Overview of the calibration methods used in the DESIS hyperspectral instrumen

Bachmann, Martin

Carmona, Emiliano

de los Reyes, Raquel

Marshall Ingram, David

Müller, Rupert

Institute of Transport Research:Publications

The DESIS Spaceborne Hyperspectral Instrument CalibrationE. Carmona1, M. Bachmann2, R. de los Reyes1, D. Marshall1, R. Müller11 Remote Sensing Technology Institute, DLR, Oberpfaffenhofen, 82234 Wessling, Germany2 German Remote Sensing Data Center, DLR, Oberpfaffenhofen, 82234 Wessling, GermanyCEOS WGCV  IVOS 35 Oberpfaffenhofen, Germany27 September 2023Mission IntroductionDESIS Instrument• Hyperspectral instrument consisting of a Three-Mirror-Anastigmat (TMA) telescope combined with an Offner-type spectrometer> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023FEE: Front End ElectronicFPA: Focal Plane ArrayTMA: Three Mirror AnastigmatPOI: Pointing Unit Sensors 2019, 19(7), 1622; https://doi.org/10.3390/s19071622Mission Instrument MUSES/DESISTarget lifetime 2018-2023Off-nadir tilting(across-track, along-track)-45° (backboard) to +5° (starboard), -40°to +40° (by MUSES and DESIS)Spectral range 400 nm to 1000 nmSpectral Sampling (res.,acc.,bands)2.55 nm, 0.5 nm,235 bands. Binning: 118 , 79 , 60 bandsSpectral response Gaussian shape, 3.5 nm FWHMSoftware Binning (samplingdistance, number bands)Binning 2 (5.1 nm, 118 bands)Binning 3 (7.6 nm, 79 bands)Binning 4 (10.1 nm, 60 bands)Radiometry (res., acc.) 13 bits, ~10%Spatial (res., swath) 30 m, 30 km (@ 400 km)SNR (signal-to-noise) 195 (w/o bin.) / 386 (4 bin.)@ 550 nmInstrument (mass) 93 kgCapacity (km, storage) 2360 km per day, 225 GBitDLR.de  •  Chart 20.14 nmFit parametersNormalization =  0.996495Spectral_shift = -1.25566 (nm)Vertical_shift = -0.0680321Wavelengthn (nm)Calibration unit• Equipped with 9 different types of LEDs. It allows to measure signal with different LED types. Calibration measurement every week for 5 years• It allows for precise spectral stability measurements. Jumps of 0.5 nm in all LEDs, correlated with different temperature gradients inside DESIS. Corrected during processing. Residual RMS ~0.1 nm for each of the two states> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023DLR.de  •  Chart 3Vicarious calibration data• Input scenes not evenly distributed in time• Particularly challenging to have abundant good quality Radcalnet (RCN) scenes• Calibration updates arrive several months after data acquisition> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.20230102030405060RCN Scenes Uniform Scenes00,511,522,533,544,52019-022019-032019-042019-052019-062019-072019-082019-092019-102019-112019-122020-012020-022020-032020-042020-052020-062020-072020-082020-092020-102020-112020-122021-012021-022021-032021-042021-052021-062021-072021-082021-092021-102021-112021-122022-012022-02Period #1 Period #2 Period #3 Period #4 Period #5RCN Data Used for calibrationPeriod #2Period #1 Period #3 Period #4 Period #5“Vicarious calibration of the DESIS imaging spectrometer“, E. Carmona et al., IGARSS2021DLR.de  •  Chart 4Vicarious calibration Input data> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023DLR.de  •  Chart 5RadCalNet Scenes Uniform ScenesGobabeb (“GONA”) La Crau (“LCFR”)Railroad Valley (“RVUS”) Gobabeb (“GONA”)Correction Steps II: Flat-fielding> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• Flat-fielding spatial: In homogeneous scenes all pixels across-track shall have the same value within a band• Flat-fielding spectral: In homogeneous scenes all across-track pixels shall deliver the same spectra as the central pixelsRadiance pix 316 / Radiance central pixelsRadiance pix 316 / Radiance central pixelsOriginal Flat-fielding spatial + striping correctionPixel 316 across-track Pixel 316 across-trackOriginal Flat-fielding spectralDLR.de  •  Chart 6Correction Steps III: L2A spectral smoothing• Fine tuning of individual pixels radiometric factors obtained using L2A data to avoid atmospheric features• Compute correction to minimize pixel to pixel fluctuations. Effect visible at lower wavelengths. Fluctuations at larger wavelengths dominated by spectral calibration errors and etaloning/fringing effect in the detector> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023DLR.de  •  Chart 7Correction Steps IV: Spectral adjustment vicarious• Spectral calibration very difficult to obtain in-flight. Only small adjustments on central wavelengths possible> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023LTG calibration measurements0.1 nm• Global shift: based on LED calibration measurements. Change of trend in September 2019. More stable since then. 0.10 nm shift included in calibration update• Spectral adjustment: Certain L2A spectral features after calibration can only be fixed adjusting central wavelengths (around strong atmospheric absorption features)Spectral shift w.r.t. laboratory calibrationDLR.de  •  Chart 8Mission Introduction Vicarious Calibration• Use flat-fielding over uniform areas for pixel-to-pixel relative adjustment. Use RadCalNet sites for absolute calibration> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023DLR.de  •  Chart 9Results from 3 calibration periods• Absolute calibration adjusted with RCN data for 3 different periods• Absolute calibration uses only part of RCN scenes (19)• good atmospheric conditions• below 50 degrees Sun Zenith Angle• These summary plots show 19 RCN scenes used for calibration> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023RCN TOA reflectance TOA reflectance from RCN BOA reflectanceDLR.de  •  Chart 10Results from 3 calibration periods: All RCN Data Results• Absolute calibration adjusted with RCN data for 3 different periods• Absolute calibration uses only part of RCN scenes (19)• good atmospheric conditions• below 50 degrees Sun Zenith Angle• These summary plots show allRCN scenes (30 scenes)> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023RCN TOA reflectance TOA reflectance from RCN BOA reflectanceDLR.de  •  Chart 11Latest Vicarious calibration data• New calibration periods continue using baseline vicarious calibration• Data in periods #4 and #5 with calibration for period #3:> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• Large variation of calibration below 500 nm as seem in other periods• Magnitude is smallerRCN TOA reflectance TOA reflectance from RCN BOA reflectanceCalibration period #3applied to #4 and #5DLR.de  •  Chart 12Latest Vicarious calibration data• New calibration periods continue using baseline vicarious calibration used in DESIS• Data in period #4 calibrated with calibration in period #4 (preliminary):> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• Similar results as seen in other periods• After calibration bias is corrected, but as usual RMS below 500 nm is significant larger than above 500 nmRCN TOA reflectance TOA reflectance from RCN BOA reflectanceDLR.de  •  Chart 13Latest Vicarious calibration data• New calibration periods continue using baseline vicarious calibration used in DESIS• Data in period #5 calibrated with calibration in period #5 (preliminary):> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• For the first time different behavior below 500 nm• More stable during Period #5, reduced RMS compared to all previous periods• Degradation below 500 nm is reducedRCN TOA reflectance TOA reflectance from RCN BOA reflectanceDLR.de  •  Chart 14Vicarious Calibration> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• Radiometric calibration shows high variability for wavelengths above 480 nm for data until July 2021• More stable (better than 3.4% / year) for wavelengths above 500 nmDLR.de  •  Chart 15Crosscheck with Calibration Unit> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• Change of behavior below 500 nm observable in August 2021. Degradation reduces• More Stable until February 2022• Rapid increase until December 2022 and more stable (again) since then• Very stable in other wavelengthsDLR.de  •  Chart 16> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023A B CA BCDESISA BCEnMAPDESIS and EnMAP – spectral comparisonDESISEnMAPGobabeb, Namibia; DESIS + EnMAP cal/val site, DESIS: SAA 72°, SZA 47°, 02/10/2022, 07:52 (UTC)DLR.de  •  Chart 17ExtraLevel 1A ProcessorTranscriptionLevel 1B ProcessorSystematic and Radiometric CorrectionLevel 1C ProcessorOrthorectificationLevel 2A ProcessorAtmospheric CorrectionL1B Product*L1C Product*L2A Product*Long Term ArchiveL1A DataRaw DataEarth datatakesExperimental datatakesCalibration mesaurementsOrbit and AttitudeProductsAuxilary DataIn-flight Calibration ProcessCalibration &ReferenceProductsGlobalDEM DatabaseAtmospheric LUTUpdateCal TablesPublishCal MeasurementsScreening Calibration &ReferenceProductsGlobalREF DatabaseProducts:• Level 0 (L0)• Raw data (Datatakes up to 100 tiles 30x30 km², trajectory files, DC)• Level 1A (L1A)• Tiled images, browse image, metadata, quality flags <= archived• Level 1B (L1B)*• Top of Atmosphere (TOA) radiance (W·m-2·sr-1·μm-1)• Systematic and radiometric correction (rolling shutter, smile, …)• All metadata attached for further processing• Level 1C (L1C)*• Level 1B data ortho-rectified, re-sampled to a specified grid• Global DEM (SRTM, 1arcsec), sensor model refinement using global reference image (Landsat-8 PAN with acc. 18m CE90)• Level 2A (L2A)*• Ground surface reflectance (i.e. after atmospheric corrections)• With and w/o terrain correctionL0 Data* Delivery ProductData Products, Quality and Validation of the DLR Earth Sensing Imaging Spectrometer (DESIS) Sensors 2019, 19(20), 4471; https://doi.org/10.3390/s19204471DESIS Data Products DESIS – Operational processors (DLR + Amazon Cloud)> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023DLR.de  •  Chart 19Correction Steps I: Radiometric adjustments• Most steps performed with uniform scenes with L1B products averaged in the along-track direction • 235 bands × 1024 spatial pixels• Most corrections are performed after smile correction (confusion of spectral and radiometric corrections)> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023• Striping correction: Compute adjustment to radiometric coefficient using spline fits. Use iterative process until convergence• Rad./Spc. correction: Use all pixels across track in one single spectrum. Compute minimum deviation to common spectrumDLR.de  •  Chart 20Results First Vicarious calibration (2018-10 − 2019-09) • Crosscheck using independent scene from RCN LCFR• TOA reflectance (left, 2 references)• BOA reflectance (right, 1 reference)> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023L1C DESIS product L2A DESIS productBOA Reflectance (0.0 –1.0)DLR.de  •  Chart 21Results from 3 calibration periods: All RCN Data Results• Bias is kept <1-2% on average• Limitation of the method, probably never better than 1-2%• Differences between the two TOA calculations ~1%• RMS is typically ~4% outside strong absorption bands• Smaller for reduced Sun zenith range and good atmospheric conditions• Problematic area below 450 nm:• Sensor not very stable• Degradation of up to ~20% / year at 400 nm> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023RCN TOA reflectance TOA reflectance from RCN BOA reflectanceDLR.de  •  Chart 22> CEOS WGCV IVOS 35 > E. Carmona et al.  >  27.09.2023Crosscheck with Calibration UnitDLR.de  •  Chart 23

The DESIS Spaceborne Hyperspectral Instrument Calibration

https://elib.dlr.de/199476/1/IVOS_2023_DESIS.pdf

The DESIS Spaceborne Hyperspectral Instrument Calibration

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