Using lunar observations for calibrating the solar channels

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) EUMETSAT (2) USGS EUM/ Issue <No. > <Date>

The Meteosat Second Generation SEVIRI instrument Launches • MSG 1: launched on 28/08/2002 • MSG 2: launched on 21/12/2005 • MSG 3: summer 2012 • MSG 4: ? è 15 min repeat cycle / 10 bits coding Sampling distance at SSP: • 3 km for VIS 06 / VIS 08 / NIR 16 • 1 km for HRVIS / 4 solar channels èSEVIRI solar band calibration requirements: 10% absolute accuracy for NRT applications + 5% long-term stability No on-board calibration system Vicarious calibration only EUM/ Issue <No. > Slide <Date>

The SEVIRI instrument solar bands HRVIS EUM/ Issue <No. > Slide <Date> VIS 06 VIS 08 NIR 16

Vicarious calibration • SEVIRI Solar Channel Calibration system offline vicarious calibration Ø Reference = RTM simulations of Top-Of-Atmosphere radiances Ø Comparison with TOA measured signal • 2 target types used for comparison 1. Desert bright targets (18 targets) 2. Dark sea targets (10 targets) (checking purposes) EUM/ Issue <No. > Slide <Date> What about the Moon?

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 Source : Pix. Heaven. net / Wikipedia • Stability of the Moon Model valid for any time Calibration reference = model To date, USGS has the only established lunar reference model, based on the ROLO observation program (more than 8 years of observations). EUM/ Issue <No. > <Date>

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 ÞMore 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 EUM/ Issue <No. > <Date>

The USGS lunar irradiance model (courtesy T. Stone) ROLO lunar images spatially integrated to irradiance and converted to disk reflectance: èEmpirical reflectance model — a function of geometric variables only: ref: Astronomical Journal 129, 2887 -2901 (2005) èBy fitting the observations, the model parameters are estimated èEstimated uncertainty: • 5 -10% in absolute irradiance scale (due to measurement of Vega by the ROLO telescopes) • 1% relative accuracy EUM/ Issue <No. > <Date>

What about lunar observations with SEVIRI? Fields of regard for Low Resolution Channels: • Level 1. 0: raw measurement data + auxiliary data èVisible Moon (up to 5 observations in a row) èFOR covers a rectangle of 22 N/S and 18 E/W • Level 1. 5: data rectified to an uniform grid è No visible Moon anymore èFOR = Earth disk Lunar observations available in the 4 image corners (more than 100 potential observations / year) SEVIRI Level 1. 5 1. 0 image (forward and backward scan) EUM/ Issue <No. > Slide <Date>

Lunar calibration with SEVIRI From L 15 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 NIR 16: Moon Min/Max Count = 46/ 403 Moon Mean Count = 166 Moon Std Dev = 86 EUM/ Issue <No. > <Date>

Sensor spectral responses. . . ROLO versus MSG 1/SEVIRI EUM/ Issue <No. > Slide <Date>

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 EUM/ Issue <No. > Slide <Date>

How to extract the lunar observations from the SEVIRI L 1. 0 images? 1. 2. 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) 3. 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 EUM/ Issue <No. > <Date>

Why is the Moon rectified? Rectification Corrected apparent motion L 1. 0 image (here from GOES) Fitted curve derive overall reflectance EUM/ Issue <No. > <Date> Figures: courtesy T. Stone

Example of cut-off for 09/02/2004 – 14: 30 UTC (SEVIRI / MSG 1) Extracted L 1. 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 EUM/ Issue <No. > Slide <Date> • 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 MSG 1 (between 2004 and end of 2005) and MSG 2 (from mid 2006 till early 2009): ØVarious phases / librations / positions in the SEVIRI Field Of Regard (potential for looking at scan-angle dependency of the calibration? ) ØOver 30 potential lunar observations for MSG 1 ØOver 50 potential lunar observations for MSG 2 • Not all potential lunar observation yet available EUM/ Issue <No. > Slide <Date>

Preliminary results from Meteosat 8 (MSG 1) 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 VIS 06 and VIS 08 are too low. But absolute accuracy only “ 5 -10%” However, VIS 08 performs better than VIS 06 (Better inter-channel relative accuracy) in contradiction with current findings with DCCs EUM/ Issue <No. > Slide <Date> Note: End at the end of 2005 after the start of Rapid Scan Service

Preliminary results for Meteosat 9 (MSG 2) Results for MSG 2 consistent with the ones for MSG 1 (VIS 08 performs slightly better) EUM/ Issue <No. > Slide <Date>

Conclusions and future work What was achieved? • Lunar observation database for MSG 1 and MSG 2 • Systematic extraction from the Level 1. 0 • MSG 1/SEVIRI and MSG 2/SEVIRI = very stable in time • Systematic underestimation for VIS 06 and VIS 08 • Amplitude of the bias in disagreement with findings from other groups using DCCs (VIS 08 performs better than VIS 06) • 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) VIS 06 / VIS 08 / NIR 16 (MSG 1 and MSG 2) • Setup of a database for the HRVIS band (MSG 1 and MSG 2) • Investigate the availability of lunar observations during the Rapid Scan Service of MSG 1 • Analysis of the current results to understand the variations in the time series • What about Meteosat First Generation? (MET 6 for instance) EUM/ Issue <No. > <Date>

What about the HRVIS band the Rapid Scan Service? Fields of regard for High Resolution Channel: • Level 1. 0: èHow many visible Moon ? èLimited FOR • Level 1. 5: No visible Moon anymore as for Low Resolution Lunar observations = available but lower frequency + always on the East EUM/ Issue <No. > Slide <Date> SEVIRI Level 1. 5 1. 0 image (forward and backward scan)