157 research outputs found
Processing Techniques for Nadir Echo Suppression in Staggered Synthetic Aperture Radar
Synthetic aperture radar (SAR) is a class of high-resolution imaging radar particularly suitable for satellite remote sensing with diverse applications, such as biomass and ice monitoring, generation of digital elevation models, and measuring of subsidence. Staggered SAR is a novel mode of operation under consideration for the next-generation SAR missions such as Tandem-L and NASA-ISRO SAR. It uses digital beamforming and a continuous variation of the pulse repetition interval (PRI) to achieve high azimuth resolution over a much wider, continuous swath than is traditionally possible. This PRI variation renders infeasible use of existing methods to avoid nadir echoes, which might impair the quality of staggered SAR images. This work proposes processing techniques that mitigate the impact of nadir echoes in staggered SAR through localization and thresholding-and-blanking of these echoes in range-compressed data and recovery of part of the underlying useful signal through interpolation. The performance of these processing techniques is evaluated through simulations using real TerraSAR-X data. The proposed technique can be implemented as an optional stage in the processing chain of future staggered SAR missions and leads to improved image quality at a reasonable additional computational cost
Characterization of Nadir Echoes in Multiple Elevation-Beam SAR with Constant and Variable Pulse Repetition Interval
Multiple-elevation-beam synthetic aperture radar (SAR) is a concept based on digital beamforming (DBF) in elevation and simultaneous recording of the echoes of multiple transmitted pulses. It enables high-resolution imaging of wide areas and is therefore ideal for the systematic observation of dynamic processes on the Earth’s surface. Furthermore, if the pulse repetition interval (PRI) is continuously varied (staggered SAR), it is possible to map a wide continuous swath rather than multiple subswaths separated by “blind” ranges. Within the design of multiple-elevation-beam SAR, however, it is fundamental to consider how nadir echoes affect the mapping capabilities of systems with constant PRI and the image quality of staggered SAR systems, where nadir echoes are intrinsically smeared due to the PRI variation. This paper addresses the characterization of nadir echoes in multiple-elevation-beam SAR with constant and variable PRI by presenting a parametric model for the nadir echo profile based on real radar measurements, a formulation of the nadir echo location and smearing in staggered SAR, and realistic simulations based on TerraSAR-X data, which show that nadir echo are likely to be barely visible in staggered SAR images. The results of this work are relevant to both the design of future SAR systems and the interpretation of the acquired data
Investigation of Fixed Across-Track Baselines for Distributed Spaceborne SAR Systems
Spaceborne Synthetic Aperture Radar (SAR) greatly benefits from distributed concepts. This work investigates the opportunity of establishing formations with fixed baselines. In particular, a fixed baseline in the across-track direction can enhance the performance of SAR interferometry for the generation of accurate digital elevation models or enable multi-platform range ambiguity suppression. Achieving a fixed across-track baseline requires a continuous forced solution of the relative motion, different from the Helix configuration, adopted for TanDEM-X, which relies on a natural solution of the dynamics. Continuous control is given by low-thrust engines, which continuously compensate for the natural oscillations of the relative motion in the across-track direction and maintain the desired spacecraft separation. This study aims to evaluate the feasibility of continuous control, in terms of delta-velocity budget, and the impact on SAR interferometric performance. The analyses yield promising results in terms of SAR applications and can be used for preliminary mission analysis and design, including both SAR and mission control consideration
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
On the Exploitation of CubeSats for Highly Accurate and Robust Single-Pass SAR Interferometry
Highly accurate digital elevation models (DEMs) from spaceborne synthetic aperture radar (SAR) interferometry are often affected by phase unwrapping errors. These errors can be resolved by the use of additional interferograms with different baselines, but this requires additional satellites in a single-pass configuration, resulting in higher cost and system complexity, or additional passes of the satellites, which affects mission planning and makes the system less suitable for monitoring fast-changing phenomena. This work proposes augmenting a bistatic SAR interferometer with one or more receive-only CubeSats, whose images are used to form an additional interferogram with a small baseline, making the system robust to unwrapping errors. In spite of the lower quality of the CubeSat images due to their small antenna aperture, this additional information can be used to detect and resolve phase unwrapping errors in the DEM without impacting its resolution or accuracy. A processing scheme for the phase unwrapping correction is presented along with a theoretical model for its performance. Finally, a design example is presented and discussed along with a simulation based on TanDEM-X data. It is also shown that CubeSat add-ons allow further increasing the baseline and thus improving the accuracy of DEMs. This concept represents a cost-effective solution for the generation of highly accurate, robust DEMs and paves the way to distributed SAR interferometric concepts based on CubeSats
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
MirrorSAR: An HRWS Add-On for Single-Pass Multi-Baseline SAR Interferometry
This paper reports the Phase A study results of the interferometric extension of the High-Resolution Wide-Swath (HRWS) mission with three MirrorSAR satellites. According to the MirrorSAR concept, small, low cost, transponder-like receive-only satellites without radar signal demodulation, digitization, memory storage, downlink, and synchronization are added to the planned German X-band HRWS mission. The MirrorSAR satellites fly a triple helix orbit in close formation around the HRWS orbit and span multiple single-pass interferometric baselines. A comprehensive system engineering and performance analysis is provided that includes orbit formation, MirrorLink, Doppler steering, antenna pattern and swath design, multi-static echo window timing, SAR performance, height performance and coverage analysis. The overall interferometric system design analysis of Phase A is presented. The predicted performance of the global Digital Elevation Model (DEM) is improved by one order of magnitude compared to presently available global DEM products like the TanDEM-X DEM
SNR and Noise Variance Estimation in Polarimetric SAR Data
The problem of estimating the signal-to-noise ratio (SNR) of the cross-polarised channels and the noise variance in polarimetric synthetic aperture radar (SAR)
data is dealt with. The Cramer-Rao Lower Bound (CRLB) is evaluated and a maximum likelihood (ML) estimator is derived, which jointly estimates the SNR of
the cross-polarised channels and the noise variance. The performance of the joint estimator is assessed and a comparison with a coherence-based (CB) SNR
estimator and an eigenvalue-based (EB) noise variance estimator is carried out. As far as the SNR estimation is concerned, both the ML and the CB estimator
are biased, but the bias of the ML estimator is smaller than the bias of the CB estimator, while the accuracies are very similar. As far as the noise variance
estimation is concerned, the ML estimator is unbiased and its variance is equal to the CRLB, while the EB estimator is biased. The difference in the biases is
also shown using TerraSAR-X fully-polarimetric data, acquired during the Dual Receive Antenna (DRA) campaign
Accounting for Azimuth Ambiguities in Interferometric Performance Analysis
Azimuth ambiguities affect the interferometric performance of SAR systems, causing a bias in the interferometric phase and a modulation of the interferometric coherence, as also visible in some TanDEM-X interferograms. This paper provides an explanation for this phenomenon and derives the analytical expressions for the phase bias and the coherence, resorting to the interferogram statistics for jointly circular Gaussian processes. The impact of azimuth ambiguities on the overall system performance is then considered. Plots are provided, which display the standard deviation of the phase bias, as well as the expected value and the standard deviation of the coherence loss component due to azimuth ambiguities. These plots can be useful for interferometric performance analysis
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