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MSG RAO Annex III

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					ANNEX III : Definition of SEVIRI channels and characteristics of MSG
Images


General

          MSG is a spin-stabilised satellite in geostationary orbit, as its predecessor Meteosat. Its main
          payload will be the Spinning Enhanced Visible and Infrared Imager (SEVIRI) providing earth
          observation of the earth in 12 spectral channels.


Spectral channels

          SEVIRI has eight channels in the thermal infrared (IR), three Visible and Near InfraRed (VNIR)
          channels in the solar spectrum, and a high resolution visible (HRV) channel.

            Channel        Centre          Nominal spectral           Dynamic range               Radiometric Noise
                           wavelength      Bandwidth (µm),
                           (µm)            integrated energy (%)
            HRV            between 0.5-    Broadband channel          0 – 459 W/m2 µm             S/N > 4.3 for target of 1%
                           0.9 µm          similar to Meteosat        (scaled at centre           of max dynamic range
                                                                      wavelength)
            VIS 0.6        0.635           0.56 - 0.71                0 – 533 W/m2 µm             S/N > 10.1 for target of 1%
                                           98%                                                    of max dynamic range
            VIS 0.8        0.81            0.74 - 0.88                0 – 357 W/m2 µm             S/N > 7.28 for target of 1%
                                           99%                                                    of max dynamic range
            IR 1.6         1.64            1.50 - 1.78                0 – 75 W/m2 µm              S/N > 3 for target of 1% of
                                           99%                                                    max dynamic range
            IR 3.9         3.92            3.48 - 4.36                0 – 335 K                   0.35 K @ 300 K
                                           98.6%
            WV 6.2         6.25            5.35 - 7.15                0 – 300 K                   0.75 K @ 250 K
                                           99%
            WV 7.3         7.35            6.85 - 7.85                0 – 300 K                   0.75 K @ 250 K
                                           98%
            IR 8.7         8.70            8.30 - 9.10                0 – 300 K                   0.28 K @ 300 K
                                           98%
            IR 9.7         9.66            9.38 - 9.94                0 – 310 K                   1.50 K @ 255 K
                                           99%
            IR 10.8        10.8            9.80 - 11.80               0 – 335 K                   0.25 K @ 300 K
                                           98%
            IR 12.0        12.0            11.00 - 13.00              0 – 335 K                   0.37 K @ 300 K
                                           98%
            IR 13.4        13.40           12.40 - 14.40              0 – 300 K                   1.80 K @ 270 K
                                           96%




          Table 1: Spectral channels of MSG. Spectral bandwidth refer to percentage of total integrated energy received in this
          band, except for the HRV channel that is defined merely as a successor of Meteosat.




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            These spectral channels build upon the heritage of other satellites as outlined below:



            0.6 and 0.8 µm: Known from the Advanced Very High Resolution Radiometer (AVHRR). They
            are essential for cloud detection, cloud tracking, scene identification, aerosol observation,
            vegetation monitoring.
            1.6 µm: Some recent heritage from the Along Track Scanning Radiometer (ATSR).
            Discriminates between snow and cloud, ice and water clouds, and provides aerosol
            information.
            3.9 µm: Known from AVHRR. Primarily supports low cloud and fog detection. In addition, it will
            support measurement of land and sea surface temperature at night. In order to improve signal-
            to-noise ratio, the channel on MSG is broader and has been shifted towards higher
            wavelength.
            6.2 and 7.3 µm: Continues the mission of the Meteosat broadband water vapour channel for
            observing water vapour and winds, but is split into two channels peaking at different levels in
            the troposphere. Height allocation of semitransparent clouds will also be supported.
            8.7 µm: Known from the High resolution Infra Red Sounder (HIRS) instrument on the polar
            orbiting NOAA satellites. The channel will deliver quantitative information of thin cirrus clouds
            and support the discrimination between ice and water clouds
            9.7 µm: Total Ozone channel. As an experimental channel, it will be used for tracking of ozone
            patterns that should be representative for wind motion in the lower stratosphere. Its radiances
            are considered as an essential input to future Numerical Weather Prediction (NWP).
            10.8 and 12.0 µm: Well-known split window channels from AVHRR, essential to measure earth
            surface and cloud top temperatures. Detection of cirrus and inference of total precipitable water
            vapour over sea.
            13.4 µm: CO2 absorption channel known from former GOES VAS instrument. It will improve
            height determination of transmissive cirrus clouds. In cloud free areas, it will provide
            temperature information from the lower troposphere for static instability assessment.




            Figure 1: Contribution functions of SEVIRI IR channels based on spectral response curves of the Engineering Model.




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Imaging Area

          The imaging area of SEVIRI will be the same as for Meteosat. The sampling distance in N-S
          and in E-W direction will be 3 km for the VNIR channels and 1 km for the broadband visible
          channel (HRV), referring to a location at the nominal subsatellite point at zero degrees
          longitude over the equator. Fields of view are 4.8x4.8 km for the VNIR channels and 2.67x2.0
          km for the HRV channel.

          The scan area of the HRV channel will be restricted in the E-W direction by scanning only half
          the nominal scan line of the other channels. For the northern quarter of the image area, the
          scan line will in the nominal case be centred. For the lower part, the observations area can be
          adjusted in the E-W direction by ground command, as shown in the example below.

                                All Channels except HRV               HRV (nominal and alternative coverage)

                                Figure 1: Nominal Earth coverage of MSG Image Channels




                         All Channels except HRV                HRV (nominal and alternative coverage)

                 Figure 2: Nominal Earth coverage of MSG Image Channels

          The nominal repeat cycle of SEVIRI for scanning full disk images is 15 minutes. Shorter repeat
          cycles will be possible by scanning a reduced number of scan lines.


Level 1.0 data

          Level 1.0 data are the samples observed by SEVIRI and transmitted to ground in digital form in
          10 bits per pixel. SEVIRI Level 1.0 data will be archived and will be processed by the IMPF into
          Level 1.5 data.

Level 1.5 data

          Level 1.5 data is the primary product of the MSG system and is derived from Level 1.0 image
          data by radiometric and geometric processing by the IMPF of the MSG Ground Segment.
          Level 1.5 data will be disseminated in near real time through the HRIT and LRIT schemes, and
          will be archived.

          Radiometric processing

          Radiometric processing includes linearisation and equalisation of detector data. Signal
          contribution from straylight in the solar channels is also removed.

          Calibration

          Calibration coefficients for the IR channels are derived in real-time based on the black body
          information of each detector, accounting for the contribution of the front optics. This contribution




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ANNEX III


            is modelled using temperature measurements at relevant locations within the SEVIRI
            instrument. The target value for absolute radiometric accuracy is 1K.

            Calibration coefficients for the solar channels are derived regularly through a vicarious
            calibration scheme based on selected and well characterised ground targets. The target value
            for absolute radiometric accuracy is 5%.



            Geometric rectification and quality assessment

            In a single step, the radiometrically corrected image is geometrically corrected, remapped into
            a standard geostationary projection, and the registration between channels is performed.
            Space image data (in the corners of the images) are masked out.

            The channel registration to a common reference grid is expected to be with an accuracy of 600
            m at SSP for the HRV and VNIR channels. The IR channels will be registered within 750 m at
            SSP. The registration of the HRV, VNIR and IR channels is ensured via the geometric
            accuracy. The quality control of the geolocation process will be based on the extraction of
            landmarks. The processing scheme is designed so that the quantisation error from
            linearisation, equalisation and re-sampling processes should affect the image radiometric
            quality by no more than 0.5 digital count RMS. Basic geometric quality figures are given in
            Table 2 below.

                                 Geometric quality criterion                               RMS
                     Absolute accuracy                                                     < 3 km SSP
                     Relative accuracy (image to image)                                    < 1.2 km SSP
                     Relative accuracy within an image (500 pixels)                        < 3 km SSP
                     Relative accuracy within an image (16 pixels)                         < 0.75 km SSP
            Table 2: Geometric quality accuracy of the level 1.5 image data at the Sub-Satellite Point.

            The resulting Level 1.5 image is a data set with 37123712 pixels per channel except for the
            HRV channel which has a size of 111365568 (NSEW). All channels are coded in 10 bits per
            pixel. All pixels are geolocated on a common grid in the geostationary projection. The
            geostationary projection used for the level 1.5 image is defined with sub-satellite point (SSP) at
            zero degree longitude and latitude. This SSP has the image coordinates at (1856, 1856) for
            non-HRV channels and (5566, 5566) for HRV.

            The origin has image coordinates at (1, 1).

            Level 1.5 image data description

            Level 1.5 image data will be transmitted to external users or to the MPEF in the MSG ground
            segment as the basic input data for the derivation of meteorological products.

            In order to allow full interpretation, validation, calibration, reprocessing and further processing,
            Level 1.5 data will contain header and trailer blocks with the following information:

                      Status information of level 1.0 image acquisition;
                      The satellite, Sun and celestial bodies position at the time of image acquisition;
                      Status information on the radiometric and geometric processing;
                      The radiometric image quality for both level 1.0 and 1.5;
                      The geometric image quality of level 1.5;
                      Information related to the calibration schemes applied.




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                Detailed information on MSG image data contents, characteristics and data formats can be
                found in the document MSG/ICD/105 Level 1.5 Data Format Description that is available for
                download from the site

                                                          http://www.eumetsat.de/msg/




                Illustration of MSG image sampling

                Due to the nature of the scanning mechanism and the curvature of the Earth, the sub-satellite
                point sampling distance increases as the satellite viewing angle increases. Figure 3 shows the
                East-West sampling distance over of the MSG disk for a satellite nominal position of 0 degree
                longitude. This distance remains in the range of 4 km over most part of Europe and Africa. The
                corresponding South-North sampling distance is shown on Figure 4. In this case, the sampling
                distance remains almost unchanged along the Equator. Figure 5 shows the pixel area
                normalised by the sub-satellite point pixel area.




Figure 3: MSG East-West sampling distance in km. The result is shown in an equidistant cylindrical projection.




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Figure 4: MSG South-North sampling distance in km. The result is shown in an equidistant cylindrical projection.




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Figure 5: MSG normalised pixel area. The result is shown in an equidistant cylindrical projection.




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posted:10/2/2011
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