Calibration and characterisation of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on Meteosat Second Generation (MSG).
02 October 2020
01 October 2020
The SEVIRI Level 1.5 data includes calibrated radiances, which have been rectified to a fixed grid after geolocation. The following sections describe the characterisation and calibration needed to understand the instruments’ radiometric response.
Pre-launch instrument characterisation
Initial calibration is performed, pre-launch, on the SEVIRI instrument of each detector of the 12 channels — in terms of spectral, polarisation and radiometric characterisation, spatial frequency response and spatial sampling. These responses are non-linear and may change over the lifetime of the instrument. It is these changes that must be corrected for after launch, when producing images from the system.
Spectral responses are derived for all 12 channels of the SEVIRI instrument.
Spectral responses for Meteosat Second Generation (MSG). Note: SEVIRI PFM is onboard Meteosat-8, SEVIRI FM2 is onboard Meteosat-9, SEVIRI FM3 is onboard Meteosat-10, and SEVIRI FM4 is onboard Meteosat-11.
In the MSG product processing the conversion from the observed radiances of the 'cold' channels (IR3.9, IR6.2, IR7.3, IR8.7, IR9.7, IR10.8, IR12.0, IR13.4) to equivalent black-body temperatures (so-called brightness temperatures) is done with the help of an analytical relation between these two quantities. These are expressed as a look-up table in the document Effective Radiance and Brightness Temperature Relation, and in the zipped Excel spreadsheet Tables relating EBBT to effective radiance. These tables converting radiance to brightness temperature can be accurately approximated by analytic functions described by The Conversion from Effective Radiances to Equivalent Brightness Temperatures.
Note: Original SEVIRI Level 1.5 image processed before May 2008 used spectral black-body radiances, which were defined in a different way from the effective radiances used in data processed later and for reprocessed data now available from the Data Centre before this change. The differences between these radiance definitions, together with other processing changes, and their impacts are described in the document On Differences in Effective and Spectral Radiance MSG Level.
SEVIRI Modulation Transfer Function (MTF) characterisations for MSG describes the spatial response of SEVIRI’s pixels as characterised during pre-launch testing.
Note: SEVIRI PFM is onboard Meteosat-8 (MSG-1), SEVIRI PFM2 is onboard Meteosat-9 (MSG-2), SEVIRI FM3 is onboard Meteosat-10 (MSG-3), and SEVIRI FM4 is onboard MSG-4 (Meteosat-11).
Post-launch commissioning calibration validation
After launch a number of characteristics of the instrument will be measured to check for changes in instrument response due to launch and a different operating environment. These include measuring the noise and medium-term drifts and an assessment of the Level 1.5 Image quality using an independent tool, the Image Quality Ground Segment Equipment (IQGSE).
On-board operational calibration and vicarious calibration
Calibration of the 'cold' channels (IR3.9, IR6.2, IR7.3, IR8.7, IR9.7, IR10.8, IR12.0, IR13.4) is performed several times a day. An on-board black-body source is placed in the optical path of the instrument, at two different known temperatures. The response of the instrument at these two temperatures is then fed into a mathematical model, representing the instrument optics, which gives the required scaling and offset needed to be applied to the instrument output in order to determine the corresponding radiance for any output.
However the on-board black body can not be used for the 'solar' or 'warm' channels (HRV, VIS0.6, VIS0.8, IR1.6). For these channels, on-ground vicarious calibration is used. Data from outside the MSG system, from other satellites or from calibration campaigns, about known stable sites on the Earth and meteorological conditions, are used, along with a Radiation Transfer Model (RTM), to predict the response of the instrument. These predicted responses are compared to the actual response and, again, offsets and scaling required for the instrument output are calculated.
The calibration coefficients derived from this process are given in the level 1.5 image headers.
On-ground operational calibration (equalisation and linearisation)
For each scan of the instrument, three lines of Level 1.0 image are collected (nine for the HRV channel), one for each detector. Each of these detectors may have different responses and offsets which, if left uncorrected, would cause a visible striping of the image. Furthermore, the detector response is not necessarily naturally linear.
Hence, on-ground processing is performed to:
- equalise the detectors;
- linearise the level 1.5 image (i.e. a doubling of the radiance of the target (Earth) produces a doubling in the pixel value).
The image is then geometrically corrected to a standard projection.