CBERS-4 and CBERS-4A

CBERS-4 and CBERS-4A multispectral EO data products are calibrated using the Optical Products Calibration (OPT-Calib) service. The process applied is the same for CBERS-4 and CBERS-4A acquisitions.

Figure 1 - True Color composite derived from calibrated CBERS-4 AWFI L4 data acquired on 27/07/2023 over the gulf of Bejaia in Algeria. Credits: INPE, AEB, CAST.

Input

The input is an ingested CBERS-4 and CBERS-4A dataset. Find details here for CBERS-4 and here for CBERS-4A.

DN to Reflectance

CBERS-4 and CBERS-4A TOA radiance for a band b is derived from DN values using the formula below¹:

\[ L_{(b)} = DN_{(b)} \times abscalibcoeff_{(b)} \]

where :

\(L_{(b)}\) is the TOA radiance for a CBERS-4 or CBERS-4A band b.
\(DN_{(b)}\) is the digital number for a band b. The digital number DN=0 represents the “NO_DATA” value.
\(abscalibcoeff_{(b)}\) is the scaling factor for the band b. This value is extracted from the ABSOLUTE_CALIBRATION_COEFFICIENT field in the product annotation.

Radiance to Reflectance

To convert TOA radiance to TOA reflectance, the following formula is used :

\[ R(b) = { {\pi \times L(b) \times d \times d} \over {ESUN(b) \times cos(θ)} } \]

where :

\(L(b)\) is the spectral radiance for band b
\(pi\) is the famous mathematical constant
\(d\) is the earth-sun distance (in astronomical units) and depends on the acquisition’s day and month
\(ESUN(b)\) is the mean TOA solar irradiance (or solar illumination) in \(W/m^2/micrometers\)
\(θ\) is the solar zenith angle in degrees.

The conversion from elevation angle to solar zenith angle \(θ\) (in degrees) is:

\[ θ = { 90 - elevation_angle } \]

The values for the \(ESUN(b)\) for CBERS-4 are provided in Table 1:

Common band name (BAND name)	λ(nm)	Thuillier 2003
pan (BAND1)	510-890	1259.85
blue (BAND5, BAND13)	450-520	1984.65
green (BAND2, BAND6, BAND14)	520-590	1823.40
red (BAND3, BAND7, BAND15)	630-690	1536.38
nir (BAND4, BAND8, BAND16)	770-890	981.91

Table 1 - ESUN value employed in the conversion of radiance to reflectance for CBERS-4 products.

The values for the \(ESUN(b)\) for CBERS-4A are provided in Table 2:

Common band name (BAND name)	λ(nm)	Thuillier 2003
pan (BAND0)	450-900	1258.38
blue (BAND1, BAND5, BAND13)	450-520	1984.65
green (BAND2, BAND6, BAND14)	520-590	1823.40
red (BAND3, BAND7, BAND15)	630-690	1536.38
nir (BAND4, BAND8, BAND16)	770-890	981.91

Table 2 - ESUN value employed in the conversion of radiance to reflectance for CBERS-4A products.

Output

The output is a STAC item GeoJSON² including single band and overview assets in COG format. Below is an example of assets included in the STAC item of a CBERS-4 AWFI L4 4-band multispectral BAND16151413 calibrated dataset.

  "assets": {
    "blue": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/blue.tif",
      "eo:bands": [
        {
          "name": "BAND13",
          "common_name": "blue",
          "center_wavelength": 0.485,
          "solar_illumination": 1984.65
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 64
          }
    },
    "green": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/green.tif",
      "eo:bands": [
        {
          "name": "BAND14",
          "common_name": "green",
          "center_wavelength": 0.555,
          "solar_illumination": 1823.40
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 64
          }
    },
    "red": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/red.tif",
      "eo:bands": [
        {
          "name": "BAND15",
          "common_name": "red",
          "center_wavelength": 0.66,
          "solar_illumination": 1536.38
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 64
          }
    },
    "nir": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/nir.tif",
      "eo:bands": [
        {
          "name": "BAND16",
          "common_name": "nir",
          "center_wavelength": 0.83,
          "solar_illumination": 981.91
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 64
          }
    },
    "overview-trc": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
        "reflectance",
        "visual"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/overview-trc.tif"
    },
    "overview-civ": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
        "reflectance",
        "visual"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/overview-civ.tif"
    },
    "overview-trc-low-res": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
        "overview",
        "reflectance"
      ],
      "href": "https://.../Call956_AWFI_20220731_111_063_L4_BAND16151413-calibrated/overview-trc-low-res.tif"
    }
  }

Pinto C., Ponzoni F., Castro R., Leigh L., Mishra N., Aaron D., Helder D. (2016), "First in-Flight Radiometric Calibration of MUX and WFI on-Board CBERS-4", Remote Sens. 2016, 8, 405. DOI:/10.3390/rs8050405. ↩
Radiant Earth Foundation, STAC item specifications, a single spatiotemporal asset as a GeoJSON feature plus datetime and links, available at:https://github.com ↩