From GIS to Remote SensingI'm continuing the development of the new version 7 of the Semi-Automatic Classification Plugin (SCP) for QGIS.
In this post I'll briefly describe the download and processing of Sentinel-1 GRD images in SCP.
Synthetic Aperture Radar (SAR) is a technique of active remote sensing that is the sensor platform emits microwaves in order to acquire images of the ground (ESA, 2020). In fact, the sensor platform emits the radiation (at a specific wavelength) and measures the magnitude and the phase of radiation that bounces back from the ground to the sensor.
Sentinel-1 is a Copernicus mission of satellites that operate at C-band to provide SAR imagery at medium resolution (about 10m).
The Sentinel-1 constellation provides high revisit time (about 5 days), a wide swath (250 km), and acquires images in different operational modes.
Ground Range Detected (GRD, Level-1 data with multi-looked intensity only) is a product systematically delivered by Copernicus. Sentinel-1 supports dual polarization, which are horizontal (H) or vertical (V); VV and VH polarimetric channels are available to classify and analyse land cover such as built-up areas or vegetation.
SCP will include the search and download of Sentinel-1 GRD images, in the same interface that allows for the download of other satellite images, such as Sentinel-2 and Landsat and more.
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| Sentinel-1 download |
SCP 7 will preprocesses Sentinel-1 GRD images performing basically the same methodology applied for the images included in Google Earth Engine .
The preprocessing comprises the following phases:
- Application of the orbit file;
- Removal of GRD border noise for low intensity and invalid data;
- Removal of thermal noise to reduce discontinuities between sub-swaths;
- Calculation of backscatter intensity using radiometric calibration;
- Orthorectification (terrain correction) using the SRTM 30 meter DEM;
- Conversion of backscatter coefficient to dB.
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| Sentinel-1 preprocessing |
These processes are performed through the SNAP Graph Processing Tool (GPT) and a graph file .xml .
Therefore, the installation of ESA SNAP will be required for the preprocessing of Sentinel-1.
It will also be possible to use custom graph files and select to project the raster to the same coordinate reference system as a band set (default WGS 84).
I am going to post other news on this blog to illustrate the progress of SCP development.
For any comment or question, join the Facebook group about the Semi-Automatic Classification Plugin.
References
- ESA, 2020. Sentinel-1 SAR Definitions. Available at https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/definitions