The purpose of calibration is to enable the calculation of the actual radiance of the Earth target from the corresponding digital counts produced by the SEVIRI instrument.

Calibration of the Level 1.5 Images comprises a number of steps:

Meteosat IR / WV Channel Calibration Methods

• (equalisation and linearisation)

SEVIRI Solar Channel Calibration Coefficients for Meteosat-8 and Meteosat-9:

Pre-Launch calibration

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 and 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.

(PDF, 388 KB).
(WinZip, 226 KB). Note: SEVIRI PFM is onboard Meteosat-8, SEVIRI FM2 is onboard Meteosat-9, SEVIRI FM3 is onboard MSG-3, and SEVIRI FM4 is onboard MSG-4 (data not yet available).

In the MSG-MPEF 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.

Please note: Information previously provided to the User community describes the process used to convert MSG radiances to temperatures. Recent analysis has revealed that an inaccuracy arises when this conversion formula is applied to the spectral blackbody radiance provided by the Level 1.5 images that is significant enough to justify the modification of the MSG Ground Segment to generate Level 1.5 Image Products in terms of effective blackbody radiance rather than spectral blackbody radiance.

A description of a more accurate conversion to brightness temperature, the planned changes to the Level 1.5 Image Product radiance definition, as well as the steps that the User community may take to prepare for the change to effective radiance are described in the document A Planned Change to the MSG Level 1.5 Image Product Radiance Definition .

Information on how the MSG level 1.5 image product in effective radiance can be interpreted in terms of equivalent brightness temperatures (EBBT) can be found in the document in the form of tables relating EBBT to effective radiance. The same information is also contained in a zipped Excel sheet .

Post-launch commissioning calibration

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.

On-ground operational calibration (equalisation and linearisation)

For each scan of the instrument, 3 lines of Level 1.0 image are collected (9 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, by the Image Processing Facility (IMPF), to

• equalise the detectors and
• 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.