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Nadir Echo Suppression in Staggered SAR

Nogueira Peixoto, Maxwell and Villano, Michelangelo and Krieger, Gerhard (2019) Nadir Echo Suppression in Staggered SAR. In: Proceedings of Workshop on Advanced RF Sensors and Remote Sensing Instruments (ARSI). Advanced Remote Sensing Instruments (ARSI), 2019-11-11 - 2019-11-13, Noordwijk, The Netherlands.

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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.

Item URL in elib:https://elib.dlr.de/130665/
Document Type:Conference or Workshop Item (Poster)
Title:Nadir Echo Suppression in Staggered SAR
AuthorsInstitution or Email of AuthorsAuthor's ORCID iD
Villano, MichelangeloMichelangelo.Villano (at) dlr.dehttps://orcid.org/0000-0002-1769-6927
Krieger, GerhardGerhard.Krieger (at) dlr.dehttps://orcid.org/0000-0002-4548-0285
Journal or Publication Title:Proceedings of Workshop on Advanced RF Sensors and Remote Sensing Instruments (ARSI)
Refereed publication:Yes
Open Access:No
Gold Open Access:No
In ISI Web of Science:No
Keywords:Synthetic aperture radar, staggered SAR, nadir echo, best linear interpolation.
Event Title:Advanced Remote Sensing Instruments (ARSI)
Event Location:Noordwijk, The Netherlands
Event Type:international Conference
Event Dates:2019-11-11 - 2019-11-13
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:Earth Observation
DLR - Research area:Raumfahrt
DLR - Program:R EO - Earth Observation
DLR - Research theme (Project):R - Vorhaben Tandem-L Vorstudien (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Microwaves and Radar Institute > Radar Concepts
Deposited By: Villano, Dr.-Ing. Michelangelo
Deposited On:15 Nov 2019 13:22
Last Modified:20 Jan 2020 14:13

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