ENMAP RADIOMETRIC INFLIGHT CALIBRATION

Transcription

1 ENMAP RADIOMETRIC INFLIGHT CALIBRATION Harald Krawczyk 1, Birgit Gerasch 1, Thomas Walzel 1, Tobias Storch 1, Rupert Müller 1, Bernhard Sang 2, Christian Chlebek 3 1 Earth Observation Center (EOC), German Aerospace Center (DLR) 2 Kayser-Threde GmbH Munich, Germany 3 Space Administration, German Aerospace Center (DLR)

2 Chart 2 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Outline Hyperspectral Imager (HSI) Instrument Characteristics Radiometric Calibration General Concept In-Flight Calibration Dark Values Relative Calibration Absolute Calibration

3 Chart 3 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 EnMAP and the HyperSpectral Imager HSI (Introduction) EnMAP - Environmental Mapping and Analysis Program The project is being developed entirely in Germany under the aegis of the Space Administration Division of the German Aerospace Center (DLR) Science leadership by the German Research Centre for Geosciences (GFZ) in Potsdam Design and construction of the EnMAP sensor HSI by Kayser-Threde company in Munich Satellite platform built by OHB in Bremen Goal: Ecological assessment of vegetation for agriculture and forestry water bodies for coastal zones and inland waters land surfaces for geology and soil applications

4 Chart 4 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 HSI - Instrument Characteristics - Overview Orbit Sun-synchronous 11:00 equator crossing time Inclination 98 Height 653km Swath width 30km Swath length 1000km/orbit Spectral range Number of bands Spectral sampling VIS 420nm 1000nm SWIR 900nm-2450nm nm 10nm FWHM 8.1nm 12.5 nm SNR > 400:1 > 150:1 rad. accuracy 5% 5%

5 Chart 5 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Pushbroom Principle 2-dimensional Focal Plane Array Spatial dimension Spectral dimension Second spatial dimension through movement over Earth

6 Chart 6 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Calibration general approach Calibration is the map between the instrument figures onto the nature figures Nature: Object reflected light radiance spectra L[λ] natural map Optics Electronics A/D-C Calibration Map Instrument: Transmitted light through optics and electronics onto FPA CCD A/D converted Numbers DN[i,j] Important Goal: to have a linear as possible relation L i,j = ( DN i,j - Dark i,j ) *C i,j

7 Chart 7 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 On-board Radiometric Calibration - 4 Parts: Dark Values (all measurement modes: earth observation, calibration ) internal calibration (relative radiometric calibration, main sphere) internal calibration (linearity control, focal plane LEDs) Sun Calibration (absolute radiometric calibration, sun diffuser plate) -Dependencies of Calibration measurements

8 Chart 8 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 OBCA On Board Calibration Assembly Technical tools and operational modes to perform the necessary measurements for in-orbit calibration throughout the mission: Shutter/calibration mechanism for dark value and calibration measurements Full aperture diffuser for Sun calibration (radiometric, absolute) Main integrating sphere (white Spectralon ) for relative radiometric assessment Secondary sphere (doped Spectralon ) for spectral calibration assessment Focal plane LEDs for linearity measurements

9 Chart 9 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Dark Value Analysis Two Modes: Shutter und Deep Space (SWIR-influence ~ 0.3%) Temperatures of focal planes: VNIR: T = 293 K SWIR: T = 150 K Shutter: T ~ 293 K at begin and end of any image mode 128 frames outlier filter mean value calculation comparison of start and end measurements time interpolation, when significant differences

10 Chart 10 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Dark value analysis example Example instrument MCS 255 channels 100 readouts, 3 sec integration time Histogram of dark values ---- Gaussian-curve with act. mean and variance -cumulative distribution of dark values ---- error function (Erf) with act. mean and variance no outliers (no bit flap ) very good Gaussian behaviour

11 Chart 11 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Sun Calibration absolute calibration basic equations: sun L i, j = Ci, j DN i, j L i, j = E0, i, j BRDFi, j K sun C i, j = ( E0, i, j BRDFi, j K SE ) / DN i, j SE L i,j Radiance of pixel i in spectral channel j C j,k calibrationsoefficient of pixel i in spectral channel j DN sun i,j sun measurement E 0,I,j extraterrestrial sun irradiances (new Kurucz) K SE correction factor for the Earth Sun- distance BRDF i,j bidirectionl reflectance function of the diffuser plate (pre-launch values)

12 Chart 12 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Sun Calibration diffusor aging o Reduction of UV-load of the Spectralon -Diffuser by adaption of repetition rate o Standard rate 1meas/month Δ = DN meas DN ref 2 min-max m max -requirement max. trend line at t < t n+1 -Δt m av average trend line at t < t n+1 : New average trend line after t < t n+1 t n+1 calibration measurement at time t n+1 Intersection with requirement Δ defines time span Δt to generate new calibration tables t t = t n+1 t n+1 new calibration measurement request -Δt: estimated span of time at t n+1 to generate new calibration table estimation of trend window und estimation of an optimal update-time der Sun-Calibration t n+1 maximum time span 2 months

13 Chart 13 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Relative Radiometric Calibration (Main Sphere) uses mini-lamps and/or LEDs as light sources inside the main sphere operated at different currents and illuminating the entire entrance slit using 5 (baseline) illumination levels covers only a part of the optical path (starting at the entrance slit of the spectrometer) of the instrument (in difference to sun calibration) check for radiometric stability of the instrument and to assess relative changes in radiometric performance - as well as single pixel outliers (bad/dead pixels)

14 Chart 14 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Focal plane LEDs LEDs close to the focal plane, which will directly illuminate the detector arrays. driven by highly stabilized current used for the characterization of non-linearity and for stability check the entire dynamic range of the instrument can be covered using 40 different integration times in addition, these measurements allow for assessing of possible causes for changes in instrument performance by comparison to Sun calibration and sphere measurements. LED for direct illumination of the detectors in the focal plane (drawing Kayser-Threde)

15 Chart 17 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Summary time and frame frequency Calibration type Time Frames Data Volume Frequency Dark (shutter) 23 sec 2 * 128 0,27 GB each datatake Dark (deep space) 30 sec 1 * ,38 GB every 3 months Relative radiance calibration 17 min 13 sec 1 * 512 (5steps) 1,66 GB weekly Sun calibration 140 sec 2 * ,38 GB monthly Spectral calibration 5 min13 sec 1 * ,83 GB monthly Linearity measurement < 5 min 2 * 128 * GB monthly

16 Chart 18 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 Summary general calibration scheme Dark Current Deep space Dark Current Shutter Linearity Nonlinearity- Corr. Matrix Bad Pixel Mask Relative radiometric calibration X Spectral calibration Sun calibration DN reference matrix Bad Pixel Mask X Spectral shift vector/matrix Sun DN matrices Calibration tables

17 Chart 19 > IGARSS2014> Harald Krawczyk IGARSS2014-HK-v1> July 2014 There is no unique picture of reality. Stephen Hawking