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

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

2. 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 Outline

3. 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 What is Needed to Improve the Moon as an SI traceable reference?

4. 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 Some Typical Bands

5. Are Current Requirements Met? 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

6. 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 Going Beyond ROLO — Goals of LUSI

7. 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 Example of Lunar Libration for One Lunation

8. Effect of Libration on Lunar Irradiance • With a few months of measurements in under 3 yrs the majority of the libration space can be covered

9. Atmospheric Transmittance(terrestrial lunar irradiance measurements) • 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

10. Deducing Atmospheric Transmittance– VIS/NIR/SWIR Molecular absorbers H2O, CO2, O2 & CH4 • Easy to distinguish spectrally • Atmospheric models (e.g. MODTRAN) predict band structure given air mass Alt. 4 km 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

11. 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 East South West Atmospheric Variability and Homogeneity

12. LUSI System Design Telescopes Lunar Spectrometers Wide FOV Tracker Stellar Spectrometers LED Source Lunar Integrating Sphere

13. 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 LUSI System Design—VIS/NIR/SWIR

14. 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 LUSI System Design—VIS/NIR/SWIR

15. 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 Calibration and Characterization Protocol

16. 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) Uncertainty Goals for Lunar Measurements

17. Comparison of EOS Instruments Lunar Calibration vs. ROLO model This represents the best current practices available in remote sensing.

18. 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) Summary

19. Just the Beginning….. The Earth from 33 km (120,000 feet)