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
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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
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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
Wavelength (nm)
2500
4
6
8
Wavelength (um)
10
12
• 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
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Are Current Requirements Met?
Climate Variable
Temperature: Tropospheric
Stratospheric
Water Vapor
Ozone:
Total Column
Stratospheric
Tropospheric
Aerosols
Carbon Dioxide
Clouds
Surface: Snow/Sea Ice
Ocean Color
Vegetation
Sea Surface Temp
Spectral Range
Accuracy
Per Decade Stability
Microwave/IR
0.5 K
0.04 K
UV/VIS
2% (abs) 1% (rel)
3%
3%
0.2%
0.6%
0.1%
VIS
3%
1.5%
IR
3%
1%
VIS/NIR
IR
2-5%
1K
0.5-2%
0.2 K
VIS
VIS
VIS
IR
12%
5%
1%
0.1 K
10%
1%
0.8%
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.
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NISTIR 7047
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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
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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
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Effect of Libration on Lunar Irradiance
Relative Irradiance
1.015
1.01
1.005
1
0.995
0.99
0.985
0
1000
2000
3000
4000
5000
Day
6000
( 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
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Atmospheric Transmittance
(terrestrial lunar
irradiance
measurements)
Atmospheric
Transmittance
0 km
2 km
4 km
33 km
1
Transmittance
0.8
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
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Deducing Atmospheric
Transmittance
– VIS/NIR/SWIR
Atmospheric Constituent
Transmittance
Aerosol
Rayleigh
O3
H2O
O2
CO2
CH4
Transmittance
1
0.8
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
• Relatively spectrally smooth and flat at
long wavelengths
• Can deduce net loss via Langley method
• Some “loss” is scattered light
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Molecular absorbers H2O, CO2, O2 & CH4
• Easy to distinguish spectrally
• Atmospheric models (e.g. MODTRAN)
predict band structure given air mass
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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
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LUSI System Design
Telescopes
Lunar
Spectrometers
Wide FOV
Tracker
Lunar Integrating
Sphere
LED
Source
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Stellar
Spectrometers
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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
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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
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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
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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
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0.6% (k=1)
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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)
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Just the Beginning…..
The Earth from 33 km (120,000 feet)
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