# 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.

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"
}
}


1. 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

2. Radiant Earth Foundation, STAC item specifications, a single spatiotemporal asset as a GeoJSON feature plus datetime and links, available at:https://github.com