# Presentation: 430075

## CLARREO: The GSFC RoleSolar Reflectance Calibration andFilter Radiometer Issues Related to Inter-calibration 

*CLARREO GSFC: Butler, Platnick, Xiong*

**CLARREO: The GSFC Role**Solar Reflectance Calibration andFilter Radiometer Issues Related to Inter-calibration

**GSFC Team**
- Jack Xiong: Lead, MODIS Characterization Support Team; VIIRS calibration support
- Jim Butler: EOS Calibration Scientist, NPP Deputy Project Scientist for calibration
- Steve Platnick: MODIS, VIIRS atmosphere team science and algorithms

**Notes:**
Overview: What is it?
Background: Because CLARREO is quite different from any existing or proposed ES mission - advocates will call it a new paradigm for climate monitoring - so I think it useful to provide some historical context 
Status: Activity since the DS was released

## Deliverables (~ Sept. 2008)

- Documents/files including Relative Spectral Response w/wout out-of-band knowledge as available for MODIS and other radiometers of interest [lead: Xiong]
- Assess new calibration/characterization measurement methodologies with potential applications to CLARREO for accuracy and precision, as well as feasibility for CLARREO testing [lead: Butler]

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## Solar Inter-calibration

-  You can’t inter-calibrate well ...
- an instrument that isn’t well calibrated

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

- OR
- You can’t transfer a standard beyond the level to which an instrument is characterized

## Outline

- _Xiong_: MODIS spectral characterization capability, history, and lessons learned
- _Butler_: Filter radiometer characterization and stability (examples from SeaWIFS, etc.)
- _Platnick_: Example MODIS reflectance sensitivity to filter position

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## 1. MODIS Reflective Solar Band CalibrationJack Xiong

*Page *

- 1. MODIS Reflective Solar Band CalibrationJack Xiong

- Outline
- Instrument Background
- Reflective Solar Bands (RSB) Calibration and Characterization
- Radiometric (on-orbit)
- Spectral (pre-launch)
- Lessons and Challenges

## 20 Reflective solar bands (RSB): 0.41-2.2m
16 Thermal emissive bands (TEB): 3.7-14.4m
3 spatial resolutions at nadir: 250m, 500m and 1000m
4 Focal Plane Assemblies (FPA): VIS, NIR, SMIR, LWIR
5 On-Board Calibrators: SD, SDSM, SRCA, BB, and SV port

Two MODIS (Terra and Aqua): Complementary morning and afternoon observations
A broad range of applications: land, oceans, and atmosphere

**16 Thermal emissive bands (TEB): 3.7-14.4********m**
**3 spatial resolutions at nadir: 250m, 500m and 1000m**
**4 Focal Plane Assemblies (FPA): VIS, NIR, SMIR, LWIR**
**5 On-Board Calibrators: SD, SDSM, SRCA, BB, and SV port**
**Two MODIS (Terra and Aqua): Complementary morning and afternoon observations**
**A broad range of applications: land, oceans, and atmosphere**

- MODIS Instrument

*Page *

- Terra

- Aqua

## MODIS Key Specifications

*Page *

## MODIS RSB Calibration

- Radiometric
- Pre-launch
- Calibration source: SIS-100
- Calibration activities: gain, nonlinearity, noise characterization, dynamic range (A/B electronics. 3 instrument plateaus for thermal vacuum test)
- Solar diffuser BRDF calibration
- On-orbit
- Spectral
- Pre-launch
- On-orbit
- On-board calibrator: SRCA
- Calibration activities: center wavelengths and bandwidths

*Page *

## BB for IR calibration
SD/SDSM for VIS/NIR calibration
SRCA for spatial and spectral
Lunar observations

- Moon

- SDSM

- Scan
- Mirror

- Solar
- Diffuser

- SRCA

- Blackbody

- Space
- View

- SDSM

***BB for IR calibration***
***SD/SDSM for VIS/NIR calibration***
***SRCA for spatial and spectral***
***Lunar observations***

*Page *

## RSB SD/SDSM Calibration

- Scan Mirror (MODIS)

- Sun

- 1.44% Screen

- SDSM

- SD

- Optional 7.8% Screen

*Reflectance Factor*

- SD: SD degradation factor;
- SD: SD screen vignetting function
- d: Earth-Sun distance
- dn*: Corrected digital number
- dc: Digital count of SDSM

## RSB Lunar Stability Monitoring 

- RSB Lunar Stability Monitoring

*SD*
*Calibration *

*Lunar*
*Calibration *

*Geometric Factors*

- Lunar Model Results: USGS (H. Kieffer and T. Stone)

*Page *

## Aqua MODIS Response Trending

- Wavelength and AOI dependent response change
- Little mirror side difference for Aqua MODIS; Large change for Terra mirror side 2

## Aqua MODIS SD Degradation

- SDSM detectors
- Center wavelength (nm)
- D1	411.97
- D2	465.69
- D3	529.74
- D4	553.75
- D5	646.14
- D6	746.62
- D7	856.49
- D8	904.29
- D9	936.23

- Wavelength dependent SD degradation
- Similar for both Terra and Aqua MODIS when operated under the same conditions

## RSB Calibration Issues

- Calibration Requirements
- 2% for reflectance and 5% for radiance for MODIS RSB
- SD BRF calibration uncertainty (major contributor to RSB calibration uncertainty)
- Pre-launch to on-orbit transfer
- SD on-orbit degradation (for MODIS: VIS bands experienced more degradation)
- Optics degradation (response versus scan angle)

## MODIS Spectral Calibration

- Pre-launch Spectral Calibration Performed Using A Spectral Measurement Assembly (SpMA)
- Relative spectral response (RSR), center wavelength (CW), bandwidth (BW)
- Out-of-band (OOB) response
- Optical leak characterization
- On-orbit Spectral Performance Monitored Using An On-board Spectro-Radiometric Calibration Assembly (SRCA)
- RSR (partial, covering FWHM), CW, BW (for VIS/NIR only)

## MODIS VIS/NIR RSR

## OOB Characterization

## SWIR Optical Leak

- Terra
- MODIS

- Aqua
- MODIS

## MODIS On-orbit Spectral Performance

- Band 2 not recoverable

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## MODIS Spectral Calibration Issues

- Stability monitoring during RSR characterization
- In band and out-of-band stitching
- Normalization, filter characterization
- RSR characterization steps ()
- In band and OOB, especially overlapping region
- Atmospheric impact (for ambient measurements)
- “Smile effect” (for detectors aligned along-track)

## Lessons and Challenges

- Absolute calibration accuracy can be improved, but still limited in the existing approaches
- Calibration and observations should be made through the same optical configuration (path)
- Effectively monitoring the detector response change over time, and time dependent response versus scan-angle (RVS)
- High quality RSR characterization (component and system level)

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## 2. Satellite Filter Instrument Radiometric Accuracy, Stability, and Intercomparisons: Status and Lessons LearnedJim Butler

*Page *

- 2. Satellite Filter Instrument Radiometric Accuracy, Stability, and Intercomparisons: Status and Lessons LearnedJim Butler

- Radiometric Performance: Accuracy
- Radiometric Performance: Stability
- On-orbit Intercomparisons of Filter Instruments
- Current and future intercomparison developments

## Radiometric Performance:  Accuracy

- Begins at the instrument system and subsystem specification/design stage
- No subsystem is immune: filters, optics, electronics, static and  dynamic structures, all coupled with anticipated on-orbit thermal influences
- Initially established during pre-launch calibration and characterization
- Directly limited by the ability to calibrate/characterize GSE sources, optics, detectors and data acquisition electronics
- Source (e.g. MODIS integrating sphere) validated calibration at the 2 to 3% (k=1) accuracy level has resulted in pre-launch instrument-level absolute calibration accuracies of 4 to 5% (absolute) for radiance. Reflectance calibrations have been validated at the 2% (relative) level using piece-parts (e.g. EOS BRDF round-robins).  There is little to no margin on both.

- Advances in GSE realized through the use of tunable intensity stabilized
- laser-fed spheres/collimators (e.g. NIST SIRCUS) calibrated for radiance
- using standard detectors should improve these numbers by at least a factor of 10.

## Radiometric Performance: Accuracy

- System level, end-to-end testing of on-board calibration systems should be required to properly validate piece-part approaches
- Calibrating “like with like,” “as you fly, ” and “using multiple measurement methodologies” must be adopted as pre-launch testing mantras and applied where and when possible.
- Calibration against tungsten lamp-based uniform sources does not represent scenes remotely sensed.  LED-based spectrally tunable sphere sources (capable of producing output with spectral content reproducing remotely sensed scenes) will provide improved insight into on-orbit radiometric performance.
- Development of sources such as the hyperspectral image generator will provide important information on instrument radiometric/spatial performance at the full and sub-scene level.

## Radiometric Performance: Accuracy

- Transfer to orbit calibrations utilizing the sun (e.g. SeaWiFS) provide the most detailed insight into calibration changes incurred through launch

- For SeaWIFS, prelaunch courtyard-based
- measurements of the Sun using the solar
- diffuser were used to predict the first solar
- Diffuser mmts on orbit.
- -This is essentially a check of the on-board
- reflected solar calibration system using the
- Sun

- -Difference of initial on-orbit mmt from
- pre-launch predicted mmt.
- -Main uncertainty sources of technique:
-    a. atm. transmittance mmt. 3%
-    b. blue sky radiance mmt. 0.25%
-    c. pre-launch to on-orbit alignment 0.3%
-    d. extrapolation from pre- to post-launch 1 to 2%
- -Overall uncertainty of technique  3% (largest term)

## Radiometric Performance: Stability

- Satellite instrument calibration requires instrument system and subsystem level stability to be determined and monitored on-orbit
- Stability is an instrument characteristic which influences all areas of instrument design
- Instrument stability is often insufficiently determined and demonstrated pre-launch
- Inadequate time for proper testing (time=$)
- Testing must cover all possible on-orbit operating scenarios with respect to operating voltage, instrument and focal plane temperatures, and electronic cross-strapping configurations
- The stability of vis/nir/swir on-board calibrators (e.g. solar diffusers and monitors) are often not tested
- A well designed t/v test plan (i.e. one with T plateau repeats and transitions) can be used to demonstrate instrument stability.
- Plans for adequate stability testing should be incorporated into the design of instrument test equipment and facilities (e.g. HIRDLS t/v chamber)

## Radiometric Performance: Stability

- On-orbit stability is either monitored using dedicated on-board hardware or using specific remotely sensed targets.
- For the vis/nir/swir wavelength region, monthly lunar views have been used to monitor instrument degradation to better than 0.1%/yr  (e.g. SeaWiFS)

- This level of precision achieved using
- repeat views of the Moon at nearly
- identical phase meets the stability
- requirement for climate change
- (NISTIR 7047)

- Requires and up-front commitment
- by projects to produce fully
- maneuverable s/c and realization
- that data reprocessing is a reality
- of climate research

## On-orbit Intercomparison of Filter Instruments

- Involve views (preferably simultaneous or near simultaneous) views of common targets.
- Require knowledge of target spectral BRDF and spectral responsivities, spectral and spatial stray light responses of instruments
- In the absence of a benchmark (i.e. truth) measurement, comparison techniques provide measures of the relative on-orbit calibration scales of instruments with the “goodness” of comparisons limited to the combined on-orbit measurement uncertainties of the instruments.

## On-orbit Intercomparison of Filter Instruments

| Cross-comparison technique | Uncertainty of cross-comparisons performed | Additional Notes |
| --- | --- | --- |
| Land sites with ground mmts. | 2-5% absolute uncertainty in technique
3 to 4% relative accuracy | Numerous low and high spatial resolution instruments
Numerous EOS instruments |
| Water sites with in-situ mmts. | 5% absolute uncertainty
 | MOBY |
| Snow/ice sites | 5% relative accuracy | MODIS to AVHRR |
| Deep convective clouds | 1 to 3% relative accuracy | NOAA-16, -17 to MODIS Terra. Aqua; GEO to LEO comparisons |
| Moon | 5 to 10% absolute unc. in lunar irradiance model
3 to 4% relative accuracy
0.1% stability | Accuracy limited by # of pixels integrated (i.e size of Moon)
MODIS Terra to SeaWiFS (vis/nir)
SeaWiFS long term dataset |
| Rayleigh scatter method | 2 to 3.5 % relative accuracy in blue bands |  |
| Stable deserts | 2.5 to 5% relative accuracy
1 to 2% stability | MODIS to ATSR-2 & AVHRR |

## Current and future intercomparison developments

- The WMO’s Global Space-based Intercalibration System (GSICS) was formed in November 2005 to achieve operational intercalibration of satellite instruments within the space-based component of WMO’s World Weather Watch/Global Observing System (WWW/GOS).
- Members include CMA, CNES, EUMETSAT, JMA, KMA, NOAA, WMO, and NASA
- The Calibration Support Segment (CSS) of GSICS plans the following activities:
- Highly accurate, SI-standards traceable tests on satellite instruments and their on-board calibrations references using special satellite and ground-based instruments
- Collecting high quality in-situ data from earth-based reference sites
- Independent montoring of extra-terrestrial calibration sources (e.g. Sun, Moon, and stars) for on-orbit monitoring of instrument calibration
- Simulations of satellite radiances, computed from NWP analyses of atmospheric conditions, compared with satellite instrument observations.
- GSICS work to date has concentrated on the comparison of thermal infrared data from on-orbit operational and research satellite instruments (i.e. LEO and GEO)
- AIRS and IASI have been designated as the benchmark instruments for these comparisons
- Future work will include comparisons of satellite instruments working in the vis/nir/ir/swir and protocols/reviews of pre-launch instrument calibration/characterization.
- CLARREO should be “one stop shopping” for this (and other) groups?

## Summary

- With respect to pre-launch calibration, the same measurement methodologies and calibration/characterization instrumentation used over the past 30 years have largely been recycled.
- This has contributed to a “stagnation” of remote sensing radiometric accuracies in the vis/nir/swir at the 4 to 5% absolute radiance levels.
- We have been slow to develop (and hopefully adopt) new technology to decrease these accuracies to levels required by climate science.
- With respect to on-orbit calibration systems, system level pre-launch testing must be required to validate on-orbit radiometric models.
- On-orbit calibration systems must be tested as if they are instruments in themselves.
- To realize a 10x improvement in remote sensing accuracy/precision, a commitment for the incorporation of new measurement methodologies and technologies in instrument calibration and characterization both pre-launch and on-orebit must be made.
- The uncertainties of these new approaches must be completely validated  and cost and schedule impacts to programs accurately assessed.

## 3. Example MODIS Reflectance Spectral Sensitivities to filter position: Simulations with MODTRANSteve Platnick

- 3. Example MODIS Reflectance Spectral Sensitivities to filter position: Simulations with MODTRANSteve Platnick

- Sensitivity to filter position only (no change in spectral shape)
- Does not include out-of-band response
- Does not include other effects (spectral crosstalk, scattered light, etc.)
- Assume a perfect transfer calibrator/calibration

## MODTRAN Spectrum

*CLARREO GSFC: Butler, Platnick, Xiong*

## MODTRAN Spectrum

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## MODTRAN Spectrum

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## MODTRAN Spectrum

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## Sensitivity Results

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## Sensitivity Results

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*

## Sensitivity Results

*1 May 2008*

*CLARREO GSFC: Butler, Platnick, Xiong*