Introduction

Radiometric image quality is expressed in terms of :

• calibration accuracy,
• resolution,
• Modulation Transfer Function (MTF).

Radiometric calibration is used to assign a physical meaning to the digital value of each pixel, by relating it to a reference value called radiance at the top of the amosphere (expressed in W.m^-2.sr^-1.mm^-1) by means of an absolute calibration coefficient.

The value of this coefficient, which is specific for each spectral band, can be estimated by several methods using :

• an onboard calibration device consisting of two redundant tungstan filament lamps and an optical assembly mounted on a carbon rod. For about 2 minutes a motor rotates the rod step-by-step in front of the cameras, thus illuminating the whole of the instrument's field of view with a stable reference radiance.
  
• viewing terrestrial targets : these are areas for which the radiance at the top of the atmosphere, which are therefore seen by the instrument, may be calculated with sufficient accuracy. Test sites are used, for which ground measurements are taken at the same time as the satellite pass. Other phenomenon may also be used such as atmospheric scattering (Rayleigh value), the reflection of sunlight off the ocean's surface (glitter) and off high altitude clouds.
  
• simultaneous HRVIR/VGT image acquisition to enable transfer of calibration settings from one instrument to another.

A combination of all these methods is used to obtain the calibration coefficient at a given time, to monitor changes over time and to determine the calibration accuracy. For VGT the calibration accuracy is estimated to be 8% at the beginning of life and 5% after a few months.

Radiometric resolution represents the smallest radiance variation which the instrument can detect. It is estimated by measuring different values of noise on the images (noise along columns, on a digital quick-look, on the complete image) while the instrument is observing a uniform area.

The modulation transfer function characterises the sharpness of the image and is obtained through spectral analysis of simultaneous HRVIR and VGT images.

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Performance characteristics

Radiometric performance is expressed at the system level in terms of :

• radiometric resolution : mission performance characteristics in the form of NeDr are converted into noise-equivalent radiance (NeDL), which is the only significant unit at the instrument level, while taking into account specific solar and atmospheric conditions. When the instrument is observing a perfectly uniform area, measurements are then take of :

        * the column noise (NeDLD) which is the radiance variation at the entrance to the instrument, corresponding to the standard deviation measured along a column of the raw image data,
        * the digital quick-look noise (NeDLH) which is the radiance variation at the entrance to the instrument, corresponding to the standard deviation measured on a digital quick-look normalized to 60 x 60 pixels,
        * the image noise (NeDLS) which is the radiance variation at the entrance to the instrument, corresponding to the standard deviation measured on an image normalized to 1 728 x 2 000 pixels. As this characteristics cannot be directly measured, it is estimated by combining 2 noises (column and normalization).

Radiometric performance characteristics :

• radiometric calibration, in other words, the relative accuracy :

* of the absolute calibration coefficient used to convert the instrument's output data into physical values (radiance at the top of the atmosphere), is referred to as absolute calibration,
* of the ratio of absolute calibration coefficients for the HRVIR and VGT instruments for a given spectral band, is known as HRVIR/VGT inter-instrument calibration.

Calibration accuracy :

Geometric accuracy :

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  page updated on the 05-07-04