Sébastien Wagner (1) Tom Stone (2), Gary Fowler (1), Tim Hewison (1) Using lunar observations for calibrating the solar channels of the METEOSAT SEVIRI imager Sébastien Wagner (1) Tom Stone (2), Gary Fowler (1), Tim Hewison (1) (1) EUMETSAT (2) USGS

The Meteosat Second Generation SEVIRI instrument  Launches MSG1: launched on 28/08/2002 MSG2: launched on 21/12/2005 MSG3: summer 2012 MSG4: ? 15 min repeat cycle / 10 bits coding  Sampling distance at SSP: 3 km for VIS06 / VIS08 / NIR16 1km for HRVIS / 4 solar channels SEVIRI solar band calibration requirements: 10% absolute accuracy for NRT applications + 5% long-term stability No on-board calibration system  Vicarious calibration only

The SEVIRI instrument solar bands HRVIS VIS06 VIS08 NIR16

Vicarious calibration SEVIRI Solar Channel Calibration system  offline vicarious calibration Reference = RTM simulations of Top-Of-Atmosphere radiances Comparison with TOA measured signal 2 target types used for comparison Desert bright targets (18 targets) Dark sea targets (10 targets) (checking purposes) What about the Moon? Slide: 4

The Moon as a radiometric calibration source (courtesy T. Stone) INTRINSIC PROPERTIES: Exceptionally stable Non-uniform appearance, varying hemisphere seen (lunar librations) Non-Lambertian reflectance Smooth reflectance spectrum Continuous and periodic changes in apparent brightness (e.g. phase)  Can be characterized and modeled UTILITY AS A CALIBRATION REFERENCE Requires an analytic model with a continuous predictive capability Stability of the Moon  Model valid for any time  Calibration reference = model Source : PixHeaven.net / Wikipedia To date, USGS has the only established lunar reference model, based on the ROLO observation program (more than 8 years of observations).

Development of the USGS lunar calibration reference (courtesy T. Stone) Scope : Capture the cyclic brightness variations of the Moon sufficiently for modeling Identified need: sample ~25% of the 18.6-year phase/libration cycle More than 8 years of observations done at the Robotic Lunar Observatory (ROLO) Site : USGS in Flagstaff, Arizona Dedicated observatory, located at altitude 2143 m Twin telescopes 23 VNIR bands, 350−950 nm 9 SWIR bands, 950−2450 nm imaging systems — radiance database of >85 000 Moon images (phase coverage from eclipse to 90°) More than 800 000 star images, for nightly atmospheric transmission measurements

The USGS lunar irradiance model (courtesy T. Stone)  ROLO lunar images spatially integrated to irradiance and converted to disk reflectance: Empirical reflectance model — a function of geometric variables only: By fitting the observations, the model parameters are estimated Estimated uncertainty: 5-10% in absolute irradiance scale (due to measurement of Vega by the ROLO telescopes) 1% relative accuracy ref: Astronomical Journal 129, 2887-2901 (2005)

What about lunar observations with SEVIRI? Fields of regard for Low Resolution Channels: Level 1.0: raw measurement data + auxiliary data Visible Moon (up to 5 observations in a row) FOR covers a rectangle of 22 N/S and 18 E/W Level 1.5: data rectified to an uniform grid No visible Moon anymore FOR = Earth disk Lunar observations available in the 4 image corners (more than 100 potential observations / year) SEVIRI Level 1.5 image SEVIRI Level 1.0 image (forward and backward scan)

Lunar calibration with SEVIRI From L15 trailer (24/02/2008 – slot 14:27) VIS 06: Moon Min/Max Count = 50 / 170 Moon Mean Count = 86 Moon Std Dev = 27 VIS 08: Moon Min/Max Count = 50 / 186 Moon Mean Count = 91 Moon Std Dev = 31 NIR16: Moon Min/Max Count = 46/ 403 Moon Mean Count = 166 Moon Std Dev = 86

Sensor spectral responses...ROLO versus MSG1/SEVIRI

How to extract the lunar observations for SEVIRI images? Information available in Lev 1.0 + Lev 1.5 images Level 1.5 image headers/trailers: Flag to indicate if the Moon is in the FOR Statistics on the Moon counts (min / max / mean / std dev) Level 1.0 images: raw images to be decoded and processed

How to extract the lunar observations from the SEVIRI L1.0 images? Location of the Moon within the SEVIRI Level 1.0 image Realignment of the Earth image Location of the sub-satellite point using horizon detection Location of the Moon with respect to the centre of the Earth Realignment of the Moon image (to correct for the apparent motion of the Moon) Location of the Moon centre using horizon detection Extraction of the Moon imagettes From a centre of the Moon  estimated radius of the Moon extraction circle (Moon radius + margin) Scaling of the Level 1.0 counts to equivalent Level 1.5 radiances Calculation of the equivalent Level 1.5 counts (correction for small potential instrument non-linearities) Conversion from the equivalent Level 1.5 counts to radiance, using calibration coefficients from the Level 1.5 headers

Why is the Moon rectified? Rectification Corrected apparent motion L1.0 image (here from GOES) Fitted curve derive overall reflectance Figures: courtesy T. Stone

Example of cut-off for 09/02/2004 – 14:30 UTC (SEVIRI / MSG1) Extracted L1.0 counts Space count + other (stray light, stars, etc.) Moon count distribution Mask (edge fitting) Space count sanity check Offset setting is correct within 0.5 count Potential of using such an approach to check the Point Spread Function (using the sharp edge of the Moon)

Current achievements and preliminary results Automated extraction tool to build up a lunar observation dataset Dataset generated for MSG1 (between 2004 and end of 2005) and MSG2 (from mid 2006 till early 2009): Various phases / librations / positions in the SEVIRI Field Of Regard (potential for looking at scan-angle dependency of the calibration?) Over 30 potential lunar observations for MSG1 Over 50 potential lunar observations for MSG2 Not all potential lunar observation yet available

Preliminary results from Meteosat 8 (MSG1) Lunar calibration method and instrument are stable. SD <1% - consistent with expected performance of ROLO  BUT is it affected by seasonality? As expected, radiances in the VIS06 and VIS08 are too low.  But absolute accuracy only “5-10%” However, VIS08 performs better than VIS06 (Better inter-channel relative accuracy)  in contradiction with current findings with DCCs Note: End at the end of 2005 after the start of Rapid Scan Service

Preliminary results for Meteosat 9 (MSG2) Results for MSG2 consistent with the ones for MSG1 (VIS08 performs slightly better)

Conclusions and future work What was achieved? Lunar observation database for MSG1 and MSG2 Systematic extraction from the Level 1.0 MSG1/SEVIRI and MSG2/SEVIRI = very stable in time Systematic underestimation for VIS06 and VIS08 Amplitude of the bias in disagreement with findings from other groups using DCCs (VIS08 performs better than VIS06) What does remain? Update the current database (systematic use of the Level 1.5 headers to check the availability of the Moon in the FOR)  VIS06 / VIS08 / NIR16 (MSG1 and MSG2) Setup of a database for the HRVIS band (MSG1 and MSG2) Investigate the availability of lunar observations during the Rapid Scan Service of MSG1 Analysis of the current results to understand the variations in the time series What about Meteosat First Generation? (MET6 for instance)

What about the HRVIS band and the Rapid Scan Service? Fields of regard for High Resolution Channel: Level 1.0: How many visible Moon ? Limited FOR Level 1.5: No visible Moon anymore as for Low Resolution Lunar observations = available but lower frequency + always on the East SEVIRI Level 1.5 image SEVIRI Level 1.0 image (forward and backward scan)