Impact of DEM-Assisted Coregistration on High-Resolution SAR Interferometry (original) (raw)
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Evaluation of DEM-assisted SAR coregistration
2008
Image alignment is without doubt the most crucial step in SAR Interferometry. Interferogram formation requires images to be coregistered with an accuracy of better than 1/8 pixel to avoid significant loss of phase coherence. Conventional interferometric precise coregistration methods for full-resolution SAR data (Single-Look Complex imagery, or SLC) are based on the cross-correlation of the SLC data, either in the original complex form or as squared amplitudes. Offset vectors in slant range and azimuth directions are computed on a large number of windows, according to the estimated correlation peaks. Then, a two-dimensional polynomial of a certain degree is usually chosen as warp function and the polynomial parameters are estimated through LMS fit from the shifts measured on the image windows. In case of rough topography and long baselines, the polynomial approximation for the warp function becomes inaccurate, leading to local misregistrations. Moreover, these effects increase with the spatial resolution and then with the sampling frequency of the sensor, as first results on TerraSAR-X interferometry confirm. An improved, DEM-assisted image coregistration procedure can be adopted for providing higher-order prediction of the offset vectors. Instead of estimating the shifts on a limited number of patches and using a polynomial approximation for the transformation, this approach computes pixel by pixel the correspondence between master and slave by using the orbital data and a reference DEM. This study assesses the performance of this approach with respect to the standard procedure. In particular, both analytical relationships and simulations will evaluate the impact of the finite vertical accuracy of the DEM on the final coregistration precision for different radar postings and relative positions of satellites. The two approaches are compared by processing real data at different carrier frequencies and using the interferometric coherence as quality figure.
Accuracy assessment of DEMs derived from multi-frequency SAR images
Digital elevation model (DEM) is useful for land surface terrain analysis and is an important requirement for topographic correction of SAR backscatter data in hilly region. SAR interferometric (InSAR) techniques are very useful for deriving DEM of a large patch of land area with considerable accuracy in a faster and cost-effective manner. Today, various orbital radars operating in different frequencies and orbital periods are capable of providing data suitable for interferometric analysis. In the present study we have generated DEMs of Jharia coalmine region, India using InSAR techniques from ALOS-PALSAR, Radarsat-2 data and compared them with SRTM (Shuttle Radar Topography Mission) DEM and then, validated the DEMs against DGPS elevations measured as Ground Control Points (GCPs). We have then attempted to use the InSAR generated high resolution DEM for topographic corrections of SAR backscatter data. Index Terms-Digital elevation model (DEM); SAR Interferometry; Multi-frequency SAR; DGPS I.
Sensitivity of topography on InSAR data coregistration
In interferometric data coregistration, both the windows and polynomial approaches influence the offsets calculation and coregistration results because of the topography in the area of a SAR image. In this paper, by using precise orbit data of ERS satellite and topographic information, we analyze qualitatively the relation between offsets difference and several influence factors as elevation, distance from near range to far range and perpendicular baseline and the selection of the order of polynomial in different topographies. The results show the DEM influence in range direction has a direct ratio relation with the elevation and perpendicular baseline but an inverse ratio relation with the distance from near to far range, the DEM influence in azimuth direction has a direct ratio relation with the elevation but small correlation with the perpendicular baseline and the distance from near to far range. Moreover, in the polynomial approach different orders of the polynomial are needed ...
Digital elevation models (DEM) are numerical representation of terrain elevation data that has been used in a wide range of spatial analysis applications. The principles for acquisition, storage, management, update, spatial analysis, visualization as well as integration with other systems are reasonably well known. However, as DEM applications are becoming increasingly more widespread, so does concern about the quality of the available elevation data and the propagation of DEM errors through the analysis. It is now well known by analysts and researchers that the results of many DEM-based quantitative operations are significantly affected by the magnitude and also by the spatial distribution of elevation errors in DEM. However, currently available DEM's frequently report only the average magnitude of errors as the root mean square error (RMSE), which does not provide information on systematic bias nor on the spatial patterns of the DEM errors (Heuvelink, 1998). A very important ...
SAR Imaging and Interferometry Using Parameters Estimated from Raw Data
Photogrammetric Engineering & Remote Sensing, 2014
Due to the costs, sensitivity, and export policies of many governments, universities, and research institutions, particularly in developing countries, the ability to purchase, install and maintain high-accuracy inertial navigation system (INS) and global positioning system (GPS) is restricted. This paper presents a new method for SAR imaging and interferometry using parameters estimated from raw data. First, methods for determining the order of real and imaginary parts within the raw data images, and for determining chirp rate polarity are proposed. Second, a selection of parameters, including the forward effective velocities of the sensor, the near range distance, and the squint angle, are extracted using the Doppler centroid and Doppler rate. Finally, we create single look complex (SLC) images, coherence maps, digital elevation models (DEMs), and dual-pass differential unwrapped phase maps. The level of accuracy shown in this comparative study suggests that the proposed method is acceptable for creating the featured SAR products and is suitable for real world applications. This method and result is particularly relevant for systems which suffer from a lack of high-accuracy positional metadata. Due to the costs, sensitivity, and export policies, many universities or institutes in developing countries, including China, are unable to buy and install the high-accuracy INS and GPS systems on airborne SAR platforms required for high accuracy orbit calculations (
Quality assessment of interferometric SAR DEMs
2000
A new interferometric SAR (InSAR) procedure for DEM generation was employed to generate different DEMs from ERS SAR image pairs. The procedure was validated comparing the InSAR DEMs with a suited reference DEM. In the first part of the paper the principal features of the procedure are briefly summarised. The second part is focused on the quality assessment of the InSAR DEMs. They cover the same test area and come from one ascending SAR image pair, one descending pair and from the fusion of data coming from ascending and descending images. The analysis includes the influence of the SAR image coherence, the degradation of the DEM quality related to the terrain topography and the artefacts due to atmospheric effects.
Topography Correction for Airborne Synthetic Aperture Radar
International Conference on Aerospace Sciences and Aviation Technology
Airborne synthetic aperture radar (SAR) is an essential tool for modern remote sensing applications. The motion compensation (MOCO) in SAR processing is usually carried out by assuming a reference level to compute the range displacements and phase corrections to apply to each received echo. This means that phase histories of targets at heights different from the reference level cannot be matched accurately, which might yield several effects in the final compressed image. Topography correction for airborne SAR accommodates topography variations during SAR data processing, using an external digital elevation model (DEM). The aperture-dependent MOCO is compensation the phase error of all targets before azimuth compression, resulting in an enhanced image quality. In this paper, analysis the effect of topography variations in focused image (after range and azimuth compression). Then presented an efficient way to use the information given by an external DEM to take into account the motion of the aircraft along the whole synthetic aperture is presented. Finally, real simulated-data experiments show that the proposed approach is appropriate for highly precise imaging of airborne SAR.
DEM by Ground-Based SAR Interferometry
IEEE Geoscience and Remote Sensing Letters, 2000
In this letter, a ground-based synthetic aperture radar (SAR) interferometer was used to generate digital elevation maps (DEMs) of the illuminated area. With respect to other ground-based data processing techniques, here, the effect of the propagation through the atmosphere is considered. An algorithm similar to multipass satellite SAR techniques was developed in accordance with the phase model used in the ground-based interferometry. Many images taken from different viewing angles were collected and combined to form different interferograms at a test site in Austria. Results from this technique have been compared with an existing geographic model of the test area.
Topographic SAR interferometry formulation for high-precision DEM generation
IEEE Transactions on Geoscience and Remote Sensing, 2002
In repeat-pass synthetic aperture radar (SAR) interferometry, the approximations, allowing the phase-to-height conversion, prevent high-resolution mapped relief. In this paper, we present a more general and exact formulation giving a new relationship between the interferogram phase and the target height. It is based on the interferometric SAR geometry and on a better expansion of the path length difference between the sensor and the target. This formulation emphasizes the impact of baseline uncertainties on digital elevation model (DEM) accuracy.
CRUCIAL POINTS OF INTERFEROMETRIC PROCESSING FOR DEM GENERATION USING HIGH RESOLUTION SAR DATA
Data collection for digital elevation model (DEM) generation can be carried out by two main methods in space-borne remote sensing such as stereoscopy using optical or radar satellite imagery (stereophotogrammetry, respectively radargrammetry) and interferometry based on interferometric synthetic aperture radar (InSAR) data. These techniques have advantages and disadvantages in comparison against each other. Especially filling the gaps which arise from the problem of cloud coverage in DEM generation by optical imagery, InSAR became operational in recent years and DEMs became the most demanded interferometric products. Essentially, in comparison, DEM generation from synthetic aperture radar (SAR) images is not a simple manner like generation from optical satellite imagery. Interferometric processing has several complicated steps for the production of a DEM. The quality of the data set and used software package come into prominence for the stability of the generated DEM. In the paper, the interferometric processing steps for DEM generation from InSAR data and the crucial threshold values are tried to be explained. For DEM generation, a part of Istanbul (historical peninsula and near surroundings) was selected as the test field because of data availability. The data sets of two different imaging modes (StripMap ~ 3 m resolution and High Resolution Spotlight ~ 1 m resolution) of TerraSAR-X have been used. At the implementation, besides the determination of crucial points at interferometric processing steps, to define the effect of computer software, DEM production have been performed using two different software packages in parallel and the products have been compared In the result section of the paper, besides the colorful visualizations of final products along with the height scales, accuracy evaluations have been performed for both DEMs with the help of a more accurate reference digital terrain model (DTM). This reference model has been achieved by large scale aerial photos. Normally, it has a 5 m original grid spacing, however it has been resampled at a spacing of 1 m towards the needs of the research.