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Product overview
Backscatter is the portion of an outgoing radar signal that is reflected back to the sensor. Backscatter coefficient or sigma nought (SIG0) is the normalised measure of the radar power reflected from a distributed target, which is calculated per unit area on the ground. In Austrian Data Cube, Sentinel-1 input data, which is the C-band Interferometric WideSwath (IW), Ground Range Detected (GRD) products are processed in order to retrieve the sigma nought images for every polarisation on every timestamp. For example, an input data file which has dual polarisation Vertical-Vertical (VV) and Vertical-Horizontal (VH) will end up with two SIG0 images: one for VV and one for VH polarisation.
Product description
The processing of SIG0 product is carried out using the TU Wien SAR Geophysical Retrieval Toolbox (SGRT), which is the software for geophysical parameters retrieval from SAR imagery developed at TU Wien. In the following sub-sections, the main characteristics and data format of Sentinel-1 SIG0 product provided by TU Wien will be presented.
Product variables
The following table presents the data format of the SIG0 product after the processing at TU Wien.
| Code name | Scaling factor | Units | Type | Pixel size (m) | Image dimension | No-data | Number of bands |
|---|---|---|---|---|---|---|---|
| SIG0 | 100 | Decibel | int16 | 10,10 | 10000x10000 | -9999 | 1 |
Overview of Sentinel-1 Sigma nought product
Area and time period
SIG0 product is provided over the area of Austria from 01st January 2016 to the present.
Temporal resolution
The temporal resolution of SIG0 product is up to 3-4 days over Austria in practical.
Polarisation
As discussed above, the polarisation of SIG0 product depends on the original polarisation of the input data. The SIG0 images for each polarisation will be stored independently of each other. Therefore, there will be one SIG0 image if the input data comes with single polarisation, and there will be two SIG0 images if the input data is dual-polarised, respectively.
The figure below is a visualisation of VV and VH-polarised SIG0 for the same area, with the same color scale.
 |  |
Sentinel-1 Sigma nought or backscatter coefficient product at 10m spatial sampling, retrieved with VV polarisation (left) and VH polarisation (right). The scene covers the area of western Innsbruck, Austria, taken on 21st April 2016, 5:26:30'.
Image timestamps
The timestamps of SIG0 product can be found in the filename, according to Sentinel file naming convention. It can also be found in the GeoTIFF header.
Spatial resolution and sampling
After the pre-processing at TU Wien, the SIG0 images are tiled and georeferenced using the TU Wien Equi7(see Data specifications and formats) and have easting and northing coordinates with the pixel spacing of 10m.
Original data format
The SIG0 data is originally stored and delivered in GeoTIFF format. GeoTIFF is a standard which allows storing georeferencing information within a TIFF raster image.
Sentinel-1 Gamma nought product at 10m spatial sampling, retrieved with VH polarisation. The scene covers the area of western Innsbruck, Austria, taken on 21st April 2016, 5:26:30.
| Product Name | Sigma nought |
|---|---|
| Code Name | GMR |
| Category | Sentinel-1 products |
| Spatial Sampling | 10m |
| Temporal Resolution | 3-4 days |
| Unit | Decibel |