Interferometric Synthetic Aperture Radar Processing Research Articles

Comparative Study of Speckle Noise Reduction Approaches for Interferometric Synthetic Aperture Radar Images

Speckle noise appearing in the interferometric SAR (InSAR) phase image degrades the quality of interferogram seriously and makes interferogram reflect the scattering characteristics of the target inaccurately, reducing the capability of extracting DEM information of target areas. Therefore, speckle noise reduction plays a major role in InSAR processing by using interferogram filtering. First, according to a terrain model with the assumed geometrical parameters in InSAR system, the paper simulated an interferometric SAR phase image with noise, which can be characterized by the multilook phase distribution based on the circular Gaussian assumption [1]. Second, the paper explores three interferogram filtering algorithm to remove speckle noise: Goldstain filter, Rotating Kernel Transformation and Lee filter; the proper implementation of three methods is given. Last, the paper discusses about the performance comparatively based on experimental results and gives broad conclusions and presents recommendations.

Interferometric Phase Denoising by Pyramid Nonlocal Means Filter

Interferometric phase denoising is a crucial step of interferometric synthetic aperture radar processing flow because it has significant influence on the following steps. Traditional interferometric phase denoising algorithms have a similar drawback that they will wipe off some texture details in phase images while denoising. Nonlocal (NL) means filter, in contrast, can reach a balance between denoising and texture preserving because it utilizes the feature of recursive structures in the whole image. Taking the characteristics of interferometric phase image into consideration, this letter proposes a modified NL means filter algorithm for phase denoising. Moreover, in order to preserve texture to the biggest extent, an iterative algorithm is invented. In the end, experiments on synthetic and real data validate that this algorithm outperforms other traditional denoising methods.

Dynamic deformation of Seguam Island, Alaska, 1992–2008, from multi-interferogram InSAR processing

We generated a time-series of ERS-1/2 and ENVISAT interferometric synthetic aperture radar (InSAR) images to study ground surface deformation at Seguam Island from 1992 to 2008. We used the small baseline subset (SBAS) technique to reduce artifacts associated with baseline uncertainties and atmospheric delay anomalies, and processed images from two adjacent tracks to validate our results. Seguam Island comprises the remnants of two late Quaternary calderas, one in the western caldera of the island and one in the eastern part of the island. The western caldera subsided at a constant rate of ~1.6cm/yr throughout the study period, while the eastern caldera experienced alternating periods of subsidence and uplift: ~5cm/year uplift during January 1993–October 1993 (stage 1), ~1.6cm/year subsidence during October 1993–November 1998 (stage 2), ~2.0cm/year uplift during November 1998–September 2000 (stage 3), ~1.4cm/year subsidence during September 2000–November 2005 (stage 4), and ~0.8cm/year uplift during November 2005–July 2007 (stage 5). Source modeling indicates a deflationary source less than 2km below sea level (BSL) beneath the western caldera and two sources beneath the eastern caldera: an inflationary source 2.5–6.0km BSL and a deflationary source less than 2km BSL. We suggest that uplift of the eastern caldera is driven by episodic intrusions of basaltic magma into a poroelastic reservoir 2.5–6.0km BSL beneath the caldera. Cooling and degassing of the reservoir between intrusions result in steady subsidence of the overlying surface. Although we found no evidence of magma intrusion beneath the western caldera during the study period, it is the site (Pyre Peak) of all historical eruptions on the island and therefore cooling and degassing of intrusions presumably contributes to subsidence there as well. Another likely subsidence mechanism in the western caldera is thermoelastic contraction of lava flows emplaced near Pyre Peak during several historical eruptions, most recently in 1977 and 1992–93.

Impacts of geoid height on large-scale crustal deformation mapping with InSAR observations

An ellipsoidal height datum (e.g. WGS84) is required in SAR Interferometric processing, whilst DEMs are often referenced to a geoid datum (e.g. EMG96). Therefore, geoid height error is introduced due to the inconsistency of height datums in InSAR derived displacement maps. In this paper, the relationship between geoid heights and uncertainties in InSAR displacements is investigated using 27 scenes of Envisat ASAR ScanSAR images and 44 scenes of Image-mode images covering the western Altyn Tagh Fault, northern Tibetan Plateau. A typical error of 8.8 m in SRTM DEM leads to 3mm uncertainty in an InSAR interferogram with a perpendicular baseline of 100m, whilst a geoid height of 20m can result in 10mm uncertainty. Geoid height ranges from one place to another, and are highly correlated with topography. The impacts of geoid height on InSAR displacements increase with perpendicular baselines, but SAR images with a small perpendicular baseline may not be available in some cases. In addition, it may also not always be feasible to use a best-fitting plane to remove the impacts of geoid height. It is thus highly recommended in this paper that DEMs with an ellipsoidal height datum should be employed in interferometric processing, particularly when long-wavelength crustal deformation is targeted.

레이더 간섭위상의 정밀고도변환 알고리즘 개선

InSAR (Interferometric SAR) 기법을 이용한 DEM 추출은 두 SAR 영상의 정합, 간섭도 생성, 위상 필터링, 위상 unwrapping, 고도변환, 좌표계 변환 등 매우 복잡한 단계를 필요로 한다. 본 연구에서는 unwrapped 위상을 고도값으로 변환하는 과정에 대한 정밀도 향상을 위한 알고리즘을 개발하였다. 개발된 알고리즘은 지구타원체를 고려하여 수정된 ambiguity 방법, Schw<TEX>$\ddot{a}$</TEX>bisch 방법과 더불어 두 SAR 자료의 궤도가 평행하지 않을 때 발생하는 오차를 현저히 줄일 수 있는 개선된 ambiguity 방법이다. JERS-1 궤도를 이용한 알고리즘 성능 분석을 통해 기존 ambiguity 알고리즘은 두 개의 SAR 영상 궤도가 평행하지 않은 경우, 간섭위상의 고도변환 과정 중에 약 40 m 이상의 오차가 발생할 수 있음을 확인하였다. 개발된 알고리즘은 JERS-1과 같은 위성궤도의 불안정 또는 ERS2-ENVISAT Cross-InSAR를 위한 궤도 제어 등 다양한 원인에 의해 발생할 수 있는 평행하지 않는 두 위성 궤도 자료의 간섭쌍을 이용한 정밀 InSAR DEM 생성에 매우 효과적으로 활용될 수 있다. The InSAR (Interferometric SAR) processing steps for DEM generation consist of the coregistration of two SAR data, interferogram generation, phase filtering, phase unwrapping, phase to height conversion, and geocoding, etc. In this study, we developed the precise algorithm for phase to height conversion, including the ambiguity method taking into account Earth ellipsoid, Schw<TEX>$\ddot{a}$</TEX>visch method, and the refined ambiguity method suitable for the interferometric pair with non-parallel obit. From the testing with JERS-1 orbit we found that the height error by traditional ambiguity method reaches to about 40 m during phase to height conversion. The proposed methods are very useful in generating precise InSAR DEM;especially in the case of using non-parallel InSAR pair due to unstable orbit control such as JERS-1 or intentional orbit control such as Cross-InSAR pair between ERS2 and ENVISAT satellite.

Inter-agency comparison of TanDEM-X baseline solutions

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is the first Synthetic Aperture Radar (SAR) mission using close formation flying for bistatic SAR interferometry. The primary goal of the mission is to generate a global digital elevation model (DEM) with 2m height precision and 10m ground resolution from the configurable SAR interferometer with space baselines of a few hundred meters. As a key mission requirement for the interferometric SAR processing, the relative position, or baseline vector, of the two satellites must be determined with an accuracy of 1mm (1D RMS) from GPS measurements collected by the onboard receivers. The operational baseline products for the TanDEM-X mission are routinely generated by the German Research Center for Geosciences (GFZ) and the German Space Operations Center (DLR/GSOC) using different software packages (EPOS/BSW, GHOST) and analysis strategies. For a further independent performance assessment, TanDEM-X baseline solutions are generated at the Astronomical Institute of the University of Bern (AIUB) on a best effort basis using the Bernese Software (BSW).Dual-frequency baseline solutions are compared for a 1-month test period in January 2011. Differences of reduced-dynamic baseline solutions exhibit a representative standard deviation (STD) of 1mm outside maneuver periods, while biases are below 1mm in all directions. The achieved baseline determination performance is close to the mission specification, but independent SAR calibration data takes acquired over areas with a well known DEM from previous missions will be required to fully meet the 1mm 1D RMS target. Besides the operational solutions, single-frequency baseline solutions are tested. They benefit from a more robust ambiguity fixing and show a slightly better agreement of below 1mm STD, but are potentially affected by errors caused by an incomplete compensation of differential ionospheric path delays.

A Fast Method for InSAR Phase Unwrapping Based on Quality Guide

相位解缠是干涉SAR 数据处理中的关键步骤，解缠效果的好坏直接影响干涉测量的精度。该文针对质量指导的相位解缠方法需要进行大量排序操作，在干涉数据维度较大时解缠效率低下的问题，提出了一种基于堆排序的快速的质量指导相位解缠方法。首先通过干涉复数据对或干涉相位确定相位质量图。然后利用最大堆作为质量图排序的数据结构，通过对最大堆进行删除根结点、插入新结点操作的过程中始终保持最大堆的性质不变，从而实现了质量图的排序，完成了从高质量区域到低质量区域的相位解缠。与传统方法相比，基于堆排序的方法大大降低了计算的时间复杂度，对于干涉SAR 大面积测绘应用具有十分重要的意义。最后，通过仿真和实测数据验证了算法的正确性和高效性。

GPS Based Relative Navigation for the TanDEM-X Mission - First Flight Results

ABSTRACT: TanDEM-X is the first Synthetic Aperture Radar (SAR) mission using close formation flying for bistatic SAR interferometry. Within three years, TanDEM-X will enable a global mapping of Earth and production of a high resolution digital elevation model (DEM). As a prerequisite for the interferometric SAR processing, the relative position, or baseline, of the two spacecraft must be determined with millimeter accuracy. In support of this task, the two spacecraft are equipped with geodetic-grade GPS receivers. Independent baseline solutions from the German Space Operations Center (DLR/GSOC) and the German Research Centre for Geosciences (GFZ) will be merged into a quality controlled combined product prior to use in the DEM generation. This paper provides an overview of the baseline determination process and discusses specific aspects such as phase pattern calibration, ambiguity resolution, and a trade-off between single- and dual-frequency solutions. First flight results from the early commissioning phase are presented.

InSAR 정보 서비스 스마트폰 어플리케이션 설계 및 시험 구현

Nowadays, demands with respect to applications based on geo-based information on mobile device such as smartphone and those based on open platform or open source have been increasing. This trend can be regarded as significant opportunities to widen application fields and to expand industry business cases using satellite imagery. However, it needs a different approach from conventional remote sensing researches. This work focuses on SAR among many kinds of geo-based data resources. First, an application for Interferometric SAR processing based on open source was implemented. And using this, InSAR data was processed and stored into database. When smartphone users at any places request InSAR results, they can receive InSAR information and concerned metadata on their device. An smartphone application for this task was designed and implemented in this study. This provides a practical way for SAR service for smartphone, and can apply to build mobile service system of complex and compound types of remote sensing resources and their derived contents.

Open radar interferometry software for mapping surface Deformation

Over the past 15 years, software for processing interferometric synthetic aperture radar (InSAR) data into maps of surface deformation has been developed and refined. The InSAR technique is commonly used to investigate deformation associated with earthquakes, volcanoes, withdrawal of crustal fluids, and coherent ice motions [Massonnet and Feigl, 1998]. The software, called Generic Mapping Tools Synthetic Aperture Radar (GMTSAR), is an open‐source (GNU General Public License) InSAR processing system designed for users familiar with Generic Mapping Tools (GMT) [Wessel and Smith, 1998]. The GMTSAR code is written in the C programing language and will run on any UNIX® computer. It requires installation of GMT and Network Common Data Format (NetCDF) and supports several fast Fourier transform libraries.

Geodetically Accurate InSAR Data Processor

We present a new interferometric synthetic aperture radar (InSAR) processing approach that capitalizes on the precise orbit tracking that is available with modern radar satellites. Our method uses an accurate orbit information along with motion-compensation techniques to propagate the radar echoes to positions along a noninertial virtual orbit frame in which the location and focusing equations are particularly simple, so that images are focused without requiring autofocus techniques and are computed efficiently. Motion compensation requires two additional focus correction phase terms that are implemented in the frequency domain. If the images from an interferometric pair or stack are all computed along the same reference orbit, flat-Earth topographic correction is not needed, and image coregistration is simplified, obviating many difficulties that are often encountered in InSAR processing. We process several data sets collected by the ALOS PALSAR instrument and find that the geodetic accuracy of the radar images is 10-20 m, with up to 20 m of additional image distortion needed to align 100 km × 100 km scenes with reference digital elevation models. We validated the accuracy by using both known radar corner reflector locations and by the registration of the interferograms with digital maps. The topography-corrected interferograms are free from all geometric phase terms, and they clearly show the geophysical observables of crustal deformation, atmospheric phase, and ionospheric phase.

Mapping of Sand Layer Thickness in Deserts Using SAR Interferometry

This paper presents an interferometric synthetic aperture radar (InSAR) system to map the bedrock topography underneath the sand in deserts and arid areas. This is anticipated to greatly increase the efficiency of oil field and ground water exploration as well as environmental and archaeological studies. The proposed system consists of two InSAR subsystems, one operating at Ka-band to map the sand topography and the other operates in the VHF band to map the subsurface topography. The different issues associated with InSAR processing for subsurface mapping are discussed. It is shown that conventional InSAR processing produces unacceptable error in height estimation since it does not account for the refraction and the different propagation velocity in the sand. Thus, a new inversion algorithm is developed which can be used to accurately estimate the bedrock topography for arbitrary sand and bedrock geometries. A sensitivity analysis is then presented to show the effect of the different systematic and random errors. The inversion algorithm is verified experimentally for flat sand case using a scaled model that was implemented in the lab.

4-D SAR Imaging: The Case Study of Rome

Four-dimensional synthetic aperture radar (SAR) imaging, also known as differential SAR tomography, is a new research topic in the framework of coherent multitemporal/multibaseline SAR processing that extends the interferometry concept. Four-dimensional SAR imaging-based processing could improve the capability of ground-scatterer monitoring with respect to classical differential interferometric SAR processing. The first results on the applicability of such an advanced tomographic SAR processing to real spaceborne data were recently discussed in the literature. In this letter, we present the results of an experiment with a data set that demonstrates the potentialities of this new technique for monitoring complex targets, such as infrastructures.

Improvement of the Accuracy of InSAR Image Co-Registration Based On Tie Points – A Review

Interferometric Synthetic Aperture Radar (InSAR) is a new measurement technology, making use of the phase information contained in the Synthetic Aperture Radar (SAR) images. InSAR has been recognized as a potential tool for the generation of digital elevation models (DEMs) and the measurement of ground surface deformations. However, many critical factors affect the quality of InSAR data and limit its applications. One of the factors is InSAR data processing, which consists of image co-registration, interferogram generation, phase unwrapping and geocoding. The co-registration of InSAR images is the first step and dramatically influences the accuracy of InSAR products. In this paper, the principle and processing procedures of InSAR techniques are reviewed. One of important factors, tie points, to be considered in the improvement of the accuracy of InSAR image co-registration are emphatically reviewed, such as interval of tie points, extraction of feature points, window size for tie point matching and the measurement for the quality of an interferogram.

DEM Control in Arctic Alaska With ICESat Laser Altimetry

Use of Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry is demonstrated for control of a digital elevation model (DEM) that is synthesized from repeat-pass ERS-1 and 2 synthetic aperture radar (SAR) imagery using interferomet-ric SAR (InSAR). Our study area is 15 650 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of the Barrow, AK coastal plain adjacent to the Arctic Ocean; a vast expanse of tundra, lakes, and arctic wetlands of such low relief as to be nearly devoid of terrain features. The accuracy of the ICESat-derived elevation measurements is assessed by comparison with differential global positioning system (DGPS) data acquired along ICESat ground tracks. The ICESat-derived elevations have a mean accuracy, relative to the DGPS elevations, of -0.01 plusmn 0.18 m. ICESat-derived elevations on the Arctic coastal plain provide an excellent source for DEM control. We employ the ICESat-derived ground control points (GCPs) in two distinct InSAR processing steps: 1) selected points are used to perform baseline refinements, which improves the ERS-1 and 2 interfero-grams and 2) the ICESat-derived GCP position data (latitude, longitude, elevation) are then used as control in mosaicking multiple InSAR-derived DEMs. The resulting ICESat-controlled DEM has a mean accuracy of -1.11plusmn 6.3 m relative to an independent standard, which is a commercial airborne InSAR-derived DEM having 0.5 m rms accuracy. This easily meets DTED-2 standards and suggests that DEMs derived using only ICESat altimetry for ground control would meet similar standards in other regions of low relief.

Knab Sampling Window for InSAR Data Interpolation

Interferometric synthetic aperture radar processing requires interpolating a slave image onto a master one. Since the signals requiring interpolation are limited in both time and bandwidth, the Knab sampling window provides an almost optimal and viable interpolation kernel. Its performance in terms of coherence preservation and interferometric phase error as a function of the number of retained samples and oversampling factor are shown.

Multilook Coherence Estimation Using Adaptive Weighted Window in Interferometric SAR

The coherence between two SAR images is an important factor in the interferometric SAR processing. As a previous step to the phase unwrapping, multilook averaging is widely used as the coherence estimator, and in general, a k × k square window is used. In this letter, it is shown that the performance of the coherence estimator can be improved by using an adaptive weighted window, which is originally developed for SAR speckle reduction. The proposed method was tested using the InSAR data obtained by an automobile-based SAR system.

An Adaptive Contoured Window Filter for Interferometric Synthetic Aperture Radar

An adaptive contoured window filter is proposed to filter off the noise from phase images of interferometric synthetic aperture radar (InSAR) in this letter. The contoured windows can best satisfy the requirement that constrains the phase signal constant inside windows on which low-pass filtering can remove the noise well while the fringe phases are well preserved. The contoured windows are determined by tracing along the local fringe orientation. An algorithm for determining window sizes adaptive to the fringe density is also proposed. The theoretical analysis and experiments prove that the proposed filter can greatly remove decorrelation noise while preserving the fringe phase well, even for those fringes with strong curvatures for InSAR processing

Delta-K Interferometric SAR Technique for Snow Water Equivalent (SWE) Retrieval

This letter describes the concept of using delta-K technique on interferometric synthetic aperture radar (InSAR) data for deriving the snow water equivalent (SWE) of dry snow-covered ground by utilizing the presence of scatterers in both datasets. The main scattering contribution from a dry snow cover is from the snow-ground interface. Thus, the interferometric phase of two SAR images, one with no snow and one with dry snow cover, contains information on the SWE. By performing a delta-K processing of the two SAR scenes followed by averaging, an estimation of the SWE can be achieved. The first step in the delta-K InSAR processing is to split the band into two nonoverlapping subfrequency band images. The resulting two subband images then contain two new carrier frequencies with a small delta frequency or delta-K separation. The next step is to multiply the two subband images together to obtain the delta-K image, one for summer and one for winter. Finally, the delta-K interferometric SAR image is generated by multiplying the two delta-K images from summer and winter together. In this letter, experimental results using European Remote sensing Satellite 1 (ERS-1) data from a summer and winter situation show that the delta-K phase can be estimated to a few degrees accuracy for an area of 10/spl times/10 km/sup 2/ corresponding to an SWE accuracy of approximately 100 mm.

Efficient simulation of sar interferograms of large areas and of rugged terrain

Interferometric synthetic aperture radar (InSAR) techniques are today applied in many areas of remote sensing, ranging from digital elevation model (DEM) generation to surface motion mapping and InSAR tomography. To enhance the understanding of the InSAR mapping process and to test new algorithms, accurate tools for the simulation of the topographic InSAR phase are necessary. Whereas the equations for the interferometric phase of a given DEM are well known, the actual implementation is tedious. Furthermore, a straightforward implementation would take far more computation time than all the other InSAR processing steps put together. This paper presents a novel algorithm for the efficient simulation of the InSAR phase, taking into account the special problems in mountainous terrain. Simulation results are compared to and illustrated with real data from the European Remote Sensing satellite (ERS-1/2) tandem mission and the Shuttle Radar Topography Mission (SRTM). Accuracy estimates for the phase simulation are given for different terrain types. The algorithm is described in enough detail that it can be implemented as a general-purpose tool for the accurate simulation of interferograms with virtually unlimited size, taking no more processing time than other InSAR processing steps. The algorithm in the presented form is used operationally within the interferometry software GENESIS to support the processing of SRTM/X-SAR data at the German Aerospace Center (DLR).