Introduction to formats

In the Menut Viewer, data is available in the following formats: 

• L1N
  • It is a geometrically corrected, high-precision product with a digital value of the pixel captured by the sensor with only relative calibration applied. No dark current subtraction is applied.
  • Absolute calibration coefficients (laboratory OC) are present in the metadata but have not been applied. Therefore, and since there is no radiometric calibration, these images do not allow for analysis of responses from different bands, development of indices or spectral algebra.
  • Format: COG-TIFF 16bpp, LZW compression, 7 bands.
     
• L1C
  • It is obtained from L1N, by applying absolute radiometric coefficients and an illumination model with pixels converted to TOA Reflectance. The parameters, the applied equation and the output units can be consulted in the metadata.
  • Absolute radiometric calibration is incorporated, with ICGC estimation of the Dark Current levels and calibration coefficients. This format allows performing algebra between bands and generating indices. It is not recommended for estimating the biophysical characteristics of the terrain, since the atmospheric effect is still present.
  • Format: COG-TIFF 16bpp, LZW compression, 7 bands.
     
• L2A
  • Processed from L1C, with atmospheric corrections and pixels converted to the lower part of the atmosphere (BOA Reflectance).
  • They incorporate atmospheric corrections and allow for the estimation of biophysical variables, spectral analysis and interband algebra.
  • Format: COG-TIFF 16bpp, LZW compression, 7 bands.

 
Gain 

Currently, the coefficient of each band is only the average of the pixels in the RADIANCE.TIF files and, therefore, this coefficient is only suitable for multiplying the DN and generating energy (this is how the calibration process for SNAP images was defined).

Standard satellite images, however, usually provide coefficients that convert the input DNs (SCAN images, in this case) into spectral radiance. In this sense, the following parameters must be taken into account for each band:

• Exposure time (a combination of line period and readout time).
• TDI.
• Spectral width.

These parameters are taken into account to generate the appropriate gain value (GAIN) and are written in the XML. Any modifications that the OP may make to these parameters in its operations must be communicated to the IEEC so that the ICGC can adapt its processing.

Measurements and units

In the metadata there are the fields MEASURE_DESC and MEASURE_UNIT, where the equations that have been used and their units are described. As an example, see the following equation that provides us with the values ​​of the radiances: 

• RadOut = BIAS + GAIN * DNin.

Another example, in the case of Pleiades, for the passage of reflectances to TOA:

• <MEASURE_DESC>Reflectance (RHO) to TOA Radiance (L). Formulae L=RHO/GAIN+BIAS</MEASURE_DESC>.
• <MEASURE_UNIT>watt/m2/steradian/micrometer</MEASURE_UNIT>.
  It should be noted that in the case of the Menut, for the previous example, Rho will be the original DN.

The XML associated with all the processed data is generated in a standard format for all satellite data. 

BIAS

In ICGC we apply relative correction and then add offset to allow users to reprocess images without loss of information. The BIAS value of the label ensures that no significant amount of pixels will fall below zero when absolute calibration is applied.

Glossary