Introduction to satellite sensor radiometric calibration a review of FY-3 ocean color applications

Transcription

1 //3 Introduction to satellite sensor radiometric calibration a review of FY-3 ocean color applications SUN Ling NSMC, CMA sunling@cma.gov.cn -- Calibration: The process of quantitatively defining system responses to known, controlled signal inputs. It includes radiometric, spatial, and spectral aspects. Radiometric calibration converts the instrument response to an Earth scene in digital number to top of atmosphere (TOA) radiance or reflectance, the main parameter in LB. Quantitative remote sensing applications such as ocean colour are sensitive to sensor s radiometric performance. Calibration is the prerequisite to assure data's radiometric quality, especially when long time series data are used in application.

2 //3 Calibration methods: On-orbit calibration, could be realized with on-board traceable calibrator systems and the Moon. Vicarious calibration, for those without effective on-board calibration devices, includes radiative simulation with dependant geophysical parameters, "invariant" earth targets tracking and intercalibration with a reference sensor. Outline On-board calibration Vicarious calibration FY-3 Ocean color applications

3 //3 On-board calibration Instrument Background: Moderate Resolution Imaging Spectroradiometer (MODIS) Terra (EOS-AM): Launched on /8/99 First light on /4/ Aqua (EOS-PM): Launched on 5/4/ First light 6/4/ 36 spectral bands with a total of 49 detectors located on 4 focal plane assemblies (FPA). reflective solar bands (RSB): -9 and 6,.4~.um 6 thermal emissive bands (TEB): -5 and 7-36, mm Three spatial resolutions (nadir): 5 m(- ), 5 m(3-7), and km(8-36) Scan angle: ±55 (from instrument nadir) A swath of km (along-track) by 33 km (nadir along-scan) Global coverage in less than days -sided Paddle Wheel Scan Mirror.478 second each scan A broad range of applications Near 4 science data products for studies of the Earth s land, ocean, and atmosphere properties. MODIS Key Specifications Primary Use Land/Cloud/Aerosols Boundaries Land/Cloud/Aerosols Properties Ocean Color/ Phytoplankton/ Biogeochemistry Band Bandwidth (nm) Spectral Radiance Required SNR Primary Use Band Bandwidth (mm) Spectral Radiance Required NEDT(K) (3K) Surface/Cloud (335K) Temperature (3K) (3K) Atmospheric (5K) Temperature (75K) (SNR) Cirrus Clouds Water Vapor (4K) (5K) Cloud Properties (3K) Ozone (5K) Surface/Cloud (3K) Temperature (3K) (6K) (5K).5 Cloud Top Altitude (4K).5 Atmospheric Water Vapor (K) Spectral Radiance values are (W/m -µm-sr)

4 //3 Calibration Requirements: ±% in reflectance and ±5% in radiance for RSB ±% in radiance for most TEB, ±.75% for band, ±% for band, and ±.5% for bands 3 and 3 Calibration requirements are specified at typical scene radiance levels and for observations within ±45º scan angle range On-board calibrators(obcs) Solar diffuser (SD): calibration of RSB Solar diffuser stability monitor (SDSM) :monitor SD degradation Blackbody (BB): calibration of TEB Space view (SV) port: radiative background Spectro-radiometric calibration assembly (SRCA) :monitor on-orbit spatial and spectral characterization 4

5 //3 On-orbit Calibration Activities SD/SDSM: Weekly to tri-weekly Solar Diffuser SRCA: Radiometric: monthly Spatial: bi-monthly Spectral: quarterly SDSM SRCA Blackbody. Scan Mirror Space View BB: quarterly Moon: monthly (nighttime orbits) - o spacecraft roll maneuvers 55 o phase angle Scanning sequence schematic Angle of Incidence (AOI) Instrument Temperatures Terra MODIS: less than 3.5 K increase over years Aqua MODIS: less than. K increase over 9 years Similar trends for the VIS and NIR FPA temperatures 5

6 //3 Cold Focal Plane Assembly (FPA) Temperatures Terra MODIS: SMIR LWIR controlled at 83K extremely stable controlled at 83K Aqua MODIS: SMIR LWIR Small increase of CFPA temperatures in recent years (.3K) Blackbody Temperatures (nominal operation) Terra MODIS: less than 3 mk increase over years B-Side set at 9K Aqua MODIS: extremely stable set at 85K On-board BB used for Thermal Emissive Bands (TEB) calibration 6

7 //3 EV Reflectance Factor: EV Radiance: L EV On-board calibration-rsb RSB calibration using SD/SDSM E sun EV cos( EV ) m dn Sun * EV d EV cos( EV ) * m dn d ES _ EV EV ES _ EV E sun /.44% Screen Optional 7.8% Screen(Band 8~6) SD degradation dcsd SD dc SD dc dc SD Sun Sun / dc / dc 9 SD 9 Sun SDSM Calibration coefficients plbrf SD SD m cos( ) SDS * SD dnsd d ES _ SD Γ: screen attenuation function for the SDS closed mode, unity for open mode Δ : SD degradation factor determined from the SDSM. Scan Mirror (MODIS) SD EV reflectance factor: EV cos( EV ) m dn * EV d ES _ EV * dn EV : Digital number corrected for background and instrument effects. Instrument effect corrections include normalizing the sensor s viewing angle and correcting for the instrument temperature dependence. dn ) / RVS * EV ( DNEV DNSV) ( kinst TInst _ EV EV K : instrument temperature correction coefficient determined pre-launch. RVS: response versus scan angle normalized at the angle of incidence (AOI) of SD. 7

8 //3 RSB Lunar Calibration Moon provides a good radiometric reference with its stable surface reflectance and irradiance, has been used to monitor detector response stability in VIS and NIR spectral regions. Moon observations, monthly (nighttime orbits) via the SV port at a near-constant phase angle(55.5 ) 9~ lunar observations each year angle of incidence (AOI) :. to the scan mirror Lunar coefficients used for RVS calculations MODIS Response Lunar coefficients f m moon vg * dnmoon View geometry correction f vg f d f phase angle libration Sun Moon dmoon MODIS 8

9 //3 SD calibration Lunar calibration. The difference in the magnitude of the trending is a direct measure of the mirror degradation rate difference at different AOI to the scan mirror. Angle of Incidence (AOI) RSB RVS Algorithms RVS is characterized by prelaunch measurement and on-orbit variation pl oo RVS B, D, M,, t RVS B, M, RVS B, D, M,, t B, D, M, θ and t represent band, detector, mirror side, AOI and time pl: pre-launch; oo: on-orbit. RVS on-orbit variation at AOI of the SV moon oo m B, D, M, t m B, D, M, t RVS B, D, M, SV, t moon m B, D, M, t m B, D, M, t Mirror side one RVS on-orbit variation a linear function of AOI RVS F: Frame oo SD oo RVS B, D,,, t RVS B, D,, SV, t SV SD The calculated RVS is fitted to a quadratic form of the frame, and the fitted coefficients form a time dependent LUT for MODIS RSB RVS L B F, t c t c t F c t F 9

10 //3 MODIS SD Degradation Trending Larger degradation at shorter l SD door anomaly on July nd, 3 Increased SD degradation due to SD door fixed at open SD door left open for 5 days due to a command drop On-board SD used for Reflective Solar Bands (RSB) calibration MODIS RSB Response Trending Larger changes at shorter l. Most degradation seen in VIS bands with largest change for Band 8 (4 nm) at SD AOI: 5%. Some NIR bands show gain increase over time Gain change for NIR and SWIR bands is generally within %. SWIR bands are located on the cold PFA controlled at 83 K.

11 //3 MODIS RSB Response Trending Most degradation seen in VIS bands with largest change seen for Band 8 (4 nm) at SD: 35%. b3 increase in gain b first drop then increase Gain change for NIR and SWIR bands is generally within %. MODIS RSB RVS Trending Terra band 8 RVS at AOI of the SV has changed about 4% in last few years

12 //3 MODIS RSB RVS Trending Aqua band 8 RVS at AOI of the SV has decreased about 8% since launch Terra MODIS Band 8 RVS Trending The RVS detector difference can be as large as.5% for Terra band 8.

13 //3 Terra EV Reflectance at Stable Targets Band MS Band 3 MS Band 9 MS Pseudo-Invariant Desert Sites Sites: Mauritania, Mali, Algeria, Algeria 3, Niger, Libya, Libya, Libya 4, Egypt, Sudan, Yemen Desert, Arabia Detail information: MODIS should see similar long-term EV reflectance trending for a given wavelength On-board calibration-teb BLACKBODY SCAN MIRROR SPACE VIEW RADIATIVE COOLER CAVITY Sensor views the blackbody at a known temperature (or radiance) and deep space through the SV port to measure the instrument s thermal background and electronic offset. 3

14 //3 Subtracting the Space View Path and solve for the on-board linear gain term b : L CAL RVS BB RVS L BB BB a b dn BB ( RVS BB cav SV ( ) L a dn BB BB RVS cav BB ) L SM RVS: Response Versus Scan-angle L: Spectral band averaged radiance dn: Digital count with background corrected EV Radiance: L a b dn a dn RVS RVS L EV EV EV SV EV SM RVSEV Calibration Coefficients: b ( LCAL a adnbb ) / dnbb L RVS L ( RVS RVS ) L RVS ( ) L CAL BB BB BB SV BB SM BB BB cav cav - for each band, detector, and mirror side - performed on scan-by-scan basis 4-scan running average Source radiance with RSR integration: L S Planck(l,T) RSR(l) RSR(l) thermistors in BB panel Warm-up and cool-down (WUCD) TBB: 7 to 35K performed quarterly provides a and a, Band b a, a derived from pre-launch or periodic warm-up/cool-down cycles 4

15 //3 Aqua b Short-term Stability /66 9/349 Aqua MODIS TEB Response Trend Band Percent Change Stable detector response (excluding sensor configuration changes and instrument reset events) 5

16 //3 Vicarious calibration-rsb To monitor the radiometric degradation and assure calibration accuracy. Vicarious calibration with RTM and input environmental parameters, to get the absolute calibration coefficient Stable targets tracking analysis, including desert, dcc, etc to get the sensor response changing trend Instrument Background: Data website: FY-3A (launched on May 7, 8),local equator-crossing time of :3 A.M. (descending southward) FY-3B (launched on Nov 5, ), local equator-crossing time of :3 P.M. (ascending northward) 6

17 //3 Medium Resolution Spectral Imager (MERSI) Band Central Wave ( m) Band Width ( m) Resolution (m) spectral bands with 35 detectors on 4 focal plane assemblies (FPA) 9 reflective solar bands (RSB):.4-. μm thermal emissive bands (TEB): -.5 μm Two spatial resolutions (nadir): 5 m(-5), and km(6-) Scan angle: ±55 (from instrument nadir) A swath of km (along-track) by 9 km (nadir along-scan) Global coverage in day One-sided 45 scan mirror in concordance with a K mirror (de-rotation).5 second each scan Range of applications Land, ocean, and atmosphere properties. 7

18 //3 Reflectrance-based VC with in-situ measurement FY-3A MERSI image CRCS Dunhuang 敦煌场循环采样反射比光谱 平均 r r r3 r4 r5 r6 r7 r8 r9 r r r r r r3 r4 r5 r6 r7 r8.6.5 反射比 波长 纳米 Surface reflectance BRDF(AMBRALS) Site measurement Satellite geometry (Surface and atmosphere) Radiative transfer TOA vicarious reflectance Aerosol, Ozone, Water vapor Satellite measurement ARef=Ref (d/d)cos(solz) (SV, EV) =Slope (SV-SV) Radiance Radiance CONS Relative difference with MODIS measurment <5% except for.3um Dif(%)est with V6S Mea Est V6S Radiance(W/m/sr/um) Wavelength(nm) Wavelength(nm) 8 MERSI calibration coefficient CV<3.% Wave nm Band Mean Std CV(%) Wave nm Band Mean Std CV(%)

19 //3 Multi-sites calibration tracking Multi-sites with stable surface properties : Gobi and desert targets: Dunhuang, Libya, Libya4 and Arabia, ocean site: Lanai (MOBY) Dunhuang (4.38 N, 94.3 E) Libya (4.4 N, 3.35 E) Libya4 (8.55 N, 3.39 E) Arab (.3 N, 5.96 E) Lanai (.49 N, -57. E) TOA reflectance simulation Calibration coefficient calculation Data within certain days from 5 sites are used to get the calibration slope: ARef i =Ref i (d/d) cos(solz) =Slope i (DN i -DC i ) Calibration coefficient trend fitting Based on the calibration slope series, a linear model is used to describe the varying trend with DSL: Slope i =a i DSL+b i DSL is the day number since launch (May 7, 8) Annual degradation rate AnnualDecayRate i =a i *365/ b i *. 9

20 Scale Scale Scale Scale //3 Multi-sites with different brightness to cover the sensor dynamic range; Multi-sites and multi-days to decrease the random error. Days= FY-3A MERSI Reflectance Calibration Scale.4.39 SolZ<7. Deg; SenZ<6. Deg; CV<.5;.38 DSL(45-885) B: Scale=.447*DSL+.33 R = Date(/mm) FY-3A MERSI Reflectance Calibration Scale.3.9 SolZ<7. Deg; SenZ<6. Deg; CV<.5;.8 DSL(45-885) B3: Scale=-.*DSL+.44 R = FY-3A MERSI Reflectance Calibration Scale SolZ<7. Deg; SenZ<6. Deg; CV<.5;.33 DSL(45-885) B: Scale=.4*DSL R = Date(/mm) FY-3A MERSI Reflectance Calibration Scale SolZ<7. Deg; SenZ<6. Deg; CV<.5;.33 DSL(45-885) B4: Scale=-.38*DSL R = Date(/mm) Date(/mm) The calibration coefficients present a linear trend with DSL.

21 DecayRate(%) //3 Apparent Reflectance = Scale * (DN_EarthView - DN_SpaceView) (%) Scale = b + a* Days = b*( + RatePerDay/ * Days) (%/DN) Band b (%/DN) a (%/DN/Days) Sigma Sigma/M (%) Degrading rate per day (%) Degrading rate per year (%) Degrading rate(%) Band (47nm) E-6 4.4E Band (55nm) E-6 3.5E Band3 (65nm) E-7.4E Band4 (865nm) E-7.79E Band8 (4nm) E-6 5.6E Band9 (443nm) E E Band (49nm).45.87E-6 3.9E Band (5nm).987.9E-6.9E Band (565nm).484.6E-6.3E Band3 (65nm) E-8.E Band4 (685nm) E-8.98E Band5 (765nm) E-7.69E Band6 (865nm).47 5.E-8.45E Band7 (95nm).47.36E E Band8 (94nm) E-6.93E Band9 (98nm) E E Band (3nm) E-6.9E * Days = Day Count since FY-3A Degrading rate: Count from launch to ARef Scale tracking trend AnnualDecayRate TotalDecayRate Wavelength(nm) The short-wave channels have large degradation, especially channel 8 with the annual decay rate up to 4%. In the red and near-infrared bands (6 ~ 9nm), the calibration coefficients almost have no change with the annual decay rate below %.

22 sigma/mean* //3 ARef Scale tracking trend Wavelength(nm) The uncertainty (σ/mean) for the trend analysis is nearly below 4% except for water vapor channels (7, 8 and 9). TOA Radiation Simulation Test with Terra/MODIS Short wave (CH3,4,) more noisy. Scatter diagrams between MODIS measurement and simulated radiance

23 //3 Percentage difference between simulated and MODIS measurement The precision (Mean+-Std) is almost within 6% except for CH5. Performance is good for Libya and Libya4, relatively good for Arabia, poor for Dunhuang. 3

24 Dif(%) B B B4 B4 B6 B7 B8 B9 B B B B3 B4 B5 B6 B7 B8 B9 B //3 MERSI Re-calibration Performance Analysis Calibration slope test with Dunhuang calibration experiment DH vs Multi-sites Channel The percent difference between estimated coefficients and vicarious calibration with in-situ data are below 3.5% (with exceptions of 4.8% for band 5 and.8% for band 8). 4

25 //3 Comparison between re-calibrated and simulated radiance Radiance Radiance Radiance CH CH CH4 CH6 CH8 CH CH8 CH3 CH Radiance Radiance CH9CH9 CH CH3 CH5 CH7 CH CH Dunhuang Dunhuang Dunhuang Libya Libya Libya Libya4 Libya4 Libya4 Arabia Arabia Arabia est est MERSI (W/m /sr/um) MERSIest(W/m /sr/um) MERSIest(W/m /sr/um) MERSI (W/m /sr/um) 4 Dunhuang Dunhuang Libya Libya Libya4 Libya4 Arabia Arabia 5 est est MERSI (W/m /sr/um) mea mea mea MERSI MERSI MERSI (W/m (W/m (W/m /sr/um) /sr/um) /sr/um) mea mea MERSI MERSI (W/m(W/m /sr/um) /sr/um) Double Difference Analysis with MODIS DPDif Simulated Re cal Simulated Observation ( Rad MERSI Rad MERSI ) ( Rad MODIS Rad MODIS ) Observation Rad MODIS RMS = 5.6% RMS = 3.5% MERSI systematically larger RMS = 3.5 % RMS = 3.4% 5

26 Reflectance(%) Reflectance(%) (MODIS -MERSI)/MODIS* Reflectance(%) Reflectance(%) Reflectance(%) Reflectance(%) Reflectance(%) //3 5 Radiance Multi-sites Prelaunch Wavelength(nm) Mean percentage difference through double difference analysis with MODIS Radiation is underestimated with the pre-launch calibration coefficients; degradation trend is effectively corrected using the results from multi-sites method. After recalibration, DPDif s means are within % except for band ~3.6% overestimated. FYC/D TOA Ref at Dunhuang FY-C/D CH FY-C/D CH FY-C/D CH6 FY-C/D CH7 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal nm 6 869nm 6 599nm nm Date Since Launch(999-5-) Date Since Launch(999-5-) Date Since Launch(999-5-) Date Since Launch(999-5-) FY-C/D CH8 FY-C/D CH9 FY-C/D CH 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal 9 SolZ<7. Deg; SenZ<7. Deg 8 FY-C OriCal FY-C ReCal FY-D OriCal FY-D ReCal nm nm nm Date Since Launch(999-5-) Date Since Launch(999-5-) Date Since Launch(999-5-) The degradation trend with time is corrected with new coefficients Data is consistent between satellite. 6

27 //3 Deep Convective Cloud tracking Albedo Albedos vary with COD Broadband Ratio Shortwave Broadband.65 um.86 um T(µm) < 5 K ± latitude... Cloud Optical Depth Cloud Optical Depth Albedo Albedos vary with SZA SZA SZA SZA Cloud Optical Depth Clout reflectance with high optical depth is stable. COD>, differential coefficient approaches. DCC reflectance angle dependent Apply initial calibration coefficient Convert DCC pixel radiances to overhead sun using CERES Angle Distribution Model (ADM) F( ) s L( s R(, s, v, v, ) ) Plot and analyze the mean monthly DCC radiances 7

28 Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance Nominally Reflectance // Band DSL Band DSL DSL Band Band Band DSL Band DSL DSL Band3 Band Band DSL Band DSL DSL Band4 Band Band DSL Band DSL Band DSL Band9 Band Degrading rate per year (%) Band Sigma /Mean (%) B (47nm) B (55nm).7. B3 (65nm) B4 (865nm).4.65 B8 (4nm).5.9 B9 (443nm) 5.8. B (49nm) B (5nm).4.4 B (565nm).9 B3 (65nm) B4 (685nm) B5 (765nm).6.5 B6 (865nm)..79 B7 (95nm).67 B8 (94nm).6. B9 (98nm).4.57 B (3nm) DSL DSL DSL DSL DSL Vicarious calibration-teb VC with in-situ measurement Site measurement (water temperature, skin radiance, aerosols, atmospheric profile or total column water) Lake,Qinghai Satellite geometry Radiative transfer (skin temperature or radiance, atmospheric profile, geometry) Instrument RSR TOA vicarious radiance Rad v =Slope(EV-SV) Satellite measurement (SV, EV) 8

29 DN // Terra,TIR, Y = *X radiance[mw/m *sr*cm - ] To evaluate and improve data quality, rather than get the calibration coefficient. MODIS-Terra TOA radiance: TIR, no bias, abs. acc..k Lake Tahoe, Terra,TIR, - SNO inter-calibration Based on comparison of collocated observations from instrument pairs during Simultaneous near-nadir Overpasses. Hyperspectral sensor METOP/IASI and AQUA/AIRS are used as reference standard. well-specified, contiguous spectral coverage. Collocation map ( FY-E (4.5 E) with METOP) 9

30 //3 FY-E IR SRF (red line) Overlay on AIRS Spectrum Green:valid Purple:failed Orange:gap Band Name FY-E/VISSR METOP/IASI AQUA/AIRS Wavelength (um) Resolution IR.3~.7 5 km TIR Win IR.3~3. 5 km Wavelength (um) Resolution Wavelength( um) Resolution 5.5~8.6.55cm - 5.4~8.8.5cm - IR3 WV 6.~7.7 5 km 8.6~5..cm - 8.~6..5cm - IR4 MWIR 3.4~4. 5 km 5.~3.6.cm - 4.6~3.74.5cm - Observation time difference t FY t sounder < dt max (5 mins) Satellite zenith angle difference cos( SZA sounder ) / cos( SZA FY ) < MaxRate (clear:.; cloudy:.3) Environment uniformity STDV(FY Rads in ENV_BOX) < MaxSTDV Normality check MEAN(FOV_BOX) MEAN(ENV_BOX) 9/ STDV(ENV_BOX) < Gaussian Orbital Prediction Colloc. Criteria SRFs, PSFs, Masks, flags, IUT Level Data Ref Level Data Collocate Collocated Data Transform Comparison Data Key Point Match in time, location, and angle Difficul t Point Match in space and spectrum Filter, smooth, regression, etc. Monitoring Correction Diagnosis Plots and Tables GSICS Correction Paper, briefings Products IUT Lvl Data Re-Cal Data 3

31 //3 FY-E Calibration Slope Series 3

32 Probability Distribution Probability Distribution Probability Distribution //3 FY-E TBB bias with IASI GSICS : 9% bias<k, most Temp (7K<T<K) bias<k; accuracy is improved K at least.8 IR.8 IR.8 IR3 Calibrated Tbb bias with IASI in Top : variation with temperature Right: probability distribution.6.4. GSICS/IASI Operation.6.4. GSICS/IASI Operation.6.4. GSICS/IASI Operation BT Bias/ K BT Bias/ K BT Bias/ K Typhoon Example SangDa FY-D :5 8 K FY-E :3 4 K Typhoon monitoring system from CMA NMC FY-D(left) and FY-E(right) 3

33 //3 K GSICS 5K Operation TBB PDF of Typhoon Cloud System OPERATION 5 4 眼区云墙 GSICS DTBB GSICS -Oper FY-E TBB Comparison of Typhoon Cloud Wall

34 //3 FY-3 ocean color applications Ocean color product parameters: ρ w (λ) Chlorophyll a concentration Total suspended mater concentration (TSM) Absorption coefficient at 443nm of CDOM and NAP(YS) Data Format: HDF5 FY-3A ocean color product specification. Type Projection Coverage Day Geographic Longitude/Latitude Global, per breadth Spatial Resolution.. Ten days Ditto Global.5.5 Month Ditto Global

35 //3 OPP RGB with band 3/4/ on June 8, 8 35

36 //3 HAB at Pacific coastal region of the Central America Resuspended sediment variation caused by storm surge Thank you for your attention! Acknowledgements Xiu-Qing Hu, Na Xu, Min Min, Jack Xiong, Simon J. Hook, More information about MODIS onboard calibration could be found at 36