classificationdigital image processingradarremote sensingsentinel-1
In a Sentinel-1 Level 1 Ground Range Detected SAR data product, Intensity = Amplitude * Amplitude.
Should the intensity image or the amplitude image be used for radiometric calibration when using the ESA Sentinel-1 Toolbox software? The ultimate purpose is the classification of geophysical features, e.g. land, water, etc.
I did this type of thing for a college project some years back using 25cm aerial photography. It is a difficult thing to accomplish. I ran a number of texture analysis on the imagery and added the bands to the RGB imagery to have more information during the classification process. While it is not a substitute for the NIR band, it did provide some additional information that increased the classification accuracy.
This is a known issue and caused by inaccurate access to the SRTM 3Sec (Auto Download) data which is required for the terrain correction. Usually, it is automatically downloaded but the access changed.
Solution: Please install the latest internal updates (see image below) of SNAP and restart the software. Additionally, I recommend using the SRTM 1Sec (Auto Download) because of its higher resolution and smaller tile size (mostly reduces downloading time). Furthermore, it is not affected by the issue mentioned above.
Best Answer
The intensity image should be used for calibration and subsequent classification of geophysical features. To radiometrically calibrate the intensity, use the Calibrate tool in the Sentinel-1 Toolbox (SAR Processing > Radiometric > Calibrate). The S-1 Level 1 GRD product includes several Look-Up Tables (LUTs) to convert intensity values into sigma or gamma naught values. Use these LUTs as the processing parameters for the Calibration tool.
Using amplitude would produce a calibration result of a lower magnitude than using intensity because intensity equals amplitude squared. The calibration is applied using the equation
For more information about the calibration process see the ESA Sentinel-1 Technical Guide.
Intensity depends on the scattering properties of surfaces (if the spatial extent of the scattering medium is large relative to the signal wavelength) or objects (if the spatial extent of the scattering medium is smaller than the signal wavelength). The effectiveness of a scatterer is determined by the radar cross section (RCS). Many parameters influence the RCS: surface roughness, local geomorphology, scatterer density, 3-D distribution of scatterers, and the dielectric properties of the scene. These variables can help classify features in an intensity image.