns of areas. The paper demonstrates several new developments in high resolution SAR interferometry using Ter-raSAR-X as an example. Of particular interest is the very high resolution spotlight mode, which requires some care in interferometric processing. Results from interferometry, Persistent Scatterer Interferometry (PSI), and tomographic SAR in urban environment are presented. The high resolution of TerraSAR-X also supports accurate speckle and feature tracking. An example of glacier monitoring is shown and discussed.

Potential of SAR sensors TerraSAR-X, ASAR/ENVISAT and PALSAR/ALOS for monitoring sugarcane crops on Reunion Island

Multi-temporal TerraSAR-X, ASAR/ENVISAT and PALSAR SAR data acquired at various incidence angles and polarizations were analyzed to study the potential of these new spaceborne SAR systems for monitoring sugarcane crops. The sensitivity of different radar parameters (wavelength, incidence angles, and polarization) to sugarcane growth stages was analyzed to determine the most suitable radar configuration for better characterisation of sugarcane fields and in particular the monitoring of sugarcane harvest. Correlation between backscattered signals and crop height was also carried out. Radar signal increased quickly with sugarcane height until a threshold height, which depended on radar wavelength and incidence angle. Beyond this threshold, the signal increased only slightly, remained constant, or even decreased. The threshold height is higher with longer wavelengths (L-band in comparison with C- and X-bands) and higher incidence angles (~ 40° in comparison with ~ 20°). The radar backscattering coefficients ( σ°) were also compared to the Normalized Difference Vegetation Index (NDVI) calculated from SPOT-4/5 images. Results showed a high correlation between the behaviors of σ° and NDVI as a function of sugarcane crop parameters. A decrease in NDVI for fully mature sugarcane fields due to drying of the sugarcane (water stress) was also observed in the radar signal. This decrease in radar signal was of the same order as the decrease in radar signal after the sugarcane harvest. In general, it is more suitable to monitor the sugarcane harvest using high incidence angles regardless of the radar wavelength. SAR data in L- and C-bands showed an ambiguity between the signals of ploughed fields and those of fields in vegetation because of the high sensitivity of the radar signal at these wavelengths to surface roughness of bare soils. Indeed, sometimes the radar signal of ploughed fields was of the same order as that of harvested or mature sugarcane fields. Results showed better discrimination between ploughed fields and sugarcane fields in vegetation (sugarcane canopy) when using TerraSAR-X data (X-band). Concerning the influence of radar polarization, results showed that the co-polarizations channels (HH and VV) were well correlated, but had slightly less potential than cross-polarization channels (HV and VH) for the detection of the sugarcane harvest. Finally, SAR data at high spatial resolution were shown to be useful and necessary for better analysis of SAR images when the fields were of small size.

고해상도 SAR 영상 형성 및 품질 분석을 위한 시뮬레이터

High resolution synthetic aperture radar image could be sensitive to the various parameters of the payload, platform, and ground system. In this paper, a parameter based SAR simulator is presented for two-dimensional image formation and image quality analysis. Functional modules are implemented by Matalb code and GUI for the flexibility and expandability. Main function of this simulator includes the SAR input signal generation, range-doppler algorithm(RDA) based SAR image formation, and the SAR image quality analysis which is relevant to the SAR system design parameters. This simulator can effectively be used for the SAR image quality performance evaluation, which can be applicable to the airborne as well as spaceborne SAR system design and analysis.

Space-borne synthetic aperture radar received data simulation based on airborne SAR image data

Starting from airborne SAR image data, a method of simulating space-borne SAR raw received data is proposed, excluding the conventional method of simulating the targets’ back-scattering coefficients through the model of three-dimensional distributed targets. The airborne SAR image data are regarded as space-borne targets’ back-scattering coefficients, and raw data are generated by using space-borne SAR echo signal model. Raw data’s simulation of space-borne SAR can help us to optimum design of space-borne SAR system scheme, test and evaluate the performance of every imaging algorithm, and research motion error from satellite platform having influence on received data and image quality.

Spaceborne Polarimetric SAR Interferometry: Performance Analysis and Mission Concepts

We investigate multichannel imaging radar systems employing coherent combinations of polarimetry and interferometry (Pol-InSAR). Such systems are well suited for the extraction of bio- and geophysical parameters by evaluating the combined scattering from surfaces and volumes. This combination leads to several important differences between the design of Pol-InSAR sensors and conventional single polarisation SAR interferometers. We first highlight these differences and then investigate the Pol-InSAR performance of two proposed spaceborne SAR systems (ALOS/PalSAR and TerraSAR-L) operating in repeat-pass mode. For this, we introduce the novel concept of a phase tube which enables (1) a quantitative assessment of the Pol-InSAR performance, (2) a comparison between different sensor configurations, and (3) an optimization of the instrument settings for different Pol-InSAR applications. The phase tube may hence serve as an interface between system engineers and application-oriented scientists. The performance analysis reveals major limitations for even moderate levels of temporal decorrelation. Such deteriorations may be avoided in single-pass sensor configurations and we demonstrate the potential benefits from the use of future bi- and multistatic SAR interferometers.

Ground moving target detection and location based on SAR images for distributed spaceborne SAR

This paper investigates data processing approaches to detect and locate ground moving targets using distributed spaceborne SAR systems with long cross-track baselines. In particular, it investigates the performance of ground moving target detection for two typical satellite formations: Cartwheel and Pendulum. An approach based on SAR images and a space-time adaptive processing (STAP) algorithm is proposed in order to overcome the effects of the ground terrain on the clutter suppression. The key idea of the approach is firstly to reduce the clutter degrees of freedom greatly by using conventional SAR imaging processing. Then the ground terrain clutter within each SAR pixel can be effectively cancelled by using the very limited spatial degrees of freedom. Finally, constant-false-alarm-rate (CFAR) techniques can be used to detect the remaining target SAR pixels after clutter cancellation. An approach to relocate the detected targets is also proposed, which is based on the estimation of the Doppler spectrum shifts of ground moving targets relative to the clutter Doppler spectrum. The proposed approaches in this paper have the advantages of simplicity and high efficiency.

An empirical calibration of the integral equation model based on SAR data, soil moisture and surface roughness measurement over bare soils

The retrieval of surface parameters demands the use of well calibrated models and unfortunately, none of the existing models provide consistently good agreement with the measured data. The overall objective of this article is to propose a semi-empirical calibration of the Integral Equation Model (IEM) so as to better reproduce the backscattering coefficient measured from SAR images over bare soils. As correlation length is not only the least accurate parameter but also the most difficult to measure, we propose its empirical estimation from an experimental data set of SAR images and soil parameter measurements. Based on a first data set, correlation length behaviour was studied in terms of f rms (where f is the wave number and rms the standard deviation of surface height) and radar configuration (polarization and incidence angle). Exponential relationships between optimal correlation length and rms height were found for each radar configuration. The IEM was then tested on another set of measured data in order to validate the calibration procedure. The new calibrated version of the IEM, corresponding to the original IEM with a coupling of the empirical function of correlation length, shows a very good agreement with the backscattering measurements provided by space-borne SAR systems. This adapted version of the IEM can be used in inversion techniques for retrieving rms height and/or soil moisture from radar observations.

A new dual polarized aperture-coupled printed array for SAR applications

This paper presents a new design of dual polarized aperture-coupled printed antenna array. The finite-difference time-domain (FDTD) analysis of an aperture-coupled microstrip element is performed, and the effects of antenna parameters on its characteristics are obtained to guide the design of the printed array. Then an 8×2 dual polarized array design in X-band is introduced with configuration plots. In order to improve its isolation and cross polarization, an outphase-displacement feeding technique is adopted in the feed network. Also, the round bends are used instead of conventional right-angle bends so as to achieve better VSWR performance. Experimental results are presented, indicating the validity of the design. This dual polarized array can be applied as a sub-array of spaceborne SAR systems.

The use of satellite-based radar interferometry to monitor production activity at the Cold Lake heavy oil field, Alberta, Canada

The Cold Lake heavy oil field has been studied by geoscientists for more than 30 years and has been producing bitumen for 20 years using the cyclic steam stimulation (CSS) process. Future development options can be improved by the resolution of steam movement and the avoidance of areas of faults and fractures. To locate these features, remote sensing has recently been investigated as a cheaper alternative to four-dimensional seismic surveys. Advances in satellite and radar technology have made it possible to measure very small movements of the earth's surface found in earthquake zones and volcanic regions. The technique uses synthetic-aperture radar interferometry (InSAR), allowing the measurement of deformation using a vertical resolution, in optimal conditions, on the order of millimeters between repeat acquisitions. This accuracy has only been achieved over dry areas without significant vegetation growth. At the Cold Lake oil field, it has been reported previously that the injection of steam to mobilize the bitumen causes the pump jacks to heave and subside by as much as 30 cm during the first steam cycle. The present project was instigated to determine if such small positive and negative vertical movements could be resolved over the field. Data from three satellite radar sensors were selected: ERS (European remote sensing satellite), JERS (Japanese remote sensing satellite) and Radarsat (the Canadian radar satellite). The present article has provided the first results of repeat-pass InSAR using JERS SAR data for the land subsidence application area. Contrary to the widespread belief that this technology can only be used for dry areas, we show that accurate results, on the order of a centimeter in resolution, can be obtained in a forested area such as Cold Lake using the JERS L-band spaceborne SAR system. (Begin page 782)

Compression of SAR data through range focusing and variable-rate vector quantization

In spaceborne SAR systems, some form of data compression is required to reduce the bandwidth of the downlink channel. Compressing the raw data is very inefficient because such data exhibit little or no correlation. Data focusing would restore the original dependencies of the image, but it is too complex to be implemented onboard. In this paper we propose to carry out a partial, low-complexity focusing onboard, and then compress the resulting data by means of a suitable algorithm based on variable-rate adaptive vector quantization. Experiments show a performance improvement of 0.5-0.7 dB over compression techniques proposed in the literature.

Aperture-Coupled Microstrip Patch Phased Arrays in C- and X-Band: A Contribution to Future Multi-Polarization Multi- Frequency SAR Systems

Future spaceborne SAR systems will use active phased array antennas with multi-polarization and multi-frequency capabilities. For high resolution SAR images of high quality the antennas have to provide a large bandwidth and an extreme polarization purity. These requirements can be met by aperture-coupled microstrip patch phased arrays based on low-permittivity, low-weight and low-cost foam substrates. The paper presents their design and their outstanding measured performance in C-band (5.3 GHz) and X-band (9.6 GHz). The constructed arrays show bandwidths of more than 150 MHz in C-band and more than 300 MHz in X-band. The polarization purity is typically in the range of 40 dB.

Passive and active calibrator characterization using a spaceborne SAR system simulator

Presents a spaceborne SAR system simulator based on a coherent approach. After a description of the original aspects of the computer simulator (i.e., digital terrain model, Earth ellipsoid rotation, space platform orbital and attitude dynamics, antenna pattern, chirp sampling, and two-dimensional compression), the characterization of passive and active calibrators embedded in different backgrounds is discussed in terms of the SAR impulse response function. An assessment of the SAR processor performance is derived by computing the one-dimensional ground range and azimuth resolutions of the simulated calibrators. Finally, various plots and numerical results are shown in order to illustrate impulse responses under clutter and no clutter assumptions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mapping of forest types in Alaskan boreal forests using SAR imagery

Mapping of forest types in the Tanana river flood-plain, interior Alaska, is performed using a maximum-a-posteriori Bayesian classifier applied on SAR data acquired by the NASA/JPL three-frequency polarimetric AIRSAR system on several dates. Five vegetation types are separated, dominated by 1) white spruce, 2) balsam poplar, 3) black spruce, 4) alder/willow shrubs, and 5) bog/fen/nonforest vegetation. Open water of rivers and lakes is also separated. Accuracy of forest classification is investigated as a function of frequency and polarization of the radar, as well as the forest seasonal state, which includes winter/frozen, winter/thawed, spring/flooded, spring/unflooded, and summer/dry conditions. Classifications indicate that C-band is a more useful frequency for separating forest types than L- or P-bands, and HV polarization is the most useful polarization at all frequencies. The highest classification accuracy, with 90 percent of forest pixels classified correctly, is obtained by combining L-band HV and C-band HV data acquired in spring as seasonal river flooding recedes and before deciduous tree species have leaves. In 17 forest stands for which actual percentages of each tree species are known, the same radar data are capable of predicting tree species composition with less than 10 percent error. From the authors' classification, they predict that current and future spaceborne SAR systems will have limited mapping capabilities when used alone. Yet, RADARSAT combined with J-ERS-1 and ERS-1 could resolve forest types with 80 percent accuracy, separate nonforest areas resulting from commercial logging or forest wildfire, and map river edges.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>