In December 2014 experts from 14 different agencies and departments attended the joint GSICS – CEOS/IVOS Lunar Calibration Workshop meeting organised by EUMETSAT in collaboration with USGS, CNES and NASA. Altogether, this represents potentially more than 25 instruments capable of observing the Moon. The main objectives of the workshop were i) to work across agencies with the GSICS Implementation of the ROLO model (GIRO) - a common and validated implementation of the USGS lunar radiometric reference, ii) to share knowledge and expertise on lunar calibration and iii) to generate for the first time a reference dataset that could be used for validation and comparisons. This lunar calibration community endorsed the GIRO to be the established publicly available reference for lunar calibration, directly traceable to the USGS ROLO model. However, further effort is required to reach inter-calibration between instruments, in particular for each instrument team to accurately estimate the over-sampling factor for their images of the Moon. A way to develop a cross-calibration algorithm and GSICS inter-calibration products is proposed. This includes key issues of fixing the GIRO calibration to an absolute scale, addressing spectral differences between instruments, and improving the existing calibration reference, which translates into future updates of the GIRO. The availability of extensive Moon observation datasets will help to further improve this reference and is expected to grow with the availability of additional lunar observations from past, current and future missions. All participants agreed on EUMETSAT pursuing its efforts in developing and maintaining the GIRO in collaboration with USGS to ensure traceability to the reference ROLO model.
The NASA VIIRS Ocean Science Team (VOST) has the task of evaluating Suomi NPP VIIRS ocean color data
for the continuity of the NASA ocean color climate data records. The generation of science quality ocean color
data products requires an instrument calibration that is stable over time. Since the VIIRS NIR Degradation
Anomaly directly impacts the bands used for atmospheric correction of the ocean color data (Bands M6 and
M7), the VOST has adapted the VIIRS on-orbit calibration approach to meet the ocean science requirements.
The solar diffuser calibration time series and the solar diffuser stability monitor time series have been used to
derive changes in the instrument response and diffuser reflectance over time for bands M1–M11. The lunar
calibration observations have been used, in cooperation with the USGS ROLO Program, to derive changes in
the instrument response over time for these same bands. In addition, the solar diffuser data have been used to
develop detector-dependent striping and mirror side-dependent banding corrections for the ocean color data. An
ocean surface reflectance model has been used to perform a preliminary vicarious calibration of the VIIRS ocean
color data products. These on-orbit calibration techniques have allowed the VOST to produce an optimum timedependent
radiometric calibration that is currently being used by the NASA Ocean PEATE for its VIIRS ocean
color data quality evaluations. This paper provides an assessment of the current VIIRS radiometric calibration
for the ocean color data products and discusses the path forward for improving the quality of the calibration.
The Lunar Calibration program at the U.S. Geological Survey (USGS) in Flagstaff, AZ, provides the radiometric
reference of the Moon as a source for calibration at reflected-solar wavelengths. To develop this capability, thousands
of multispectral images of the Moon were acquired by the Robotic Lunar Observatory (ROLO) telescope
imaging systems. During normal ROLO operations, 10 to 12 different stars were observed up to 15 times each
night, primarily to derive atmospheric transmittance corrections for the Moon observations. But additionally,
the ROLO telescope sensors are calibrated to the star Vega through a process of reduction of stellar images to
absolute irradiances. A study of the ROLO stellar imaging characteristics for this purpose has led to development
of an analytic model for the signal contained in the extended point spread function of the image data. This
model is then applied as part of the standard data reduction procedures to generate corrections for individual star
images. The resulting absolute stellar irradiance measurements allow development of a calibration history for
the entire ROLO dataset, and by extension for the lunar models that constitute the lunar radiometric reference.
This paper will discuss the image reduction techniques developed for calibration of the ROLO focal plane array
sensors, and the implications of this development on the use of the Moon as a calibration reference source.
The Moon plays an important role in the radiometric stability monitoring of the NASA Earth Observing System's (EOS)
remote sensors. The MODIS and SeaWIFS are two of the key instruments for NASA's EOS missions. The MODIS
Protoflight Model (PFM) on-board the Terra spacecraft and the MODIS Flight Model 1 (FM1) on-board the Aqua
spacecraft were launched on December 18, 1999 and May 4, 2002, respectively. They view the Moon through the
Space View (SV) port approximately once a month to monitor the long-term radiometric stability of their Reflective
Solar Bands (RSB). SeaWIFS was launched on-board the OrbView-2 spacecraft on August 1, 1997. The SeaWiFS
lunar calibrations are obtained once a month at a nominal phase angle of 7°. The lunar irradiance observed by these
instruments depends on the viewing geometry. The USGS photometric model of the Moon (the ROLO model) has been
developed to provide the geometric corrections for the lunar observations. For MODIS, the lunar view responses with
corrections for the viewing geometry are used to track the gain change for its reflective solar bands (RSB). They trend
the system response degradation at the Angle Of Incidence (AOI) of sensor's SV port. With both the lunar observation
and the on-board Solar Diffuser (SD) calibration, it is shown that the MODIS system response degradation is
wavelength, mirror side, and AOI dependent. Time-dependent Response Versus Scan angle (RVS) Look-Up Tables
(LUT) are applied in MODIS RSB calibration and lunar observations play a key role in RVS derivation. The
corrections provided by the RVS in the Terra and Aqua MODIS data from the 412 nm band are as large as 16% and
13%, respectively. For SeaWIFS lunar calibrations, the spacecraft is pitched across the Moon so that the instrument
views the Moon near nadir through the same optical path as it views the Earth. The SeaWiFS system gain changes for
its eight bands are calibrated using the geometrically-corrected lunar observations. The radiometric corrections to the
SeaWiFS data, after more than ten years on orbit, are 19% at 865 nm, 8% at 765 nm, and 1-3% in the other bands. In
this report, the lunar calibration algorithms are reviewed and the RSB gain changes observed by the lunar observations
are shown for all three sensors. The lunar observations for the three instruments are compared using the USGS
photometric model. The USGS lunar model facilitates the cross calibration of instruments with different spectra
bandpasses whose measurements of the Moon differ in time and observing geometry.
With the increased emphasis on monitoring the Earth's climate from space, more stringent calibration requirements
are being placed on the data products from remote sensing satellite instruments. Among these are stability
over decade-length time scales and consistency across sensors and platforms. For radiometer instruments in the
solar reflectance wavelength range (visible to shortwave infrared), maintaining calibration on orbit is difficult due
to the lack of absolute radiometric standards suitable for fight use. The Moon presents a luminous source that
can be viewed by all instruments in Earth orbit. Considered as a solar diffuser, the lunar surface is exceedingly
stable. The chief diffculty with using the Moon is the strong variations in the Moon's brightness with illumination
and viewing geometry. This mandates the use of a photometric model to compare lunar observations,
either over time by the same instrument or between instruments. The U.S. Geological Survey in Flagstaff, Arizona,
under NASA sponsorship, has developed a model for the lunar spectral irradiance that explicitly accounts
for the effects of phase, the lunar librations, and the lunar surface reflectance properties. The model predicts
variations in the Moon's brightness with precision ~1% over a continuous phase range from eclipse to the quarter
lunar phases. Given a time series of Moon observations taken by an instrument, the geometric prediction
capability of the lunar irradiance model enables sensor calibration stability with sub-percent per year precision.
Cross-calibration of instruments with similar passbands can be achieved with precision comparable to the model
precision. Although the Moon observations used for intercomparison can be widely separated in phase angle
and/or time, SeaWiFS and MODIS have acquired lunar views closely spaced in time. These data provide an
example to assess inter-calibration biases between these two instruments.
Production of reliable climate datasets from multiple observational measurements acquired by remote sensing
satellite systems available now and in the future places stringent requirements on the stability of sensors and
consistency among the instruments and platforms. Detecting trends in environmental parameters measured at
solar reflectance wavelengths (0.3 to 2.5 microns) requires on-orbit instrument stability at a level of 1% over
a decade. This benchmark can be attained using the Moon as a radiometric reference. The lunar calibration
program at the U.S. Geological Survey has an operational model to predict the lunar spectral irradiance with
precision ~1%, explicitly accounting for the effects of phase, lunar librations, and the lunar surface photometric
function. A system for utilization of the Moon by on-orbit instruments has been established. With multiple
lunar views taken by a spacecraft instrument, sensor response characterization with sub-percent precision over
several years has been achieved. Meteorological satellites in geostationary orbit (GEO) capture the Moon in
operational images; applying lunar calibration to GEO visible-channel image archives has the potential to develop
a climate record extending decades into the past. The USGS model and system can provide reliable transfer of
calibration among instruments that have viewed the Moon as a common source. This capability will be enhanced
with improvements to the USGS model absolute scale. Lunar calibration may prove essential to the critical
calibration needs to cover a potential gap in observational capabilities prior to deployment of NPP/NPOESS. A
key requirement is that current and future instruments observe the Moon.
In this paper, we study the feasibility of a method for vicarious calibration of the GOES Imager visible channel using the Moon. The measured Moon irradiance from 26 unclipped moon imagers exhausted all the potential Moon appearances between July 1998 and December 2005, together with the seven scheduled Moon observation data obtained after November 2005, were compared with the USGS lunar model results to estimate the degradation rate of the GOES-10 Imager visible channel. A total of nine methods of determining the space count and identifying lunar pixels were employed in this study to measure the Moon irradiance. Our results show that the selected mean and the masking Moon appears the best method. Eight of the nine resulting degradation rates range from 4.5%/year to 5.0%/year during the nearly nine years of data, which are consistent with most other degradation rates obtained for GOES-10 based on different references. In particular, the degradation rate from the Moon-based calibration (4.5%/year) agrees very well with the MODIS-based calibration (4.4%/year) over the same period, confirming the capability of relative and absolute calibration based on the Moon. Finally, our estimate of lunar calibration precision as applied to GOES-10 is 3.5%.
Solar-band imagery from geostationary meteorological satellites has been utilized in a number of important applications in Earth Science that require radiometric calibration. Because these satellite systems typically lack on-board calibrators, various techniques have been employed to establish "ground truth", including observations of stable ground sites and oceans, and cross-calibrating with coincident observations made by instruments with on-board calibration systems. The Moon appears regularly in the margins and corners of full-disk operational images of the Earth acquired by meteorological instruments with a rectangular field of regard, typically several times each month, which provides an excellent opportunity for radiometric calibration. The USGS RObotic Lunar Observatory (ROLO) project has developed the capability for on-orbit calibration using the Moon via a model for lunar spectral irradiance that accommodates the geometries of illumination and viewing by a spacecraft. The ROLO model has been used to determine on-orbit response characteristics for several NASA EOS instruments in low Earth orbit. Relative response trending with precision approaching 0.1% per year has been achieved for SeaWiFS as a result of the long time-series of lunar observations collected by that instrument. The method has a demonstrated capability for cross-calibration of different instruments that have viewed the Moon. The Moon appears skewed in high-resolution meteorological images, primarily due to satellite orbital motion during acquisition; however, the geometric correction for this is straightforward. By integrating the lunar disk image to an equivalent irradiance, and using knowledge of the sensor's spectral response, a calibration can be developed through comparison against the ROLO lunar model. The inherent stability of the lunar surface means that lunar calibration can be applied to observations made at any time, including retroactively. Archived geostationary imager data that contains the Moon can be used to develop response histories for these instruments, regardless of their current operational status.
A system to provide radiometric calibration of remote sensing imaging
instruments on-orbit using the Moon has been developed by the US Geological Survey RObotic Lunar Observatory (ROLO) project. ROLO has developed a model for lunar irradiance which treats the primary geometric variables of phase and libration explicitly. The model fits hundreds of data points in each of 23 VNIR and 9 SWIR bands; input data are derived from lunar radiance images acquired by the project's on-site telescopes, calibrated to exoatmospheric radiance and converted to disk-equivalent reflectance. Experimental uncertainties are tracked through all stages of the data processing and modeling. Model fit residuals are ~1% in each band over the full range of observed phase and libration angles. Application of ROLO lunar calibration to SeaWiFS has demonstrated the capability for long-term instrument response trending with precision approaching 0.1% per year. Current work involves assessing the error in absolute responsivity and relative spectral response of the ROLO imaging systems, and propagation of error through the data reduction and modeling software systems with the goal of reducing the uncertainty in the absolute scale, now estimated at 5-10%. This level is similar to the scatter seen in ROLO lunar irradiance comparisons of multiple spacecraft instruments that have viewed the Moon. A field calibration campaign involving NASA and NIST has been initiated that ties the ROLO lunar measurements to the NIST (SI) radiometric scale.
The RObotic Lunar Observatory (ROLO) project has developed radiometric models of the Moon for disk-integrated irradiance and spatially resolved radiance. Although the brightness of the Moon varies spatially and with complex dependencies upon illumination and viewing geometry, the surface photometric properties are extremely stable, and therefore potentially knowable to high accuracy. The ROLO project has acquired 5+ years of spatially resolved lunar
images in 23 VNIR and 9 SWIR filter bands at phase angles up to 90°. These images are calibrated to exoatmospheric radiance using nightly stellar observations in a band-coupled extinction algorithm and a radiometric scale based upon observations of the star Vega. An effort is currently underway to establish an absolute scale with direct traceability to NIST radiometric standards. The ROLO radiance model performs linear fitting of the spatially resolved lunar image data on an individual pixel basis. The results
are radiance images directly comparable to spacecraft observations of the Moon. Model-generated radiance images have been produced for the ASTER lunar view conducted on 14 April 2003. The radiance model is still experimental -- simplified photometric functions have been used, and initial results show evidence of computational instabilities, particularly at the lunar poles. The ROLO lunar image dataset is unique and extensive and presents opportunities
for development of novel approaches to lunar photometric modeling.
The Robotic Lunar Observatory (ROLO) project has developed a spectral irradiance model of the Moon that accounts for variations with lunar phase through the bright half of a month, lunar librations, and the location of an Earth-orbiting spacecraft. The methodology of comparing spacecraft observations of the Moon with this model has
been developed to a set of standardized procedures so that comparisons can be readily made. In the cases where observations extend over several years (e.g., SeaWiFS), instrument response degradation has been determined with precision of about 0.1% per
year. Because of the strong dependence of lunar irradiance on geometric angles, observations by two spacecraft cannot be directly compared unless acquired at the same time and location. Rather, the lunar irradiance based on each spacecraft instrument calibration can be compared with the lunar irradiance model. Even single observations by an instrument allow inter-comparison of its radiometric scale with
other instruments participating in the lunar calibration program. Observations by SeaWiFS, ALI, Hyperion and MTI are compared here.
The recognized need for on-orbit calibration of remote sensing imaging instruments drives the ROLO project effort to characterize the Moon for use as an absolute radiance source. For over 5 years the ground-based ROLO telescopes have acquired spatially-resolved lunar images in 23 VNIR (Moon diameter ≈500 pixels) and 9 SWIR (≈250 pixels) passbands at phase angles within ±90 degrees. A numerical model for lunar irradiance has been developed which fits hundreds of ROLO images in each band, corrected for atmospheric extinction and calibrated to absolute radiance, then integrated to irradiance. The band-coupled extinction algorithm uses absorption spectra of several gases and aerosols derived from MODTRAN to fit time-dependent component abundances to nightly observations of standard stars. The absolute radiance scale is based upon independent telescopic measurements of the star Vega. The fitting process yields uncertainties in lunar relative irradiance over small ranges of phase angle and the full range of lunar libration well under 0.5%. A larger source of uncertainty enters in the absolute solar spectral irradiance, especially in the SWIR, where solar models disagree by up to 6%. Results of ROLO model direct comparisons to spacecraft observations demonstrate the ability of the technique to track sensor responsivity drifts to sub-percent precision. Intercomparisons among instruments provide key insights into both calibration issues and the absolute scale for lunar irradiance.
Routine observations of the Moon have been acquired by the Robotic Lunar Observatory (ROLO) for over four years. The ROLO instruments measure lunar radiance in 23 VNIR (Moon diameter approximately 500 pixels) and 9 SWIR (approximately 250 pixels) passbands every month when the Moon is at phase angle less than 90 degrees. These are converted to exoatmospheric values at standard distances using an atmospheric extinction model based on observations of standard stars and a NIST-traceable absolute calibration source. Reduction of the stellar images also provides an independent pathway for absolute calibration. Comparison of stellar-based and lamp-based absolute calibrations of the lunar images currently shows unacceptably large differences. An analytic model of lunar irradiance as a function of phase angle and viewing geometry is derived from the calibrated lunar images. Residuals from models which fit hundreds of observations at each wavelength average less than 2%. Comparison with SeaWiFS observations over three years reveals a small quasi-periodic change in SeaWiFS responsivity that correlates with distance from the Sun for the first two years, then departs from this correlation.