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Optical Products Calibration

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Description

The calibration of Optical EO data is done in the ESA Charter Mapper via a dedicated processing service, named Optical Products Calibration (OPT-Calib), which derives in systematic calibrated images from ingested optical EO data products acquired from multiple EO-missions. Output optical calibrated single-band assets of TOA/BOA reflectance can be used as input for further thematic processing (e.g. co-location, change detection).

Workflow

The OPT-Calib service implements the workflow depicted below.

graph TB i[(COS-2)] style i fill:#ffde86,stroke:#333,color:#282828,stroke-width:2px i --> a1(EO data product) subgraph Inputs style Inputs fill:#e8e8e8,stroke:#333,color:#282828 a1[/Optical Dataset/] style a1 fill:#acc8ff,stroke:#333,color:#282828,stroke-width:2px end subgraph OPT-Calib a1 --> dntor[Convert DN to radiance] style a1 fill:#cfdfff,stroke:#333,color:#282828 dntor --> rtor[Convert radiance to reflectance] rtor --> cog[Convert to COG] cog --> p1(TOA/BOA Reflectance) style p1 fill:#cfdfff,stroke:#333,color:#282828 p1 --> over[Create Overview/s] over --> p2(overviews) style p2 fill:#cfdfff,stroke:#333,color:#282828 p1 --> stac[Create STAC item] p2 --> stac[Create STAC item] end subgraph Outputs style Outputs fill:#e8e8e8,stroke:#333,color:#282828 stac --> o1[/Reflectance asset for each CBN: pan, red, green, etc./] style o1 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px stac --> o2[/overview-pan, overview-trc, overview-civ, etc./] style o2 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px end

The processor employs the Optical Calibration application of Orfeo Toolbox1 or plain matrix calculations to apply the conversion of DN to radiance and reflectance.

A detailed description of each step of the OPT-Calib chain is provided below.

Digital Numbers to Radiance

Top Of Atmosphere (TOA) Radiance in Wμm-1 m-2 sr-1 is derived from DN values using the following formula:

\[ L_\lambda = gain \times DN + offset \]

where: \(L_\lambda\) is TOA Radiance in Wμm-1 m-2 sr-1.

Radiance to TOA Reflectance

TOA reflectance \(\rho_\lambda\) in spectral band \(\lambda\) is then derived from:

\[ \rho_\lambda = \frac {L \times d^2_{E,S} \times \pi} {ESUN \times cos(θ_{S})} \]

where: \(L_\lambda\) is the radiance in spectral band \(\lambda\), \(d^2_{E,S}\) is the is the earth-Sun distance in AU at given time, \(ESUN\) is the band averaged Exo Atmospheric Solar Irradiance at 1AU in mW m^-2 nm^-1, and \(θ_{S}\) is the solar zenith angle, and \(L\) is TOA Radiance in Wμm-1 m-2 sr-1.

ESUN is derived from reference Solar Spectral Irradiance and depends on radiometric resolution and Filter Spectral Response Profiles for each band of the optical EO data.

This information is usually provided in the mission handbooks or manuals. For those missions where the ESUN values were not provided, these are derived from Thuillier 2002, over nominal band spectral ranges. ESUN values can be refined once Filter Spectral Response Profiles for VHRI 100 imager sensors are known. Table 1 reports an example of ESUN for multiple assets made using Thuillier2 2002 model for Vision-13.

Band λ(nm) ESUN (Thuillier 2002) mW m^-2 nm^-1
PAN 450-650 1830.15
Blue 440-510 2002.25
Green 510-590 1822.22
Red 600-670 1613.83
NIR 760-910 948.98

Table 1 - ESUN values for multispectral bands of Vision-1 (VHRI 100 imager) sensor.

Creation of overviews

In the overview image creation, the service generates multiple full-resolution overviews derived from the assets of calibrated optical EO data. Multiple pre-defined RGB band composites are available in the ESA Charter Mapper for optical EO data. All the possible RGB composite options with the associated CBN combinations are listed in Table 2. Output visual products are given as RGBA band composites.

Code Composite RGB (CBN)
TRC True Color red, green, blue
CIV Color Infrared (vegetation) nir, red, green
LAW Land/Water nir, swir16, red
VEA Vegetation Analysis swir16, nir, red
SIR Shortwave Infrared swir22, nir, red
FCU False Color Urban swir22, swir16, red
ATP Atmospheric Penetration swir22, swir16, nir
WAD Water Depth green, blue, coastal

Table 2 - RGB Composites for ESA Charter Mapper using CBNs.

Note

More information about the creation of visual products in the ESA Charter Mapper can be found here.

Outputs

The outputs of the Optical Products Calibration service are the following products given in COG format:

  1. TOA/BOA reflectance for panchromatic and all multispectral bands,

  2. Multiple overviews from multispectral bands.

The output of OPT-Calib is a STAC Item with the all the output Assets included.

Product specifications for the Optical Products Calibration service can be found in the following following Tables 3 and 4.

Attribute Value / description
Long Name TOA/BOA Reflectance from calibrated Panchromatic or Multispectral Calibrated Optical data
Short Name pan, coastal, blue, etc. for all optical CBN for VIS, NIR, SWIR
Description TOA/BOA Reflectance from calibrated Panchromatic or Multispectral Calibrated Optical data rescaled to 10000
Processing level As source product (L1/L2)
Data Type Unsigned 16-bit Integer
Band Single for each optical CBN
Format COG
Projection Native
Units Dimensionless
Valid Range [0 - 10,000]
Scale Factor *0.0001

Table 3 - Product specification for TOA/BOA Reflectance Product of the Optical Products Calibration service.

The second type of output product (see Table 4) is a Browse image given as full resolution RGBA composite (multibands GeoTIFF in COG format).

Attribute Value / description
Long Name Full resolution RGBA composite or grayscale single band image from Panchromatic or Multispectral Calibrated Optical data
Short Name overview-pan, (grayscale), overview-trc, overview-civ, overview-law, overview-vea, overview-sir, overview-fcu, overview-atp, overview-wad (false color composite)
Description Grayscale single band geo-referenced image from PAN or RGBA composites from MS Optical data (including alpha band).
Processing level As source product (L1/L2)
Data Type UnSigned 8-bit
Band 4
Format COG
Projection Native
Valid Range [0 - 255]
Fill Value 0

Table 4 - Product specification for Browse Image Product of the Optical Products Calibration service.

  1. Orfeo Toolbox, Optical Calibration, Available at: https://www.orfeo-toolbox.org/CookBook/Applications/app_OpticalCalibration.html. ↩

  2. Thuillier, G., Hersé, M., Labs, D. et al. (2003), “The Solar Spectral Irradiance from 200 to 2400 nm as Measured by the SOLSPEC Spectrometer from the Atlas and Eureca Missions”. Solar Physics 214, 1–22. DOI: https://doi.org/10.1023/A:1024048429145. ↩

  3. Airbus Space and Defence, (2020) “Vision-1 Ideal High-Resolution Monitoring Solution”, Available at: https://www.intelligence-airbusds.com/automne/api/docs/v1.0/document/download/ZG9jdXRoZXF1ZS1kb2N1bWVudC02MDIyNg==/ZG9jdXRoZXF1ZS1maWxlLTYwMjI1/Vision-1_web_201906.pdf. ↩