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CAS500-1

CAS500-1 L1G panchromatic and multispectral EO data products are calibrated using the Optical Products Calibration (OPT-Calib) service. The process applied is the same for PAN and MS acquisitions.

Image-001

Figure 1 - True Color composite derived from CAS500-1 L1G data acquired on 10/06/2023 over Palu, Sulawesi, Indonesia calibrated and pan sharpened with the ESA Charter Mapper. Credits: NGII, KARI.

Input

The input is an ingested CAS500-1 Dataset, find details here.

DN to Radiance

For the conversion from \(DN\) (for Digital Numbers) to spectral radiance (or ‘TOA radiance’) \(L \), the following formula is used :

\[ L(b) = { DN_{(b)} \times GAIN_{(b)} } + OFFSET_{(b)} \]

\(L \) in \(W/m^2/steradians/micrometers\) with \(b\) being a band ID.

The gain and offset values are derived from yearly updated coefficients provided by the National Satellite Center of Korean National Geographic Information Institute based on ground observations.

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 \(ESUN(b)\) are based on Thuillier 2003 as described in Table 1.

Band λ(nm) ESUN (Thuillier 2003) mW m^-2 nm^-1
PAN 450-900 1258.38
Blue 450-520 1984.65
Green 520-600 1815.54
Red 630-690 1536.38
NIR 760-900 967.99

Table 6: ESUN values for CAS500-1 AEISS-C Imager bands.

Output

The output is a STAC item GeoJSON1 including the assets shown below

  "assets": {
    "pan": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/pan.tif",
      "eo:bands": [
        {
          "name": "pan",
          "common_name": "pan",
          "center_wavelength": 0.675,
          "full_width_half_max": 0.45,
          "solar_illumination": 1258.38
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 0.5,
          "statistics": {
            "minimum": 0,
            "maximum": 7562,
            "mean": 2471.5104115037,
            "stddev": 1649.5555761222,
            "valid_percent": 100
          },
          "histogram": {
            "count": 256,
            "min": -14.8274509803922,
            "max": 7576.82745098039
          }
        }
      ],
      "file:size": 1264137190
    },
    "blue": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/blue.tif",
      "eo:bands": [
        {
          "name": "blue",
          "common_name": "blue",
          "center_wavelength": 0.485,
          "full_width_half_max": 0.07,
          "solar_illumination": 1984.65
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 2,
          "statistics": {
            "minimum": 0,
            "maximum": 6595,
            "mean": 1610.1168525565,
            "stddev": 1189.4785714614,
            "valid_percent": 100
          },
          "histogram": {
            "count": 256,
            "min": -12.9313725490196,
            "max": 6607.93137254902
          }
        }
      ],
      "file:size": 71363545
    },
    "green": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/green.tif",
      "eo:bands": [
        {
          "name": "green",
          "common_name": "green",
          "center_wavelength": 0.56,
          "full_width_half_max": 0.08,
          "solar_illumination": 1815.54
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 2,
          "statistics": {
            "minimum": 0,
            "maximum": 6221,
            "mean": 1639.6850050845,
            "stddev": 1170.3699460103,
            "valid_percent": 100
          },
          "histogram": {
            "count": 256,
            "min": -12.1980392156863,
            "max": 6233.19803921569
          }
        }
      ],
      "key": "green",
      "file:size": 76923682
    },
    "red": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/red.tif",
      "eo:bands": [
        {
          "name": "red",
          "common_name": "red",
          "center_wavelength": 0.66,
          "full_width_half_max": 0.06,
          "solar_illumination": 1536.38
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 2,
          "statistics": {
            "minimum": 0,
            "maximum": 9938,
            "mean": 1460.1066545323,
            "stddev": 1100.6730889244,
            "valid_percent": 100
          },
          "histogram": {
            "count": 256,
            "min": -19.4862745098039,
            "max": 9957.4862745098
          }
        }
      ],
      "key": "red",
      "file:size": 75080358
    },
    "nir": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "reflectance",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/nir.tif",
      "eo:bands": [
        {
          "name": "nir",
          "common_name": "nir",
          "center_wavelength": 0.83,
          "full_width_half_max": 0.14,
          "solar_illumination": 967.99
        }
      ],
      "raster:bands": [
        {
          "spatial_resolution": 2,
          "statistics": {
            "minimum": 0,
            "maximum": 10000,
            "mean": 2748.6915238585,
            "stddev": 1869.6571688402,
            "valid_percent": 100
          },
          "histogram": {
            "count": 256,
            "min": -19.6078431372549,
            "max": 10019.6078431373
          }
        }
      ],
      "key": "nir",
      "file:size": 86763418
    },
    "ndvi": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/ndvi.tif",
      "raster:bands": [
        {
          "spatial_resolution": 2,
          "statistics": {
            "minimum": -0.99855470657349,
            "maximum": 0.95955508947372,
            "mean": 0.30803732848255,
            "stddev": 0.15089579239428,
            "valid_percent": 71.81
          },
          "histogram": {
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            "min": -1.00239413754613,
            "max": 0.963394520446362
          }
        }
      ],
      "file:size": 195089108
    },
    "ndwi": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "data",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/ndwi.tif",
      "raster:bands": [
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            "valid_percent": 71.81
          },
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      ],
      "file:size": 195325011
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    "overview-trc": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
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      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/overview-trc.tif",
      "raster:bands": [
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          "statistics": {
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            "valid_percent": 100
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          "statistics": {
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            "stddev": 74.74403832483,
            "valid_percent": 100
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            "stddev": 114.73645148004,
            "valid_percent": 100
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      ],
      "file:size": 111525078
    },
    "overview-civ": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/overview-civ.tif",
      "raster:bands": [
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      ],
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    "overview-pan": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
        "visual"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/overview-pan.tif",
      "raster:bands": [
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    },
    "overview-trc-low-res": {
      "type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "roles": [
        "composite",
        "overview"
      ],
      "href": "https://.../calibratedDatasets/C1_20230610025814_12315_00049489_L1G-calibrated/overview-trc-low-res.tif",
      "raster:bands": [
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          "statistics": {
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      "file:size": 2624498
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  }

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