METimage Calibration & Performance Verification. Xavier Gnata ICSO 2016

Transcription

1 METimage Calibration & Performance Verification Xavier Gnata ICSO 2016

2 METimage factsheet Mission Passive imaging radiometer (multi-spectral) 20 spectral channels ( nm) Global coverage within one day m ground pixel METimage instrument In beam scanner: ±54deg scan range spectral separation in scan direction 3 spectral bands (dichroic separation) 24 pixels in flight direction Mass about 286 kg Power 465 W Average Data Rate 18 Mbit/s (different for day and night side) METIMAGE A SPECTRO-RADIOMETER FOR THE VII MISSION ONBOARD METOP-SG 2

3 METimage Optical Path Overview M1 Scan Mirror M3 M2 Beamsplitter VIS/NIR Beamsplitter SWIR/MWIR De-Rotator Cryostat Window VIS/NIR SWIR/MWIR LWIR/VLWIR Filters, Fieldstop Masks 3

4 METimage Calibration & Verification needs Variety of parameters and wide range of wavelengths Solar channels : from 443nm to 2.25µm: world of radiances. Thermal channels : from 3.74µm to 13.4µm : world of brightness temperatures. Absolute radiometry : 5% or 0.5K. SNR verification. Relative radiometry : inter and intra channels all the pixels shall give have the same bias within 1% or 0.1%. Straylight : Requirement on sharpness of dark/bright edges but also impact on radiometry. Spectral response function : 2% where the amplitude is higher than 0.1% of peak. Steps < 1nm. Knowledge of polarisation sensitivity 0.5%. Co-registration : Better than 0.2 spatial sample. Pointing requirements : good enough to bootstrap the Image Navigation and Registration (INR). 4

5 METimage Calibration & Verification flow Fly as you test, test as you fly Comprehensive set of subsystems characterisations Secure the performances and gather knowledge Issues with errors bars adding up End to end Calibration and Verification in TVAC METimage in flight configuration. 5

6 Thermal Channels radiometric calibration Absolute radiometric calibration : One gain and an offset Assuming non-linearity has been corrected. Gain : tuneable black body : 180K to 350K. Challenging stability. A few tens of mk. Offset : Deep Space View. Cold 90K beam bump. In-orbit monitoring : Thermal CAlibration Device (TCAD). On-board black body. Temperature knowledge over lifetime. On ground calibrated black body as absolute reference 6

7 Solar Channels radiometric calibration Absolute radiometric calibration : One gain and an offset Offset : Deep Space View. Cold 90K beam bump. On-ground measurement : Relative gain between Sun View and Earth view. Sun Beam simulator in air and OGSE Spectralon diffuser. In-orbit : Sun on the Solar CAlibration Device (SCAD). Spectralon diffuser on-board. First in-orbit Sun measurement as absolute reference. 7

8 Presentation Title / Document Number runs here (go to Header & Footer to edit this text) Solar Channels radiometric calibration. MGSE Rotation accuracy Thermal constrains. 8

9 METimage Optical Spectral/Spatial characterization The needs : Spectral response function characterisation Knowledge of polarisation sensitivity Collimation assembly Pointing and co-registration verification Straylight calibration The needs translated in OGSE terms : Tune the wavelengths for 420nm to 14µm Reflective, in vacuum and cold. Image patterns on the detectors Collimation Assembly (Sentinel-4 project) Polarized or not the incoming light Master, not the absolute, but the relative radiometry The collimation Assembly is a challenge. Experience from other missions. 9

10 Straylight in METimage. Out-of-field. Out-of-field straylight Any light out of the ifov coupled down to the detectors Set of simulations including the Earth and the Sun. Nothing METimage specific. Demanding levels of contaminations (particles) and micro-roughness specifications. Link with BRDF sometimes questionable. Mechanical straylight Thermal photons generated by the instrument structure. Reduced by design but not suppressed. Very Comprehensive set of simulations. Scan angle dependency. Challenge for the calibration. Need to predict and mimic thermal gradients on-ground. 10

11 Straylight in METimage. In-field. In-field straylight Unwanted couplings between pixels in the ifov. Impact on inter and intra channels homogeneity. Example in the relay optics of the thermal channels. 3 November

12 Straylight in METimage. In-field. In-field straylight Characterization with the Collimation Assembly. Spatial selection by slit imaging. Illuminate each channel (or each pixel) individually with broadband and then monochromatic light. Distinguish between filters imperfection part of the spectral response and straylight. 2D times 2D correction algorithm Straylight signal on each pixel as a function of nominal signal on each pixel. Possible thanks to a low number of pixels. 3 November

13 From raw data to Level 1B products Simple processing algorithms but Challenge : measure the parameters with the needed accuracies. High data rate (18 Mbps) and data volume. Many small analysis tools are needed during the measurements campaigns. Often underestimated. Key not to lose time during costly campaigns. Various sources/level of measurements, various format but one goal : Accurate calibration keydata. Programmatic issue: First version of the L1b processor needed before availability of real data. Limitation of models. Non linearity deserves a talk on its own It s just a lookup table Measurements at Detector level Measurements at Detection chain level Measurements at Relay optics level Measurements at Cryostat level Measurements at Instrument level In-orbit monitoring 3 November

14 Presentation Title / Document Number runs here (go to Header & Footer to edit this text) Thank you for your attention The work described was performed on behalf of the German Space Administration with funds from the German Federal Ministry of Transport and Digital Infrastructure co-funded by EUMETSAT under DLR Contract No. 50EW November