18.104.22.168 Level 1b Engineering Quantities
The MERIS Level 1b product is given in radiance units [LU] after scaling.
The in-band reference irradiances for the 15 MERIS bands are computed by averaging the in-band solar irradiance of each pixel. The in-band solar irradiance of each pixel is computed by integrating the reference solar spectrum with the band-pass of each pixel.
The 15 Sun spectral flux values provided in the "GADS Scaling" record of the Level 1b products are the in-band reference irradiances adjusted for the Earth-Sun distance at the time of measurement.
The band-pass of each pixel is derived from on-ground and in-flight characterisation via an instrument model.
The values "Band wavelength" and "Bandwidth" provided in the SPH of the Level 1b products are the averaged band-pass of each pixel over the instrument field of view. The centre wavelength of each pixel as characterised on ground is shown in figure 2.43 below for a sample CCD row pertaining to band 11. Band 11 is dedicated to the observation of oxygen absorption and an accurate knowledge of its central wavelength is crucial for the retrieval of atmospheric pressure.
The detailed information can be found in document R-10. This workbook contains 9 data sheets:
- the central wavelengths of each FR pixel for each band (pixel 1 is East, pixel 3700 is west)
- corresponding in-band irradiances (derived using above wavelengths, computed instrument response functions and reference irradiances from sheet 4, scaled to Sun-Earth distance for MERIS reference day of year 95)
- FWHM (full width at half modulation) of the instrument response functions for each band of each camera (each camera has 740 FR pixels – there is no FWHM variation for a given band within a given camera FOV -; camera 1 is east, camera 5 is west)
- sheet 4 contains the reference Irradiance at 1 AU adopted for ENVISAT (Thuillier, G., M. Hersé, P. C. Simon, D. Labs, H. Mandel, D. Gillotay, and T. Foujols, The solar spectral irradiance from 200 to 2400 nm as measured by the SOLSPEC spectrometer from the ATLAS 1-2-3 and EURECA missions, Solar Phys., submitted, 2002.)
- sheets 5 to 9 hold the instrument response functions for cameras 1 to 5 (hence 5 sheets), for each band, computed at centre of FOV (between pixels 370 & 371). Each function is scaled to max=1.
All wavelengths and widths are expressed in nm, irradiances in mW.m-2.nm-1.
Figure 2.7 - Sample CCD row for band 11.
The instrument response can be computed by including, or not, a non-linearity and stray light corrections.
The non-linearity correction consists of transforming a 16-bit telemetry word into a floating point value before any offset and smear corrections (see CCD output amplifier. With the correction switched on, both the calibration and measurement signals are corrected before the instrument gains are computed or applied, respectively.
The stray light correction consists of the de-convolution of the signal with a stray light kernel. The kernels are shift invariant across-track and include for each spectral band a contribution from different spectral regions of the CCD. (See
Stray Light Correction Algorithm 22.214.171.124.4. .)
With the correction switched on, both the calibration and measurement signals are corrected after the instrument gains are computed/applied. During calibration processing, an iterative process is used to return the instrument gains, corrected for stray light.