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					                       LUSI (LUnar Spectral Irradiance)
                          A New Program to Reduce the Uncertainty in
                             the Absolute Lunar Spectral Irradiance

                 Steven Lorentz1, Allan Smith2, Howard Yoon3 and Raju Datla3

                                            in collaboration with

                                 Bob Barnes4, Hugh Kieffer5, Dave Pollock6,
                                 Ray Russell7, Tom Stone8 and Joe Tansock9.




      1. NIST Contractor (L-1)                     4. GSFC Contractor (SAIC)         7. Aerospace Corp.
      2. NIST Contractor (Jung R&D)                5. Celestial Reasonings           8. USGS
      3. NIST                                      6. UAH                            9. SDL



National Institute of Standards and Technology                                 Optical Technology Division
     Outline

     • What is needed to improve the lunar irradiance scale?
     • Goals of LUSI
     • Atmospheric Transmittance
        – The balloon solution
     • LUSI System Design
     • Calibration and Characterization Protocol
     • Summary




National Institute of Standards and Technology   Optical Technology Division
                     What is Needed to Improve the Moon
                        as an SI traceable reference?
  • The ability to more accurately predict the lunar irradiance in the future.

  • Lower uncertainties in the absolute scale of the lunar irradiance
     – Advantages are lower uncertainties for cross-calibration and
       filling possible gaps between satellite missions
     – If a low absolute uncertainty is achieved then a low relative
       uncertainty is assured.

  • Higher spectral resolution from 320 nm to 2500 nm
      – Reduces uncertainties in the application of the model to filter
        bands instruments—reduces interpolation
      – Aids in the atmospheric correction of lunar irradiance as
        measured from the Earth

  • For decadal scale climate data products a reference that is stable over
    that time scale is required—the Moon is that reference

National Institute of Standards and Technology         Optical Technology Division
                                    Some Typical Bands
                   VIIRS Sensor Bands M1-M11                    VIIRS Sensor Bands M12-M16

                                               M1                         M12
                                               M2                         M13
                                               M3                         M14
                                               M4
                                                                          M15
                                               M5
                                                                          M16
                                               M6
                                               M7
                                               M8
                                               M9
                                               M10
                                               M11




            500      1000          1500       2000   2500   4         6           8          10   12
                                                                           Wavelength (um)
                            Wavelength (nm)

    • Band-to-band comparisons between different instruments can be
      very challenging at the few percent level
    • Higher Spectral resolution irradiances are needed for low
      uncertainties
National Institute of Standards and Technology                            Optical Technology Division
                           Are Current Requirements Met?
          Climate Variable             Spectral Range            Accuracy          Per Decade Stability
    Temperature: Tropospheric           Microwave/IR               0.5 K                   0.04 K
                  Stratospheric
                  Water Vapor
    Ozone:         Total Column             UV/VIS          2% (abs) 1% (rel)               0.2%
                   Stratospheric                                  3%                        0.6%
                   Tropospheric                                   3%                        0.1%
    Aerosols                                  VIS                   3%                      1.5%
    Carbon Dioxide                             IR                   3%                       1%
    Clouds                                 VIS/NIR                 2-5%                    0.5-2%
                                              IR                    1K                      0.2 K
    Surface: Snow/Sea Ice                     VIS                  12%                      10%
             Ocean Color                      VIS                   5%                       1%
             Vegetation                       VIS                   1%                      0.8%
             Sea Surface Temp                 IR                   0.1 K                   0.01 K
      Source: Satellite Instrument Calibration for Measuring Global Climate, Report of a Workshop at the
      University of Maryland Inn and Conference Center, College Park, MD, November 12-14, 2002,
      Edited by George Ohring, Bruce Wielicki, Roy Spencer, Bill Emery and Raju Datla, March 2004.
                                                                                      NISTIR 7047
National Institute of Standards and Technology                                  Optical Technology Division
                Going Beyond ROLO — Goals of LUSI
 • A higher spectral resolution model of the lunar spectral irradiance (and reflectance)
     – Wavelength range: 320 nm to 2500 nm
     – Spectral resolution 1 nm to 4 nm
     – Uncertainty (k=1) GOAL <1 %
          Should be achievable using NIST SIRCUS facility for end to end calibration.

     – Instrument design and stability are key to achieving this uncertainty goal.
     – Retrievable instruments for both balloon and mountain top are critical!

 • A minimum data set covering multiple lunations to collect a range of phase and
   libration angles.
      – Ideally, most observations from Mauna Kea—altitude 4 km, stable air, low
          aerosols
      – Focus on atmospheric ―window‖ bands
      – Correct for residual attenuation—Spectral instrument will help
 • Use balloon data to validate atmospheric corrections and spectrally extend the
   model through absorption bands
      – Plan 2 flights per year minimum

 • Additional opportunities exist for measurements of the lunar thermal-IR spectral
   irradiance—The technology is available and the balloon flights afford the opportunity

National Institute of Standards and Technology                 Optical Technology Division
          Example of Lunar Libration for One Lunation




    • Based on one lunation between the two New Moons of Oct-Nov 2004, each
      frame is the phase at 00:00:00 UTC. The red dot indicates libration.
    •   Libration Animation Created By and Courtesy of Don Carona, Texas A&M Astronomical Observatory

National Institute of Standards and Technology                                          Optical Technology Division
                                 Effect of Libration on Lunar Irradiance
                        1.015

                         1.01
  Relative Irradiance




                        1.005

                           1

                        0.995

                         0.99

                        0.985
                                 0       1000     2000     3000      4000      5000        6000
                                                          Day                            ( 0= 9/1/2007)
                                • With a few months of measurements in under 3 yrs the
                                  majority of the libration space can be covered

National Institute of Standards and Technology                              Optical Technology Division
                                           Atmospheric Transmittance
                                   (terrestrial lunar irradiance measurements)
                                                 Atmospheric Transmittance
                                                     0 km              4 km
                                                     2 km              33 km
                              1



                             0.8
             Transmittance




                             0.6



                             0.4



                             0.2



                              0
                                     0.5         1           1.5               2         2.5

                                                     Wavelength (m)




    • Aerosol (i.e. haze) scattering and variability are significantly reduced
      above the boundary layer (approx 2 km above the ground)

    • Balloon measurements are ideal—but a finite resource, producing only
      6-8 hours of data over a very limited phase range per flight
National Institute of Standards and Technology                                     Optical Technology Division
                          Atmospheric Transmittance – VIS/NIR/SWIR
              Deducing Atmospheric Constituent Transmittance
                                                  Aerosol        O3            O2        CH4
                                                  Rayleigh       H2O           CO2
                               1

                              0.8
              Transmittance

                              0.6

                              0.4

                              0.2
                                          Alt. 4 km
                               0
                                    0.5                1               1.5           2            2.5
                                                             Wavelength (m)


          Observe spectral irradiance of stars through different air masses to deduce
            atmospheric transmittance (lunar observations may help too!)


    Aerosols, Rayleigh scattering and Ozone                         Molecular absorbers H2O, CO2, O2 & CH4
    • Relatively spectrally smooth and flat at                      • Easy to distinguish spectrally
      long wavelengths
    • Can deduce net loss via Langley method                        • Atmospheric models (e.g. MODTRAN)
    • Some ―loss‖ is scattered light                                  predict band structure given air mass



National Institute of Standards and Technology                                            Optical Technology Division
                Atmospheric Variability and Homogeneity




                    East                         South               West

                           Observe many stars at different sky locations

                • Rising and Setting stars
                    – Yield star TOA irradiance
                    – Atmospheric transmittance near horizon

                • Overhead stars
                   – Yield atmospheric transmittance usually nearer to
                      Moon (given previously deduced TOA irradiance)
                   – Measures short term temporal fluctuations


National Institute of Standards and Technology                    Optical Technology Division
                                 LUSI System Design
                                                  Telescopes




        Lunar
     Spectrometers




                                                               Wide FOV
                                                                Tracker
                                                    Stellar
                                       LED       Spectrometers
    Lunar Integrating
        Sphere                        Source

National Institute of Standards and Technology                   Optical Technology Division
                     LUSI System Design—VIS/NIR/SWIR
 Non-Imaging System

 • 10‖ F/4 telescope, a lens focuses a 3 mm diameter, F/1.5, lunar image into an
   integrating sphere that is fiber-coupled to monolithic spectrometer modules
      – Three Lunar spectrometers: F/3 concave flat-field gratings
           300 nm – 900 nm, 1024 Si photodiode array, 1 nm band pass

           850 nm – 1700 nm, 1024 InGaAs photodiode array, 2 nm band pass

           1500 nm – 2400 nm, 1024 InGaAs photodiode array, 4 nm band pass



 • Second 10‖ telescope feeds two separate fiber coupled stellar spectrometers
     – Similar to VIS/NIR telescope except the final lens focuses the stellar image
       directly into a spectrometer fiber
          300 nm – 900 nm 1024 silicon photodiode array, 1 nm band

          850 nm – 2400 nm 1024 InGaAs photodiode array, 6 nm band



 • Solar reference measurements
     – Allows derivation of a secondary data product of lunar reflectance.
     – Simple collector/diffuser (no imaging optics) fiber-coupled to spectrometer
        integrating sphere
     – Has separate tracker


National Institute of Standards and Technology                Optical Technology Division
                     LUSI System Design—VIS/NIR/SWIR
 Non-Imaging System (continued)
 • Multi-wavelength LED Reference Source
     – Monitors calibration of spectrometers and integrating sphere
     – LEDs mounted to separate integrating sphere that is fiber-coupled to
        lunar integrating sphere
     – Stabilized thermally and with monitor detectors
 • Rotating selector wheel between telescope and focusing lens
     – Shutter and 4 to 6 filters used primarily for calibration
 • Pointing and tracking
     – Imaging telescopes for Lunar (wide FOV) and Stellar tracking
     – Digital tilt sensor, compass and GPS
 • Lunar and Solar imager to measure atmospheric scatter
     – 8 filter bands for wavelength dependence of scatter
 • Maximize Stability
     – Sealed optical system and minimum moving parts
     – Temperature Control Everything!
           Maintain all detection and calibration equipment at room
            temperature during both mountain top and balloon
            measurements
National Institute of Standards and Technology         Optical Technology Division
                     Calibration and Characterization Protocol
• The advantage of an Earth-based and balloon-borne instrument is ―retrieval‖.
  Therefore, calibrations would occur both before and after a deployment. At least two
  systems would be built—this allows simultaneous measurements between sites and
  mitigates risk.

• Subsystem testing using NIST Standards will assure performance and understanding
  at the system level.

• System level calibration and characterization will validate the uncertainty goals.

     – SIRCUS facility at NIST provides a tunable laser source from
       300 nm to 2500 nm:
          The laser feeds an integrating sphere source at the focus of a collimator that
           overfills the instrument entrance aperture and matches the angular
           divergence of the moon.
          The irradiance scale is transferred to the instrument using Silicon trap
           detectors (0.025%, k=1) and InGaAs detectors (0.05%, k=1) that measure
           the collimator output.

     – Other Important Characterizations
          Scatter within instrument—size-of-source effect
          Spectrometer out-of-band signal
          Linearity of response and Spatial uniformity

National Institute of Standards and Technology                  Optical Technology Division
                   Uncertainty Goals for Lunar Measurements



  •     Atmospheric transmittance (―window‖ bands)             0.5%

  •     Telescope throughput (e.g. window transmittance)       0.3%

  •     LED source for stability monitoring and correction     0.1%

  •     Absolute scale transfer                                0.1%

                         Combined Standard Uncertainty         0.6% (k=1)




National Institute of Standards and Technology               Optical Technology Division
             Comparison of EOS Instruments Lunar
                 Calibration vs. ROLO model




             This represents the best current practices available in remote sensing.

National Institute of Standards and Technology                   Optical Technology Division
     Summary

     • Need
        – Absolute scale of lunar irradiance

     • Solution
        – NIST capability
        – CALIBRATABLE, STABLE & RETRIEVABLE

     • Data Product (k=1) uncertainty goal of <1%
        – Absolute lunar spectral irradiance from 320 nm to
          2500 nm (reflectance will be a secondary
          product)


National Institute of Standards and Technology   Optical Technology Division
     Just the Beginning…..




                                          The Earth from 33 km (120,000 feet)
National Institute of Standards and Technology                 Optical Technology Division