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

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

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

A CubeSat Add-On for Resolving Phase Unwrapping Errors in Single-Pass SAR Interferometry

Digital elevation models can be obtained from synthetic aperture radar (SAR) interferograms though the process of phase unwrapping. Even with high-quality bistatic interferograms, such as those produced by TanDEM-X, however, unwrapping errors occur, which are usually resolved by exploiting an additional acquisition over the same scene with a shorter baseline. This works proposes to upgrade a bistatic interferometric SAR system with a low-cost CubeSat add-on that allows detecting and resolving phase unwrapping errors in a single pass of the satellites. Simulation analyses show the effectiveness of the CubeSat add-on in spite of its much smaller antenna aperture

Nadir Echo Suppression in Staggered SAR

Conventional SAR sensors employ a constant pulse repetition interval (PRI), which results in blind ranges that limit the achievable swath width: a larger PRI yields a wider swath, but also a worse azimuth resolution. The technique of digital beamforming (DBF) allows for simultaneous imaging of multiple sub-swaths, but these sub-swaths are still separated by blind ranges. A staggered SAR employs a continuous variation of the PRI causing transmission events to no longer line up on the same ranges and allowing the consequent missing samples to be interpolated over, provided that data are sufficiently oversampled in azimuth. As a result, a staggered SAR system with DBF is capable of imaging a continuous wide swath with high azimuth resolution. Furthermore, the increased amount of data to be downlinked resulting from the azimuth oversampling can be reduced through onboard processing. Staggered SAR is the baseline acquisition mode of Tandem-L and under consideration for NISAR and the future missions of ESA’s Copernicus Programme. In a conventional constant PRI SAR nadir echoes also line up on the same ranges and appear in the focused image as bright stripes. However, these bright stripes can be avoided by selecting a PRI that causes the nadir echoes to align with the blind ranges. A drawback of the staggered SAR is that, since the PRI is varying and there are no blind ranges, the nadir echoes cannot be aligned with the blind ranges and will contaminate wider range intervals in the focused data. This raises the questions of how the nadir echo affects a staggered SAR image and what can be done to suppress nadir echoes. To answer the first question, we start by presenting an analytical description of the positioning of the nadir echo in the raw staggered SAR data. We also present a simplified nadir echo model based on a dedicated TerraSAR-X acquisition. With these in hands, a simulator is built capable of generating raw staggered SAR data from a given backscatterer distribution, e.g., a satellite SAR image acquired by an existing SAR sensor, contaminate it with nadir echo and process it into a focused image. The simulator can be run for different nadir echo parameters showing the impact on real scenes and for the constant PRI case, which is considered for comparison. For the second question, we propose a post-processing step based on identifying the samples affected by nadir echo in the range-compressed data and recovering them through interpolation of neighboring azimuth samples similarly as done in the raw data for the samples missing due to transmission. This step is especially challenging, if onboard processing has been performed, as nadir echoes are smeared along azimuth. The results of this work extend the staggered SAR theory and can be used to evaluate the impact of nadir echo in a specific staggered SAR system or to impose requirements on the nadir attenuation provided by the antenna pattern

Nadir Echo Suppression in Staggered SAR

Conventional SAR sensors employ a constant pulse repetition interval (PRI), which results in blind ranges that limit the achievable swath width: a larger PRI yields a wider swath, but also a worse azimuth resolution. The technique of digital beamforming (DBF) allows for simultaneous imaging of multiple sub-swaths, but these sub-swaths are still separated by blind ranges. A staggered SAR employs a continuous variation of the PRI causing transmission events to no longer line up on the same ranges and allowing the consequent missing samples to be interpolated over, provided that data are sufficiently oversampled in azimuth. As a result, a staggered SAR system with DBF is capable of imaging a continuous wide swath with high azimuth resolution. Furthermore, the increased amount of data to be downlinked resulting from the azimuth oversampling can be reduced through onboard processing. Staggered SAR is the baseline acquisition mode of Tandem-L and under consideration for NISAR and the future missions of ESA’s Copernicus Programme. In a conventional constant PRI SAR nadir echoes also line up on the same ranges and appear in the focused image as bright stripes. However, these bright stripes can be avoided by selecting a PRI that causes the nadir echoes to align with the blind ranges. A drawback of the staggered SAR is that, since the PRI is varying and there are no blind ranges, the nadir echoes cannot be aligned with the blind ranges and will contaminate wider range intervals in the focused data. This raises the questions of how the nadir echo affects a staggered SAR image and what can be done to suppress nadir echoes. To answer the first question, we start by presenting an analytical description of the positioning of the nadir echo in the raw staggered SAR data. We also present a simplified nadir echo model based on a dedicated TerraSAR-X acquisition. With these in hands, a simulator is built capable of generating raw staggered SAR data from a given backscatterer distribution, e.g., a satellite SAR image acquired by an existing SAR sensor, contaminate it with nadir echo and process it into a focused image. The simulator can be run for different nadir echo parameters showing the impact on real scenes and for the constant PRI case, which is considered for comparison. For the second question, we propose a post-processing step based on identifying the samples affected by nadir echo in the range-compressed data and recovering them through interpolation of neighboring azimuth samples similarly as done in the raw data for the samples missing due to transmission. This step is especially challenging, if onboard processing has been performed, as nadir echoes are smeared along azimuth. The results of this work extend the staggered SAR theory and can be used to evaluate the impact of nadir echo in a specific staggered SAR system or to impose requirements on the nadir attenuation provided by the antenna pattern