METimage Calibration Pepe Phillips GSICS Data & Research Working Groups Meeting, 24 March 2014

Contents Introduction to EPS-SG Introduction to METimage METimage performances Calibration of the solar channels Calibration of the thermal channels Plans for in-orbit monitoring

EPS-SG (Second Generation) Payload Satellite-A Missions Instrument (and Provider) Predecessor on Metop Infrared Atmospheric Sounding (IAS) IASI-NG (CNES) IASI (CNES) Microwave Sounding (MWS) MWS (ESA) AMSU-A (NOAA) MHS (EUM) Visible-infrared Imaging (VII) METImage (DLR) AVHRR (NOAA) Radio Occultation (RO) RO (ESA) GRAS (ESA) UV/VIS/NIR/SWIR Sounding (UVNS) Sentinel-5 (Copernicus, ESA) GOME-2 (ESA) Multi-viewing, -channel, -polarisation Imaging (3MI) 3MI (ESA) -/- Satellite-B Missions Instrument (and Provider) Predecessor on Metop Scatterometer (SCA) SCA (ESA) ASCAT Radio Occultation (RO) RO (ESA) GRAS (ESA) Microwave Imaging for Precipitation (MWI) MWI (ESA) -/- Ice Cloud Imager (ICI ) ICI (ESA) Advanced Data Collection System (ADCS) Argos-4 (CNES) A-DCS

Introduction to METimage

VII primary observations Products to be derived from the VII mission are:   Cloud observations including microphysical analysis Water-vapour imagery Aerosol observations Polar Atmospheric Motion Vectors (AMVs) Earth surface albedo Vegetation Cryosphere (snow, sea and land ice imagery) Fire detection Surface temperature (land and sea)

Cloud Observations Cloud optical thickness: 670 nm over land, 1.24 µm over snow and sea ice, 865 nm over ocean. Cloud drop/particle effective radius: 2.25 µm and 3.75 µm. Cloud imagery: red (670 nm) / green (555 nm) / blue (443 nm) /near infrared (NIR) (865 nm) / thermal infrared (TIR) (10.8 µm) cloud imagery composites. Cirrus clouds: 1.37 µm (land during daytime) and 8.5 µm (nighttime). Snow/cloud discrimination: 1.6 µm and 3.75 µm. Cloud phase at the cloud top: 1.6 µm, 8.5 µm and 10.8 µm. Cloud top temperature, 763 nm oxygen-A band channels (daytime), 12.02 µm and 13.3 µm (CO2 absorption) in combination with infrared sounding data at 13-15 µm. Cloud vortices, MODIS RGB imagery (courtesy NASA)

VII performance specifications The need for more sophisticated parameterisation of clouds and surface boundary conditions drives the spectral and spatial resolution of the VII. In comparison to AVHRR on EPS: Number Channels Spatial resolution AVHRR 6 1 km VII 20 500 m Comparison between AVHRR (left) and MODIS (right) imagery (courtesy NASA)

VII Spectral Channels  (nm) Δ (nm) Cloud Cloud top height Water vapour Aerosol Cirrus Vegeta-tion Surface temp Primary observations 443 30 X Clouds and aerosol 555 20 Clouds, aerosol and vegetation 670 752 10 Cloud and aerosol height assignment 763 865 914 50 Water vapour 1,240 1,365 40 High level clouds and aerosol and water vapour imagery 1,630 Cloud phase and surface properties 2,250 Cloud microphysical analysis 3,740 180 SST 3,959 60 SST and fire 4,040 6,725 370 Water vapour imagery and polar winds 7,325 290 8,540 Cirrus clouds 10,690 500 Split window for clouds and surface parameters 12,020 13,345 310 Cloud top height.

Performance specifications (cont) Bias error < 5% (solar), < 0.5K @ Ttyp (thermal) Stability < 1% (solar), < 0.15K @ Ttyp (thermal) Inter-spatial homogeneity < 1% (solar), < 0.1K @ 280 K (thermal) Inter-channel homogeneity MTF > 0.3 @ Nyquist Geolocation accuracy Sub-pixel (~100m @ nadir) Polarisation sensitivity < 5% (solar), < 11% (thermal) Channel co-registration > 80% all channels

METimage calibration concept Calibration concept addresses accuracy, stability and spectral and spatial homogeneity. Accuracy is achieved using on-board 2-point linear calibration by viewing cold space as the low dynamic range reference and a target at known temperature/radiance for the high dynamic range reference -> solar diffuser (solar channels) and black body (thermal channels). Non-linearity corrections shall also be considered (TBC). Thermal channels: Offset and gain corrections performed by viewing cold space and a black body at instrument temperature every scan line - > recalibration interval of approx 3 s. Solar channels: Offset and gain corrections performed by viewing cold space once per scan line and a solar diffuser once per orbit - > recalibration interval of approx 100 mins.

Solar diffuser monitor Due to the main diffuser degradation, METimage shall carry a second diffuser plate that shall be used only periodically to monitor the degradation of the main solar diffuser. The diffuser monitor shall be deployed at regular intervals (approx once every 2 weeks). Else it remains hidden away to reduce UV exposure and contamination which are the main sources of diffuser degradation. At end of mission, the degradation of the monitor shall be small enough that the lifetime stability requirement (1%) for the solar channels is met.

METimage Calibration – Lunar calibration Lunar calibration has been successfully used for MODIS and VIIRS to monitor in-orbit performance including: Channel dependant degradation of the solar diffuser Reflection versus scan angle (RVS) correction of primary scan mirror Band to band registration Cross-talk MTF The role of the moon for stability monitoring is currently being considered for METimage. To use lunar calibration we need: Lunar visibility without platform manoeuvres (except during commisioning) Regular incidences of (full) lunar intrusions (several times per year) Bright enough (and as constant as possible) lunar phase angle

Lunar calibration METimage – way forward Plan for as many lunar calibrations as possible during commisioning/start of operation (helps curve fit to stability trending as decay steepest at start of mission). Ensure the space view is: Large enough to view the full moon Where possible, optimise position of space view (simulations ongoing) Ensure lunar intrusions suitable for calibration are achievable without spacecraft manoeuvres (no operational outage).