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KOMPSAT-5

Radiometric calibration

KOMPSAT-5 Level-1D GTC products

Hereinafter is described the workflow for the radiometric calibration of K5 GTC products.

Kompsat-2 GTC image products in Enhanced High Resolution (EH), Standard (ES), and Wide Swath (EW) acquisition modes are given in amplitude. The first step consits on the rescaling of the Digital Number as following:

\[ DN_{R} = RF \times DN \]

where the \(RF\) is the rescaling factor, derived from product metadata file with the attribute name “RescalingFactor”. For Wide Swath acquisiton mode \(RF\) is assumed as the average of the \(RF\) of each sub swath.

In the second step the calibration factor \(K\) is derived with the following equation:

\[ K = { CALCO \over cellsize } \]

where \(CALCO\) is the calibration constant, included in the metadata with the name “CalibrationConstant”, and \(cellsize\) is obtained from the product of Azimuth and Slant Range resolutions.

The azimuth resolution is included in the product metadata file under the attribute name "azimuthInstrumentGeometricResolution”, and the slante range resolution is derived with:

\[ r_A = { c \over 2 * BW_{rg} } \]

where \(c\) is the speed of the light, equal to 299792458 m/s, and \(BW_{rg}\) is the range focusing bandwidth derived from the product metadata file with the attribute name “rangeFocusingBandwidth”.

The third step uses the Geocoded Incidence angle Mask (GIM) included into the GTC product to derive the local incidence angle in degrees:

\[ \theta = GIM_{DN} \times GIM_{RF} - GIM_{OFF} \]

where \(GIM_{DN}\) are digital numbers of the 8-bit GIM GeoTIFF product, \(GIM_{RF}\) and \(GIM_{OFF}\) are rescaling factor and offset of GIM derived both from the product metadata file with the attribute names “RescalingFactor” and “Offset”, respectively. In this computation only values of \(GIM_{DN}\) minor than 253 are taken to do not consider pixels values in layover and shadowing geometry.

After that, \(\theta\) is then converted into radians with:

\[ \theta_{rad} = { \theta\ \times {\pi \over 180 }} \]

Once the local incidence angle \(\theta\) is known, the radar brightness is then obtained using the following expression:

\[ \beta_0 = K \times | DN_{R}^2 | \]

and sigma nought is derived with:

\[ \sigma_0 = \beta_0 \times sin(\theta_{rad}) \]

Finally, the backscatter coefficient is converted to logarithmic scale with the following:

\[ \sigma_0(dB) = 10 \times log_{10}(\sigma_0) \]

Input

The input is an ingested KOMPSAT-5 Level-1D product, see details here.

Output

The output is a STAC item geojson ... including the assets ... in COG format.

Reference websites

[RW01] - Kompsat-5 Sigma Naught Equation