Inflight Radiometric Calibration: NASA Experience

Transcription

1 EOS Inflight Radiometric Calibration: NASA Experience Jack Xiong and Brian Markham Sciences and Exploration Directorate, NASA Goddard Space Flight Center Contributions: MODIS, VIIRS, and OLI Instrument Calibration Support Teams (Amit Angal, Ben Wang, Ning Lei, and Julia Barsi) Workshop on Radiometric Calibration for European Sensors, ESRIN, August 2017

2 Outline Instruments On-orbit Calibration Calibration Activities, Strategies, and Methodologies Performance Improvements and Challenges Lessons and Future Efforts MODIS & VIIRS OLI Calibration and Performance 2

3 NASA Earth-Observing Missions 3

4 MODIS, VIIRS, and OLI Instruments MODIS on Terra and Aqua Missions Terra: Dec. 18, 1999 Present Aqua: May 04, 2002 Present VIIRS on S-NPP and JPSS Missions S-NPP: Oct. 28, 2011 Present JPSS-1: Launch in late 2017 JPSS-2: Currently in sensor TVAC OLI on Landsat Missions L8: Feb. 11, 2013 L9: Launch in 2020 OLI 4

5 MODIS, VIIRS, and OLI Spectral Bands Band # Band Center Wavelength (nm) Bandwidth (nm) L 1 Coastal/Aerosol Blue Green Red NIR SWIR SWIR Panchromatic Cirrus L8 carries a separate TIR sensor (TIRS) MODIS bands with wavelengths above 12 mm 5

6 MODIS and VIIRS On-orbit Calibration MODIS SD/SDSM calibration is performed from weekly to once every 3 weeks SD door is open only during scheduled SD/SDSM calibration events Regular SRCA operations in all 3 modes (radiometric, spectral, and spatial) Quarterly BB warm-up and cool-down (WUCD) Monthly lunar observations at phase +/-55⁰ Solar Diffuser Stability Monitor (SDSM) Extended SV Port Rotating Telescope Aft Optics and HAM Solar Diffuser (SD) V-groove Blackbody (BB) VIIRS SD calibration performed each orbit (no SD door) SDSM operated on a daily basis and now 3 times/week Quarterly BB WUCD same as MODIS Lunar observations at phase -51⁰ 6

7 SD On-orbit Degradation 926 nm 555 nm 445 nm 412 nm Large at Short Wavelengths Terra (17 years): operated with SD door open since July 2003 Aqua (15 years): SD door opens only during scheduled calibration events S-NPP (6 years): no SD door (similar to current Terra MODIS) 7

8 On-board Blackbody Performance S-NPP VIIRS BB performance is extremely stale (similar to Aqua MODIS and slightly better than Terra) different T BB settings 30 mk uniformity requirement Long-term trend of daily-averaged T BB Stable to within a few mk. ~15mK offsets were due to the use of different T BB settings. Short-term stability (scan-by-scan) Orbital variations of individual thermistors up to 40mK Variations in average T BB ~ 20mK BB uniformity meets the requirement (30mK) 8

9 VIIRS Spectral Band Responses (Gains for TEB) Long-term changes in MWIR and LWIR are very small (<1.0% except band I5) Individual detector responses (F-factors) show small orbital variations: ±0.2% or less for scan-by-scan ±0.1% or less for granule average (except for I5) 1.0% 1.0% 9

10 VIIRS Spectral Band Responses (Gains for RSB) Small changes in VIS; large changes in NIR and SWIR (mirror contamination) Similar response trending from SD and lunar observations; small differences observed Lunar: symbols; Solar: curves Better RSB Performance Is Expected for JPSS-1 VIIRS 10

11 MODIS Spectral Band Responses (Gains for RSB) SD Moon SD Moon SD and lunar observations are made at different AOIs Changes in SWIR responses are extremely small (located on cold FPA) Challenge - changes are wavelength, AOI, and mirror-side dependent New RVS characterization strategies developed and implemented (in C6) 11

12 Calibration Improvements and Challenges (MODIS) Calibration maneuvers Roll (near-monthly) for lunar observations Yaw (at mission beginning) for solar diffuser BRF and SD screen transmission characterization Pitch (twice for Terra MODIS) for TEB RVS characterization and lunar calibration through EV port RVS characterization using data from OBC (including lunar observations) and EV observations at different AOIs On-orbit characterization of changes in sensor polarization sensitivity at different AOIs Corrections (to be) apllied for optical leak and crosstalk L1B calibration uncertainty Future Work 12

13 Pitch Maneuver Calibration Terra Spacecraft: 3 deep space maneuvers with two planned for lunar observations (4/14/2003 and 8/5/2017) 4/14/2003 8/5/2017 Applications: TEB RVS and lunar calibration inter-comparison Experience and lessons for S-NPP and JPSS-1 13

14 Impact of Polarization Sensitivity 412 nm 443 nm uncorrected reflectance corrected reflectance Terra MODIS VIS short wavelengths at large AOIs (mirror side dependent) Impact on RVS characterization Not observed in Aqua MODIS 14

15 Calibration Improvements and Challenges (VIIRS) SDSM screen transmission (t SDSM ) yaw data + SD screen transmission and t SD BRDF SD (SDSM_View) regular data SD screen transmission and t SD BRDF SD (RTA_View) yaw data Correction for the solar vector error SD degradation (H) Revised extrapolation of H to the mission very beginning Model H to cover SWIR wavelengths Fit H at RTA view to lunar trending Modulated RSR large impact on DNB calibration DNB offsets and stray light correction (forward processing) DNB HG offsets derived pitch maneuver data and trended with BB observations Stray light correction LUT derived from LUTs in previous years (weighted average of the same day LUTs from previous years) Use of stars for DNB HG calibration Calibration uncertainty (assessment and L1B implementation) 15

16 On-orbit Pre-launch SD Screen Transmission (t SD ) and BRDF SD Initial SD and SDSM LUTs derived from limited pre-launch measurements On-orbit improvements made using data collected during SC yaw maneuvers Latest improvements achieved by adding regular on-orbit calibration data to fill the gaps in yaws SD LUT Ratio On-orbit LUT: Larger range Better resolution Same wavelength 16

17 SDSM Screen Transmission SDSM PL_LUT H DH SDSM Yaw_LUT SDSM New_LUT Similar improvements to SD screen transmission 0.41, 0.48, 0.67, 0.86 mm 17

18 SD Degradation at SWIR Wavelengths 8 detectors (0.41 to 0.93 mm) 4 SWIR bands (1.2 to 2.3 mm) SDSM 1 H, t t 4.07 (t) derived from fitting SD degradation (H) at from D5-D8 18

19 VIIRS Spectral Band Responses (Gains for RSB) Before H Improvement (V1) After H Improvement (V2) Lunar: symbols; Solar: curves H SD Degradation Factor 19

20 Comparison of VIIRS SD and Lunar Calibration Before H Improvement (V1) After H Improvement (V2) H SD Degradation Factor 20

21 Modulated RSR Impact on DNB Calibration Lunar: symbols; Solar: curves Impact: up to 4% difference for DNB solar calibration (not the same for lunar calibration) 21

22 Measured and Normalized Lunar Irradiance I Meas_Sensor MODIS C6.1 VIIRS V1 I Meas_Sensor /I Model_Sensor M M M M I M M I M M M I M M

23 MODIS and VIIRS Calibration Inter-comparison Data: S-NPP VIIRS Aqua MODIS Terra MODIS VIIRS/A-MODIS VIIRS/T-MODIS T/A-MODIS MODIS VIIRS Nom STD Nom STD Nom STD Ratio STD Ratio STD Ratio STD 8 M M M M I M M I M (I Meas_Sensor-A /I Model_Sensor-A ) (I Meas_Sensor-B /I Model_Sensor-B ) 23

24 DNB HG Calibration Stability Assessment Using Stars Two arrays of HG detectors: HGA and HGB SD based F- factors SD for DNB LG calibration and gain ratios: L/M and M/H 24

25 Lessons and Future Effort Communication Calibration team to establish good communication and healthy working relationship with science team (project), instrument vendor, and users through the entire mission Pre-launch Calibration Comprehensive pre-launch testing to establish sensor baseline performance (functions and performance compliance) Sufficient and well designed measurements and high quality data sets to enable key (all if possible) sensor calibration/performance parameters to be derived for on-orbit use Comprehensive characterization of issues identified to support impact assessment and implementation of mitigation strategies (e.g. correction algorithms) Preparation for launch readiness and different phases of on-orbit calibration 25

26 Lessons and Future Effort Dedicated Effort and Resources Calibration never ends (through entire mission and beyond) Expect the unexpected Sensor to sensor difference (even with the same design requirements) Importance of reprocessing Continuing improvements Documentation Technical reports/memos ATBD, User Guild, Conference proceedings and journal papers 26

27 Landsat-8 Operational Land Imager OLI s radiometric stability and absolute radiometric calibration performance have been excellent CA band (443nm) and Blue band (482nm) show some degradation but have been tracked with the calibrators at about 0.1% level Performance and consistency between on-board calibration techniques are unprecedented in the Landsat program Markham et al, Landsat-8 Operational Land Imager Radiometric Calibration and Stability, Remote Sensing (2014). 27

28 OLI Calibration On-board calibrators Lamps: Two sets of tungsten halogen bulbs (primary and redundant), each with three pairs of lamps (working, backup, pristine) used with differing frequencies (daily, bi-weekly, semi-annually) Diffusers: Two Spectralon solar diffusers (working, pristine) used with different frequencies (weekly, semi-annually) Shutter: Shutter for dark acquisitions and to block the signal from the nadir-viewing aperture for calibration acquisitions Calibration maneuvers Lunar: Monthly scans of the moon; requires spacecraft to slew multiple times to capture the moon within each module Solar: Solar diffuser-based calibration requires maneuver Yaw: Tri-annual yaw maneuvers; spacecraft rotates ~90 so that the focal plane is parallel to the ground track and every detector sees close to the same target (not covered here) 28

29 spectral bands and guarantee a minimum signal (>40 counts) in all spectral bands. The stim lamps are current controlled, and both the voltage and current are included in the telemetry, so that the ground can monitor the change in filament resistance (and thus, aging) over the lifetime. Light from the stim lamp bulbs goes through a transmissive diffuser, which spreads the light out, so that it illuminates the entire focal plane array. Monitoring diodes (primary and redundant) look at the light reflected from the diffuser to provide another measure of how much the output has changed over time. A photo of the completed assemblies and a schematic are shown in Figure 5. OLI Calibration Stim Lamps Figure 5. The two stim lamp assemblies mounted to the aperture baffle (left); a schematic of the stim lamp assemblies showing the six bulbs and the monitoring diode (which looks at the stim lamp diffuser) (right). Assemblies are located at the entrance aperture of telescope When lit, a pair of lamps illuminates the whole focal plane Operated in constant current mode Monitored for stability with silicon photodiodes signal [counts] Band 5 Working Lamp Band 5 (NIR, 868nm) Working Lamp Profile detetor index 29

30 OLI Calibration Solar Diffusers Calibration Assembly is in front of the telescope Spacecraft maneuvered to center Sun in Solar Cal Port. Diffuser wheel rotated to place selected diffuser into solar beam Solar incidence angle 45 ; OLI view angle Total on-orbit exposure to date Working: ~265 minutes Pristine: < 14 minute for stability trending purposes. Additional processing for a reflectance-based cal Section 6. Data from a sample solar diffuser collect is shown in Figure 6. M Figure 6 is due to responsivity differences between the various detectors and foc normalized radiance Figure 5. Components of the calibration assembly. Figure 6. Working Diffuser On-Orbit Data for 28 April 2014, N Band 5 (NIR, 868nm) Working Diffuser Profile detector index 30

31 edicated maneuvers. The lunar scanning pattern selected for Landsat-8 allows for each FPM to image the The moon. pitch On rate even used during the image acquisition results in a factor of 8 overs umber months within the range of +5 to +9 lunar phase angle, the spacecraft slews the along-track to point the direction OLI (Figure 8a). The current lunar processing procedure uses the rad ear the moon during the Earth eclipse part of the orbit. Starting with FPM 8, the spacecraft corrected lunar maneuvers products, that is, bias-subtracted, linearized and converted to radiance. In OLI Calibration Lunar Maneuver o scan each FPM down to FPM 1, as shown in Figure 7. Image data are recorded USGS during lunar the pure irradiance pitch model for each observation include the spacecraft position, time of ortions of the maneuver from one lunar diameter off the moon on one side to one and lunar the diameter apparent off (oversampled) along-track size of the lunar disk. Currently, the OL he moon on the other side. The maneuver is designed to maintain the angular velocity irradiances of the are pitch obtained as by summing up all the lunar pixels above a threshold value of 1 pe onstant as possible during the imaging portion of the maneuver to reduce radiometric maximum uncertainties pixel value in in each image. An image mask created from this threshold is used to he lunar irradiance due to image distortion; better than ±1% was obtained. The spacecraft along track slews diameter back to of the moon as well as the position (and hence the time) of the moo oint the instruments at the Earth for the daylight pass and, during the next Earth eclipse, the image. slews The back latter to is used together with information from the spacecraft ancillary da he moon to complete the scanning pattern from FPM 8 to 14. The extended scans, the e.g., spacecraft after FPM position 3 and in J2000 reference frame. Ratios of the instrument irradiances versus t efore FPM 2, are to allow TIRS imaging of the moon. The ~0.5 lunar extent values fits well are within then used each for further analysis. A sample geometrically corrected lunar image PMs ~1.1 field of view. On odd months the same phase angles are used, but the pattern is altered so hat FPM 7 images the moon first followed by the higher number FPMs on the first orbit; on the second rbit FPM 7 is again imaged first, followed by the lower number FPMs. Figure 8. OLI Lunar Images, Pan band, 26 March 2013, FPM 7. (a) Leve (b) Level 1G, radiometrically and geometrically corrected (enlarged ~8 relative to (a Slew spacecraft during ascending node, requires two orbits to acquire moon in all 14 modules Acquire data with moon at phase angle between +5 and +9 Figure 8b. Signal is integrated over the lunar disk Figure 7. The OLI scan pattern of the moon over two orbits on even number months OLI irradiance is compared to ROLO model (gray is orbit #1; green is orbit #2). The center and edge detectors show the extent of each FPM in the outer and inner bands (bands 8 and 9). (a) Band 8 (Pan) L1G image from module at center of focal (b) plan 3. OLI Pre-Launch Radiance Calibration and Stability Monitoring 3.1. Absolute Radiance Calibration 31

32 OLI Prelaunch Calibration Radiance gains Established based on prelaunch measurements of the large water-cooled integrating sphere at Ball Aerospace Radiance-feedback controlled with selectable band filter Calibration transferred via radiometers from Small Sphere Source calibrated at NIST FASCAL Measurements were made at multiple radiance levels but gains were derived from a single high-level radiance where the sphere was controlled for the band under test. Large sphere source radiances validated by post-oli testing radiometric scale realization campaign, with NIST, Ball, GSFC and University of Arizona radiometers Indicated good agreement ~1% when controlled in-band; larger differences when not controlled in band Reflectance gains Bidirectional reflectance of diffuser panels were measured prelaunch at University of Arizona by Stu Biggar Heliostat test exposed diffusers and OLI to sunlight provided initial reflectancebased calibration Operational reflectance calibration based on initial on-orbit diffuser measurements 32

33 Early On-orbit Adjustments to Prelaunch Calibration Early 2013 Reflectance gains updated based on initial diffuser acquisitions Jan 2014 The cirrus band reflectance calibration was corrected for an error found in the calculation; this increased the reflectance in this band by ~7%, without changing the radiance calibration. The SWIR-2 band gain was adjusted for the difference observed prior to launch between the BATC and validation results; this increased the radiance by about 3%, without changing reflectance calibration. All bands gains were adjusted to correct for an outdated radiance file that was mistakenly used in the pre-launch calibration data reduction for all bands; this decreased the radiance in all bands by ~2%, without changing the reflectance calibration. The precision of the radiance to reflectance conversion was improved for all bands; this was a less than 1% effect in reflectance 33

34 OLI Lifetime Trending of Calibrators All calibrators have been far more useful and stable than on previous Landsat instruments Worst case band: calibrators revealed a slow degradation in the Coastal/Aerosol band (Band 1, 443nm), dropping by about 1% in the first year and 0.1% per year since then Other bands: All calibrators agree to within a few tenths of percent. Lunar data is less precise in the SWIR bands Response Rela ve to Mission Day 75 Response Rela ve to MIssion Day 75 OLI CA Band 1 Trends: Band Average Time Since Launch [years] s m lamp (working) s m lamp (backup) s m lamp (pris ne) solar panel (working) solar panel (pris ne) lunar OLI SWIR2 Band 7 Trends: Band Average Time Since Launch [years] s m lamp (working) s m lamp (backup) s m lamp (pris ne) solar (working) solar panel (pris ne) Lunar 34

35 Vicarious Calibration University of Arizona manned campaigns and RadCaTS Railroad Valley Infrequent use of other playas Uncertainties ~4% South Dakota State University Grassy field in Brookings SD Uncertainties ~7% Results to-date indicate there is no calibration error within the uncertainties Ratio of TOA Spectral Radiance (measured/oli) Ratio of TOA Spectral Radiance (measured/oli) OLI Vicarious Results Spectral Radiance Calibration Band RadCaTS/OLI (13) SDSU/OLI (20) OLI Vicarious Results Re lectance Calibration Band RadCaTS/OLI (13) SDSU/OLI (20) 35

36 Pseudo-invariant Calibration Sites Monitor specific CEOS PICS regions for long term trending. So far, OLI is more stable than PICS CEOS OLI CA CEOS OLI SWIR normalized reflectance 1 normalized reflectance me since launch [years] me since launch [years] Arabia-1 Sudan-1 Libya-4 Mauritania-2 Arabia-1 Sudan-1 Libya-4 Mauritania-2 36

37 OLI Lessons Learned Usefulness of multiple calibration methods Unlike ETM+, calibrators on OLI are all telling similar stories Usefulness of using calibrators at different frequencies Similarities/differences between multiple lamp trends and multiple diffuser trends help distinguish between calibrator or instrument change Usefulness of careful contamination control No apparent contamination on diffusers or focal plane; little degradation in instrument or calibration sources; no need for decontamination or short term responsivity corrections Spacecraft events can affect instrument calibration Some discontinuities in responsivity at spacecraft safehold events Pre-launch radiance feedback control of radiance calibration sources is useful, but must be done in the band being calibrated Procedure updated for OLI-2 (Landsat-9) to provide in band radiance control at 20 levels to improve linearity characterization Transfer radiometers generally more stable than lamp-based calibration sources 37

38 OLI Ongoing Challenges Calibration transfer-to-orbit assumptions what is stable through launch? Solar diffusers treated at stable through launch Comparing pre-launch heliostat-based OLI diffuser responses (with corrections) with on-orbit diffuser responses indicated OLI responsivity increases at levels greater than the estimated uncertainty in the technique Lamps not stable through launch; photodiode monitor indicated comparable change in lamps as OLI indicated in comparable bands Lunar irradiance model limitations SWIR bands have larger scatter compared to model Absolute uncertainty in lunar model irradiance appears to be too large to be useful for absolute calibration On going work to improve model Separating instrument and calibrator effects Much improvement with multiple well behaved calibrators, but still learning Tracking between calibration stability Video reference pixels (no detector present) and SWIR blind bands (masked) available for tracking bias stability between shutter measurement; Video reference pixels particularly useful for OLI-2 where 14 bits will be transmitted 38

39 VIIRS I-band dark spot during eclipse 39

40 Additional Information Available: 40