Experimental Demonstration of Nadir Echo Removal in SAR Using Waveform Diversity and Dual-Focus Postprocessing

Synthetic aperture radar (SAR) provides high-resolution images for remote sensing applications regardless of sunlight and weather conditions. The pulsed operation of SAR may lead to an occurrence of nadir echoes in SAR images that significantly affect the image quality in case the pulse repetition frequency (PRF) is not properly constrained within the SAR system design. As an alternative, pulse-to-pulse variation of the transmitted waveform and dual-focus postprocessing can be exploited to remove the nadir echo and alleviate the PRF constraints (also in ScanSAR operation). This work provides a demonstration of the latter concept through an experimental acquisition of the TerraSAR-X satellite. The experiment is designed by selecting the scene and the acquisition parameters in order to have the nadir echo appearing in the SAR image. The waveform variation is achieved by alternating up- and down-chirps on transmit. The analysis of the results shows the effectiveness of dual-focus postprocessing for nadir echo suppression

A Waveform-Encoded SAR Implementation Using a Limited Number of Cyclically Shifted Chirps

Synthetic aperture radar (SAR) provides high-resolution images of the Earth’s surfaceirrespective of sunlight and weather conditions. In conventional spaceborne SAR, nadir echoescaused by the pulsed operation of SAR may significantly affect the SAR image quality. Therefore,the pulse repetition frequency (PRF) is constrained within the SAR system design to avoid theappearance of nadir echoes in the SAR image. As an alternative, the waveform-encoded SAR conceptusing a pulse-to-pulse variation of the transmitted waveform and dual-focus postprocessing canbe exploited for nadir echo removal and to alleviate the PRF constraints. In particular, cyclicallyshifted chirps have been proposed as a possible waveform variation scheme. However, a largenumber of distinct waveforms is required to enable the simple implementation of the concept.This work proposes a technique based on the Eulerian circuit for generating a waveform sequencestarting from a reduced number of distinct cyclically shifted chirps that can be effectively exploitedfor waveform-encoded SAR. The nadir echo suppression performance of the proposed scheme isanalyzed through simulations using real TerraSAR-X data and a realistic nadir echo model thatshows how the number of distinct waveforms and therefore the system complexity can be reducedwithout significant performance loss. These developments reduce the calibration burden and makethe concept viable for implementation in future SAR systems

Ambiguity Assessment and Mitigation Approaches for the TerraSAR-X Concurrent Imaging Technique

For TerraSAR-X the concurrent imaging technique enables the simultaneous acquisition of scenes in two different modes and over two disjoint areas. The acquisition scheme exploits the capability to toggle the antenna configuration from pulse to pulse. A direct consequence of this interleaved acquisition scheme is the necessity to increase the azimuth sampling rate. This in turn affects the ambiguity performance. The paper at hand analyzes the range and azimuth ambiguity performance of the concurrent imaging technique and describes approaches to mitigate the impact of ambiguities. Additionally, some experimental results are shown

Introducing F-Scan to the concurrent imaging mode

The concurrent imaging mode is a recently proposed technique to increase the imaging capability and flexibility of SAR systems. This mode allows for simultaneous acquisitions of two areas by increasing the pulse repetition frequency (PRF) by interleaving the transmission and reception of two modes in a pulse-to-pulse manner. Due to intrinsic system limitations, this technique applied to current operational systems, such as the German satellite TerraSAR-X, comes along with strong trade-offs in terms of limited swath width and increased ambiguity levels. For future X-band missions, the frequency scanning (F-Scan) technique is one of the most promising methods considered. Therefore, this paper aims to derive timing and interference analyses for the integration of F-Scan with the concurrent imaging concept. Additionally, it will be shown that F-Scan can improve the main critical performance parameters of the concurrent mode

Concurrent Imaging for TerraSAR-X: Wide-Area Imaging paired with High-Resolution Capabilities

The concurrent imaging technique enables parallel acquisitions with different beams or modes, e.g., a wide area Stripmap mode with a high resolution Spotlight mode. Such a concurrent Stripmap/Spotlight imaging technique is investigated for TerraSAR-X. This technique employs a pulse-topulse interleaving scheme to acquire two acquisitions - even of disjunctive areas - at the same time, offering products with different resolution and coverage portfolios. This capability is especially interesting for customers interested in an overview of a larger area but at the same time observing an area of interest with higher resolution, e.g., for infrastructure monitoring or reconnaissance applications. The basic concept, as well as the driving system parameters, are discussed in detail, together with a coverage analysis revealing the high availability rate of the mode combinations on a global scale. A processing approach reusing a substantial part of the existing infrastructure is described and exemplary acquisitions are shown, together with a detailed performance analysis with respect to resolution and ambiguities

Elevation and Deformation Extraction from TomoSAR

3D SAR tomography (TomoSAR) and 4D SAR differential tomography (Diff-TomoSAR) exploit multi-baseline SAR data stacks to provide an essential innovation of SAR Interferometry for many applications, sensing complex scenes with multiple scatterers mapped into the same SAR pixel cell. However, these are still influenced by DEM uncertainty, temporal decorrelation, orbital, tropospheric and ionospheric phase distortion and height blurring. In this thesis, these techniques are explored. As part of this exploration, the systematic procedures for DEM generation, DEM quality assessment, DEM quality improvement and DEM applications are first studied. Besides, this thesis focuses on the whole cycle of systematic methods for 3D & 4D TomoSAR imaging for height and deformation retrieval, from the problem formation phase, through the development of methods to testing on real SAR data. After DEM generation introduction from spaceborne bistatic InSAR (TanDEM-X) and airborne photogrammetry (Bluesky), a new DEM co-registration method with line feature validation (river network line, ridgeline, valley line, crater boundary feature and so on) is developed and demonstrated to assist the study of a wide area DEM data quality. This DEM co-registration method aligns two DEMs irrespective of the linear distortion model, which improves the quality of DEM vertical comparison accuracy significantly and is suitable and helpful for DEM quality assessment. A systematic TomoSAR algorithm and method have been established, tested, analysed and demonstrated for various applications (urban buildings, bridges, dams) to achieve better 3D & 4D tomographic SAR imaging results. These include applying Cosmo-Skymed X band single-polarisation data over the Zipingpu dam, Dujiangyan, Sichuan, China, to map topography; and using ALOS L band data in the San Francisco Bay region to map urban building and bridge. A new ionospheric correction method based on the tile method employing IGS TEC data, a split-spectrum and an ionospheric model via least squares are developed to correct ionospheric distortion to improve the accuracy of 3D & 4D tomographic SAR imaging. Meanwhile, a pixel by pixel orbit baseline estimation method is developed to address the research gaps of baseline estimation for 3D & 4D spaceborne SAR tomography imaging. Moreover, a SAR tomography imaging algorithm and a differential tomography four-dimensional SAR imaging algorithm based on compressive sensing, SAR interferometry phase (InSAR) calibration reference to DEM with DEM error correction, a new phase error calibration and compensation algorithm, based on PS, SVD, PGA, weighted least squares and minimum entropy, are developed to obtain accurate 3D & 4D tomographic SAR imaging results. The new baseline estimation method and consequent TomoSAR processing results showed that an accurate baseline estimation is essential to build up the TomoSAR model. After baseline estimation, phase calibration experiments (via FFT and Capon method) indicate that a phase calibration step is indispensable for TomoSAR imaging, which eventually influences the inversion results. A super-resolution reconstruction CS based study demonstrates X band data with the CS method does not fit for forest reconstruction but works for reconstruction of large civil engineering structures such as dams and urban buildings. Meanwhile, the L band data with FFT, Capon and the CS method are shown to work for the reconstruction of large manmade structures (such as bridges) and urban buildings

Nadir Echo Removal in Synthetic Aperture Radar via Waveform Diversity and Dual-Focus Postprocessing

Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independent of weather conditions and sunlight illumination and is, therefore, very attractive for the systematic observation of dynamic processes on the earth's surface. However, as a consequence of the pulsed operation and side-looking geometry of SAR, nadir echoes may significantly affect the SAR image quality, if the pulse repetition frequency is not conveniently constrained in the design of the SAR system. As the nadir interference constraint typically limits both the swath width and the ambiguity performance of the SAR system, the investigation of novel concepts for nadir echo removal is of great interest. This letter describes how to design an SAR system without the nadir interference constraint and how to remove (not only smear) the nadir echoes by means of waveform diversity on transmit and appropriate postprocessing. The proposed technique yields improved image quality and can be exploited in a similar manner for range ambiguity suppression with remarkable benefits for the design of novel SAR systems

NASA Tech Briefs, May 1992

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