Repository files navigation

Algorithms and utilities for Synthetic Aperture Radar (SAR) sensors.Enables cloud-native SAR processing viaXarrayandDask.

This Open Source project is sponsored by B-Open -https://www.bopen.eu.

Sarsen is a Python library and command line tool with the following functionalities:

• provides algorithms to terrain-correct satellite SAR data
  • geometric terrain correction (geocoding)
    • fast mode: to terrain-correct images
    • accurate mode: for interferometric processing
  • radiometric terrain correction (gamma flattening)
• accesses SAR data viaxarray-sentinel:
  • supports most Sentinel-1 data products asdistributed by ESA:
    • Sentinel-1 Single Look Complex (SLC) SM/IW/EW
    • Sentinel-1 Ground Range Detected (GRD) SM/IW/EW
  • reads uncompressed and compressed SAFE data products on the local computer oron a network viafsspec -depends on rasterio>=1.3
• accesses DEM data viarioxarray:
  • reads local and remote data in virtually any raster format viarasterio /GDAL
• supports larger-than-memory and distributed data access and processing viaDask
  • efficient geometric terrain-correction for a full GRD
  • efficient radiometric terrain-correction for a full GRD.

Overall, the software is in thebeta phase and the usual caveats apply.

Current limitations:

• documentation needs improvement. See #6.

Non-objectives / Caveat emptor items:

• No attempt is made to support UTC leap seconds. Observations that include a leap second maycrash the code or silently return wrong results.

The typical side-looking SAR system acquires data with uniform sampling in azimuth and slant range,where the azimuth and range represents the time when a given target is acquired and the absolutesensor-to-target distance, respectively.Because of this, the near range appears compressed with respect to the far range. Furthermore,any deviation of the target elevation from a smooth geoid results in additional local geometric and radiometricdistortions known as foreshortening, layover and shadow.

• Radar foreshortening: Terrain surfaces sloping towards the radar appear shortened relative to those sloping away from the radar.These regions are much brighter than other places on the SAR image.
• Radar layover: It's an extreme case of foreshortening occurring when the terrain slope is greater than the angle of the incident signal.
• Radar shadows: They occur when ground points at the same azimuth but different slant ranges are aligned in the direction of the line-of-sight.This is usually due to a back slope with an angle steeper than the viewing angle.When this happens, the radar signal never reaches the farthest points, and thus there is no measurement, meaning that this lack of information is unrecoverable.

The geometric terrain correction (GTC) corrects the distortions due to the target elevation.The radiometric terrain correction (RTC) also compensates for the backscatter modulation generatedby the topography of the scene.

The easiest way to installsarsen is in aconda environment.The following commands create a new environment, activate it, install the package and its dependencies:

  conda create -n SARSEN  conda activate SARSEN  conda install -c conda-forge dask proj-data sarsen

Note that the proj-data package is rather large (500+Mb) and it is only needed to handle input DEM whosevertical coordinate is not on a known ellipsoid, for exampleSRTM DEM with heigths over theEGM96 geoid.

Thesarsen command line tool corrects SAR data based on a selected DEM and may producegeometrically terrain-corrected images (GTC) or radiometrically terrain-corrected images (RTC).Terrain-corrected images will have the same pixels as the input DEM, that should be resampledto the target projection and spacing in advance, for example usinggdalwarp.

The following command performs a geometric terrain correction:

  sarsen gtc S1B_IW_GRDH_1SDV_20211217T141304_20211217T141329_030066_039705_9048.SAFE IW/VV South-of-Redmond-10m_UTM.tif

Performing geometric and radiometric terrain correction is more demanding,but it is possible to produce the RTC of a full GRD product at a 10m resolutionin one go (and it takes approx 25 minutes on a 32Gb MacBook Pro):

  sarsen rtc S1B_IW_GRDH_1SDV_20211217T141304_20211217T141329_030066_039705_9048.SAFE IW/VV South-of-Redmond-10m_UTM.tif

The python API has entry points to the same commands and it also gives access to several lower levelalgorithms, but internal APIs should not be considered stable:

The following code applies the geometric terrain correction to the VV polarization of"S1B_IW_GRDH_1SDV_20211217T141304_20211217T141329_030066_039705_9048.SAFE" product:

>>>importsarsen>>>product=sarsen.Sentinel1SarProduct(..."tests/data/S1B_IW_GRDH_1SDV_20211223T051122_20211223T051147_030148_039993_5371.SAFE",...measurement_group="IW/VV",... )>>>gtc=sarsen.terrain_correction(...product,...dem_urlpath="tests/data/Rome-30m-DEM.tif",... )

The radiometric correction can be activated using the keycorrect_radiometry:

>>>rtc=sarsen.terrain_correction(...product,...dem_urlpath="tests/data/Rome-30m-DEM.tif",...correct_radiometry="gamma_nearest"... )

This is the list of the reference documents:

• the geometric terrain-correction algorithms are based on:"Guide to Sentinel-1 Geocoding" UZH-S1-GC-AD 1.10 26.03.2019
• the radiometric terrain-correction algorithms are based on:D. Small, "Flattening Gamma: Radiometric Terrain Correction for SAR Imagery," in IEEE Transactions on Geoscience and Remote Sensing, vol. 49, no. 8, pp. 3081-3093, Aug. 2011, doi: 10.1109/TGRS.2011.2120616

The main repository is hosted on GitHub.Testing, bug reports and contributions are highly welcomed and appreciated:

https://github.com/bopen/sarsen

Lead developer:

• Alessandro Amici -B-Open

Main contributors:

• Aureliana Barghini -B-Open

See also the list ofcontributors who participated in this project.

B-Open commits to maintain the project long term and we are happy to accept sponsorships to develop new features.

We wish to express our gratitude to the project sponsors:

• Microsoft has sponsored the support forGRD products and thegamma flattening algorithm.

Copyright 2016-2022 B-Open Solutions srlLicensed under the Apache License, Version 2.0 (the "License");you may not use this file except in compliance with the License.You may obtain a copy of the License at  http://www.apache.org/licenses/LICENSE-2.0Unless required by applicable law or agreed to in writing, softwaredistributed under the License is distributed on an "AS IS" BASIS,WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.See the License for the specific language governing permissions andlimitations under the License.