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The ROLO Lunar Calibration System
Description and Current Status
Thomas C. Stone
U.S. Geological Survey, Flagstaff, AZ USA
GSICS Lunar Calibration Workshop
EUMETSAT
01 December 2014
Introduction and Background
Timeline of ROLO:
•
Studies at the USGS Astrogeology Center, supporting the NASA
Moon landing program in the 1960s
― lunar albedo mapping, for landing site selection
― lunar cratering rates, which showed the stability of the lunar surface
― feasibility to use the Moon as a calibrated light source
•
Funding from NASA to develop the lunar calibration method
•
Studies of lunar reflectance modeling, for developing the calibration
reference
― for EOS instruments: SeaWiFS, MODIS, ASTER, MISR, Hyperion…
― established the ROLO facility: telescopes and data collection system
― showed that the most useful quantity is spatially-integrated irradiance
― showed that an analytic model is needed, to accommodate the various
geometries of instruments’ Moon observations
ROLO Telescope Facility
Acquired an extensive set of lunar and stellar observations, used to
characterize the brightness behavior of the Moon and provide the
basis dataset for the lunar model.
•
•
•
•
Located at USGS in Flagstaff Arizona, 2143m altitude
Twin telescopes, 20cm dia.
― 23 VNIR bands, 350−950 nm
― 9 SWIR bands, 950−2450 nm
Imaging cameras — radiance
Operated more than 8 years
―
―
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>110 000 Moon images
phases from eclipse to 90°
>900 000 star images
ROLO telescopes zenith-pointed at dusk
ROLO Data Processing for Modeling
The basis data for the lunar model are ROLO lunar irradiance
measurements, processed similarly to Moon images from spacecraft
instruments:
•
Radiance calibration developed from ROLO measurements of the
star Vega, tied to published absolute stellar energy distribution data
― absolute uncertainties for Vega:
1.0–1.5% VNIR, 3–4% SWIR
•
Corrections for atmospheric transmission developed for each
observing night from many star measurements
•
Empirical correction for atmospheric scattering around the Moon disk
ROLO Lunar Model
The lunar model kernel describes the disk-equivalent reflectance (A).
•
Reflectance was chosen to take advantage of the smooth lunar
spectrum, and eliminate the effects of the solar fine structure.
This introduces a dependence on the solar spectrum used!
•
Empirical formulation, a function of the geometric variables of phase
angle (g) and librations (Φ, φ, θ)
― equation designed to minimize residuals from fitting the ROLO dataset
•
•
Coefficients derived by fitting ~1200 observations in each band (k)
mean fit residual ≈0.0096 → a measure of the relative precision
Example Computation of ROLO Model
Lunar disk-equivalent reflectance at 865 nm
Spread shows the
effect of libration,
~5%
Lambert sphere
Lunar Model Operation — Inputs Processing
User inputs:
•
•
Double-Precision
Ephemeris DE421
Observation time
Spacecraft position (X,Y, Z)
SPICE Toolkit
Moon position
Phase
angle (g)
Librations (Φ, φ, θ)
Sun position
Moon orientation
Lunar Model Operation — Output Processing
Computing the model equation gives the lunar disk reflectance (Ak) at
the 32 ROLO wavelengths. A representative lunar reflectance
spectrum is then fitted to these Ak values:
Symbols □ are Ak
from the lunar
model computation
Solid line is the
reference lunar
reflectance
spectrum, fitted to
the Ak values.
Lunar Model Operation — Post-Processing
The fitted lunar reflectance spectrum is convolved with the instrument
band spectral response functions and the solar spectrum to give the
lunar irradiance (EM) at the band wavelengths:
• Important:
this step cancels the dependence on the solar spectrum.
The only valid output of the ROLO model is the lunar irradiance.
It is an error to use directly the lunar reflectance computed by ROLO!
Lunar Model Operation — Post-Processing
The model computations (Afit) and ΩM are for standard Sun–Moon and
Moon–Observer distances, i.e. the mean orbital radii of the Earth and
the Moon, respectively.
• Apply distance corrections:
The final output E′M is the lunar irradiance present at the instrument
location at the time of the observation, in each sensor spectral band.
• For comparisons to observations made by instruments, corrections
for oversampling are applied to the irradiance measurement data.
• For typical lunar calibration interactions, ROLO provides the
computed geometry parameters: phase and libration angles,
distance correction factors, Moon disk apparent size and orientation.
Current Status
• Current model version (311g) is used in many different applications:
― sensor response trending/on-orbit calibration stability
― nighttime aerosol optical depth measurements with lunar photometers
― photometric corrections for imaging instruments in orbit around the Moon
• Uncertainty of the relative irradiance, i.e. changes in lunar brightness
with phase angle and librations:
― initially specified by residuals from fitting ROLO dataset: ~1%
― comparisons made by new instruments (e.g. VIIRS, PLEIADES) show
geometry dependencies up to several percent; analyses ongoing
― the ROLO model is being revised with constraints developed from these
comparisons, particularly PLEIADES (more about this on Thursday)
• Uncertainty of the absolute irradiance: 5–10%
― demonstrated by comparisons of multiple instruments
― can be constrained by new, dedicated absolute lunar irradiance
measurements
Thank You!