Calibration and Status of MOBY

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Calibration and Status of MOBY

• Dennis Clark, NOAA/NESDIS
• Carol Johnson, NIST
• Steve Brown, NIST
• Mark Yarbrough, MLML
• Stephanie Flora, MLML
• Mike Feinholz, MLML
• Mike Ondrusek, NOAA/NESDIS
• Ken Voss, UM

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Calibration Requirements for GOES-R HES
CWI CHARACTERIZATION It will be critical for
the GOES-R HES CWI to be characterized as much
as possible before launch so that post-launch
calibration adjustments can be correlated to
physical instrument characteristics. VICARIOUS
CALIBRATION Isolates the effects of satellite
sensors systematic gain offset on the difference
in a match pair of satellite and in situ Lwn
measurements. PRODUCT VALIDATION Assure
products are CDR-quality REPROCESSING to
reflect improved knowledge of the instruments
performance changes -Ocean Color Review Panel
Report
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Vendor Calibration Task for CWI -The reflective
solar of the OO or CW tasks shall be calibrated
prior to launch to provide albedo to an accuracy
of 3 (TBR) at maximum scene radiance -This
reflective solar calibration performed pre-launch
should be stable and compared to a NIST
reference. -These reflective solar bands shall
be quantized in such a way that the signal will
not saturate (high counts or low counts) over the
life of the instrument and under worst-case
conditions. -An onboard full optical train,
full aperture calibrator shall be used to perform
calibrations during the life of the instrument
performing these tasks (cannot risk HES). -The
on board required calibration must provide
absolute accuracy of 3 or less at 100 albedo,
RMS repeatability of the band difference of
/-0.2 or less, and drift in absolute calibrated
radiances of 0.5 over the instrument lifetime.
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Prelaunch Characterization Full system and full
aperature spectral response characterization Every
component in the optical train is subject to
calibration and characterization. These include
but are not limited to Characterization of
solar diffuser BRDF and Earth shine Thermal
sensitivity At least 3 temperature
characterizations Out of Band responses, use
SIRCUS Characterization of polarization
sensitivity Stray light characterization
proximity analysis to bright targets Response
versus Scan (RVS) for each detector and each
band
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The need for a Vicarious Calibration -A
longstanding goal of ocean color satellite
Science Teams is to determine satellite-derived
LWN(l) with a combined standard uncertainty of
5. -Because water-leaving radiance contributes
only 10 (at most) of the radiance measured by a
satellite sensor above the atmosphere a 5
uncertainty in LWN(l) implies a 0.5 uncertainty
in the above-atmosphere radiance measurements.
-This level of uncertainty can only be
approached using vicarious-calibration
approaches. -In practice, this means that the
satellite radiance responsivity is adjusted to
minimize the squared differences between an
ensemble of satellite and in situ LWN(l) data
pairs. -The end result of this approach is to
implicitly absorb unquantified, but systematic,
errors in the atmospheric correction, incident
solar flux, and satellite sensor calibration into
a single correction factor to produce consistency
with the in situ data (Gordon 1981, 1987, 1988).

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MOBY - Primary Reference Standard for Climate
Quality Ocean Color Time-Series

• Seven Year Time-Series 7/20/97 to Present
• NIST Radiometric Scale Collaboration
• Verification of System Performance
• Stray Light and Thermal Characterizations
• Sensor Spectral Band Matching
• Ocean Color Sensors Supported with MOBY Scale
• Japan - OCTS
• French - POLDER
• US - SeaWiFS
• US - MODIS (Terra and Aqua)
• US - MISR (Terra)
• Europe - MERIS
• Japan- GLI

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MOBY Mooring Site
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MOBY Instrumentand spectralTime Series of MODIS
ocean color bandsAccuracy 4 -5
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Spectral Band Pass Matching
High Resolution Spectra Convolved to Sensors
Spectral Band Pass
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MOBY Calibration Process
NIST Collaborations Training NIST
Primary Lamp Standards Annual On Site
Calibration Systems Check Pre/Post Cal. System
monitoring with NIST Cal. Radiometers SIRCUS -
Stray Light Characterizations on MOBY and
Shipboard Spectrometers MOCE Calibration Systems
(OL420 OL425) now Calibrated at NIST Initiating
the development of new Radiometric Calibration
Sources for Oceans
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Internal Reference Lamps - Stability QC
Both - 0.5
Blue lt 0.5
Red lt 0.5
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Diver Reference Lamp Cals.
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Currently have Research to Operations Funding
(R2O) for MOBY, Goals are

• Transition MOBY vicarious calibration
  capabilities for NPP/NPOESS (e.g., VIIRS)
• Adapt MOBY technology for complex coastal
  validation activities for GOES-R (e.g. HES)

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Currently Concentrating on 2 issues

• Relocate power generation to mooring buoy
• raised a transfer of power issue but enables a
  reduction in size of optical buoy associated
  deployment/servicing costs
• Simultaneous measurements to reduce environmental
  sources of measurement uncertainty, system
  complexity
• required a new optical system design

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Tether Redesign and Prototype

• Electromechanical swivel
• design compete, connectors specified, swivel
  order in progress
• Tether flounder plate
• preliminary mechanical design complete, strain
  relief design in progress
• Electromechanical tether
• final design complete, majority of components
  ordered
• Guard Buoy service loop
• design in progress

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Tether Testing
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Optical System Goals

• In MOBY, the scans are discrete and sequential
  in the new system, they are simultaneous
• MOBY requires 20 min for upwelling radiance
  measurements (multiple Es and Lu scans). This is
  a undesirable sampling feature. There can be
  variability due to changing solar zenith angle
  and atmospheric conditions, requiring
  normalization procedures that introduce
  measurement uncertainty.
• The new system eliminates this problem by
  simultaneous observations with multiple inputs.
  In addition, a comparable sequence as for MOBY
  takes about 20 sec, not 20 min.

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Instrument Layout, at Sea Testing
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Optical System Breadboard

• ISA f/2 spectrograph
• Andor 1024x256 cooled CCD array
• Four separate optical fiber inputs along entrance
  slit

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Breadboard System Performed Well
Spectral stray light from optical system is
better than MOBY
Stability, system response, and signal to noise
ratio adequate for ocean color measurements.
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Successful Correction for Spectral and Spatial
Stray Light
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At Sea Tests
The breadboard system was implemented with four
inputs and tested in Case 1 waters off Oahu in
August 2005. The inputs were Es, Eu, Lu (0.75m)
and Lu (3.25 m).
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Preliminary Conclusions

• Breadboard System
• Superior stray light (compared to MOBY)
• A simple 2D stray light model was implemented
• Satisfactory dynamic range and sensitivity
• Successfully balanced individual throughputs
  resulting in the same integration time,
  independent of Es or Lu
• All fiber optical input simplifies optical design
• Outstanding issues
• Desirable to have six or eight fiber inputs
• Increased spectral resolution
• Prototype procurement

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