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Lunar Calibration

By EROS CalVal Center of Excellence (ECCOE)

The USGS Lunar Calibration project hosts a system to use the Moon as a reference light source for sensor radiometric calibration and stability monitoring. This capability was developed for solar reflective wavelengths, primarily to meet the on-orbit calibration needs of remote sensing imaging instruments.

USGS Lunar Calibration Goals 

The goal of the USGS lunar calibration program is to utilize the Moon as an on-orbit standard, both absolute and transfer, for radiometric calibration of remote sensing satellite sensors. There is a recognized need for an on-orbit spectral standard in the solar reflectance wavelength region (0.35 to 2.5 micron): instruments in flight commonly experience changes in responsivity from their pre-launch calibrations, on-board calibration systems typically do not use the same optical path as the Earth view, and on-board systems also degrade in the space environment. 

The exceptional stability of the lunar surface reflectance (better than one part in 108 per year (Kieffer, 1997)) makes the Moon a favorable calibration source -- its apparent brightness is knowable with high precision and absolute accuracy. The current capabilities of lunar calibration include: 

  • Long-term sensor drift monitoring with sub-percent per year relative accuracy, which meets calibration stability requirements for remote sensing for climate 
  • Providing a common target for inter-calibration of instruments 
  • Providing a stable radiometric reference that is valid for any time, enabling back-calibration and offering a strategy for bridging potential gaps in data records
     

Lunar Calibration Challenges 

There are significant complications to using the Moon as a radiometric standard source, primarily resulting from the variegation of the surface albedo, the constantly changing lunar phase and librations, and the strong dependence of the surface reflectance function on phase angle. However, the lunar surface reflectance properties are extremely stable, and therefore knowable to high precision. The practical considerations of using the Moon for instrument calibration call for the use of a model, which can predict the lunar brightness for the precise geometry of illumination and viewing of the instrument. Such a model, once established, is valid for any observation of the Moon within the geometry range, including those made in the past. 

The principal challenges to using the Moon as a light source are the non-uniformity of the lunar surface albedo, the brightness variations arising from lunar phase and libration, and the strong dependence of the surface reflectivity on phase angle. The complexity of these dependencies requires the use of a lunar radiometric model to compare against spacecraft observations of the Moon. Such a model must be developed from a collection of radiometric measurements of the Moon covering a practical range of lunar phases and a sufficient portion of the 18.6-year libration cycle. The USGS lunar calibration program has acquired the necessary observational data for development of operational lunar models. 

A basic requirement for lunar calibration is that the instrument must view the Moon. This poses another challenge for nadir-viewing spacecraft in that often an in-flight attitude maneuver is needed.
 

Lunar Irradiance Modeling 

The USGS ECCOE Project lunar calibration effort hosts a system to use the Moon as a reference light source for sensor radiometric calibration and stability monitoring. This capability was developed for solar reflective wavelengths, primarily to meet the on-orbit calibration needs of remote sensing imaging instruments. However, the lunar calibration technique is applicable to any solar-wavelength sensor that views the Moon.

The basis of the USGS lunar calibration irradiance modeling is an extensive database of radiance images acquired by the ground-based RObotic Lunar Observatory (ROLO[∗]), that was located at the USGS Astrogeology Science Center in Flagstaff, AZ. ROLO observed the Moon on clear nights between First Quarter and Last Quarter lunar phases for over 6 years. Twin telescopes on a common mount cover the Visible and Near-infrared (VNIR) range (350-950 nm) in 23 bands, and the shortwave infrared (SWIR - 950-2350 nm) in 9 bands. Substantial observing time was dedicated to imaging stars, for the purpose of determining atmospheric extinction corrections. Calibration to radiance is based on measurements of the star Vega, although efforts to tie the ROLO data to the SI radiometric scale are ongoing, involving the radiometry group at NIST. The ROLO database contains over 85,000 individual lunar images, and several hundred-thousand-star images. 

Modeling emphasis has been on the disk-integrated lunar irradiance. A spatially resolved radiance model has been developed, but the irradiance quantity has been found preferable for spacecraft calibration work due to the higher accuracy and precision achievable. The USGS lunar irradiance model was developed from fitting ROLO observational data that have been calibrated to exoatmospheric radiance and spatially integrated over the entire lunar disk, regardless of the illuminated fraction. The model analytic form was determined empirically through study of the fit residuals, with the goal of reducing correlations seen in the residuals. 

The lunar irradiance model fits the ROLO observational data with an average residual over all bands of just under 1%. This value constitutes a measure of the model precision with which the model can predict the variations in lunar irradiance due to view geometry, including phase, libration, and the lunar photometric function, over the useful range of the geometric variables. These range in phase from 90 degrees (before and after Full Moon) to near eclipse (~1.5 degrees), and virtually all libration angles viewable from the Earth's surface. 

The USGS model can specify the lunar irradiance with relative precision ~1%, based on the fit residuals and data error analysis. The absolute scale of the ROLO data and lunar model has an uncertainty currently estimated ~5-10%, based on comparisons of a number of spacecraft instrument calibrations. A dedicated effort is underway to reduce the absolute uncertainty and tie the ROLO scale to SI units; the program uncertainty goals are 1% (VNIR) to 2% (SWIR) absolute. However, a number of important instrument characterizations can be achieved independent of the absolute scale, such as tracking of instrument degradation over time, and intercomparison among instruments that have viewed the Moon, regardless of the proximity in time and location of the observations.  Given a time series of lunar views taken by a spacecraft instrument, smooth sensor degradation curves can be fitted to the irradiance model comparisons, resulting in relative response trending with sub-percent precision over the series. 
 

Lunar Calibration Support for Instrument Teams 

The USGS ECCOE Project lunar calibration team will discuss providing technical assistance to instrument teams for lunar calibration.  Lunar calibration is applicable to any sensor system that has viewed the Moon, even ground-based instruments. 

To accommodate a wide variety of sensor configurations having different operational objectives, a set of Information Exchange Items with formalized Exchange File Formats has been developed to support instrument teams.  The procedures for spacecraft instrument teams to participate in lunar calibration have also been largely formalized; an overview of the information exchange between a Spacecraft Calibration Team (SCT) and the USGS Lunar Calibration Team (LCT) can be requested by instrument teams interested in lunar calibration. 

The provided lunar calibration results are reported as the fractional discrepancy between the irradiance measured by an instrument that has viewed the Moon and the model prediction for the ephemeris, view geometry, and band wavelength of the spacecraft observation.  Although the LCT has assisted some instrument teams with lunar image processing, generally the SCT is expected to provide their observations calibrated to irradiance.
 

Lunar Calibration Mission Considerations 

Many spacecraft teams have chosen to observe the Moon at near 7 degrees phase, both before and after Full Moon. This geometry provides good S/N while avoiding the “opposition effect” enhanced backscatter at low phase angles. However, there is no requirement to restrict observations to a narrow range of phase angles -- the lunar irradiance model is valid for any phase angle between eclipse and 90 degrees, with precision on the order of 1 percent over the entire range. The radiance of the Full Moon is comparable to that of clear land viewed from space. 

A typical lunar observing sequence for a nadir-viewing spacecraft involves an attitude pitch maneuver, starting approximately when the spacecraft enters the Earth's shadow. The spacecraft is rotated to the Moon, then scans the Moon at a constant rate such that the image acquired is oversampled in the down-track direction. The spacecraft is then pitched back toward Earth, regaining its normal nadir-viewing attitude before passing out of the shadow. This adds (or subtracts) one complete revolution to the normal nadir-locked pitch rate. The scanning past the Moon should be at a constant rate, typically 4 to 8 times slower than a normal nadir scene, and it should extend at least one degree past the edge of the Moon to allow adequate sampling of the space level beyond the extended point-spread function in all bands. 

To assist spacecraft instrument teams with planning lunar views, the USGS Lunar Calibration Team has developed a tool that can predict the maximum radiance expected for a given lunar observation geometry, for a particular instrument band spectral response and spatial resolution.  Supporting instrument teams with these predictions can be discussed with the ECCOE Project. 

The visible channels of geostationary meteorological imagers typically lack on-board calibration hardware, relying instead on vicarious and cross calibration techniques. The Moon appears regularly in the margins of full-disk operational images acquired by GEO instruments with rectangular field of regard. The USGS Lunar Calibration Team has developed a tool to predict the appearance of the Moon in a GEO image based on the satellite Two-Line Element (TLE) orbital parameters. This can be used to find images of the Moon in a data archive, or to determine future Moon capture opportunities. A time 
series of lunar images can be used to develop calibration histories for these instruments, regardless of their current operational status. Supporting instrument teams with these predictions can also be discussed with the ECCOE Project.  

[∗] The acronym ROLO was created by Bob Wildey, and is used in his memory 
[1] Applied Optics 43, 5838-5854 (2004) 

 
Historical references 

  • T. Berkoff, M. Sorokin, T. Stone, T. Eck, R. Hoff, E.Welton and B. Holben, “Nocturnal Aerosol Optical Depth Measurements with a Small Aperture Photometer Using the Moon as a Light Source,” J. Atmos. and Oceanic Tech. 28, 1297–1306 (2011). 
  • B. J. Buratti, M. D. Hicks, J. Nettles, M. Staid, C. M. Pieters, J. Sunshine, J. Boardman and T. C. Stone, “A wavelength-dependent visible and infrared spectrophotometric function for the Moon based on ROLO data,” J. Geophys. Res.–Planets 116, E00G03, doi: 10.1029/2010JE003724 (2011).
  • Jay D. Goguen, T. C. Stone, H. H. Kieffer and Bonnie J. Buratti, “A New Look at Photometry of the Moon,” Icarus 208, 548–557 (2010).
  • T. C. Stone, “Stellar Calibration of the ROLO Lunar Radiometric Reference,” Proc. SPIE 7807, 7807T-1–10 (2010).
  • T. C. Stone, “Absolute Stellar Photometry on Moderate-resolution FPA Images,” Metrologia 46, S224–S227 (2009). 
  • T. C. Stone, “Radiometric Calibration Stability and Inter-calibration of Solar-band Instruments in Orbit Using the Moon,” Proc. SPIE 7081, 70810X-1–8 (2008).
  • T. C. Stone, “Use of the Moon for in-flight calibration stability monitoring,” Committee on Earth Observing Satellites (CEOS) Quality Assurance Framework for Earth Observation (QA4EO) Guideline/Procedures document (2008). 
  • T. Stone, “The Moon as a Calibration Source,” in Achieving Satellite Instrument Calibration for Climate Change, G. Ohring, ed. (T. Stone, contributing author), NOAA publication (2007). 
  • Thomas C. Stone, "Radiometric Calibration Stability and Inter-calibration of Solar-band Instruments in Orbit Using the Moon, "Proc. SPIE 7081 70810X-1-8 (2008). 
  • T. C. Stone and H. H. Kieffer, "Use of the Moon to support on-orbit sensor calibration for climate change measurements," Proc. SPIE 6296 62960Y-1-9 (2006). 
  • Hugh H. Kieffer and Thomas C. Stone, "The Spectral Irradiance of the Moon", Astronom. J. 129, 2887-2901 (2005). 
  • T.C. Stone, H.H. Kieffer, and I.F. Grant, "Potential for Calibration of Geostationary Meteorological Satellite Imagers using the Moon", Proc. SPIE 5882 (2005). 
  • T.C. Stone and H.H. Kieffer, "Assessment of Uncertainty in ROLO Lunar Irradiance for On-orbit Calibration", Proc. SPIE 5542, 300-310 (2004). 
  • T. C. Stone, H. H. Kieffer, and K. J. Becker, "Modeling the Radiance of the Moon for On-orbit Calibration", Proc. SPIE 5151, 463-470 (2003). 
  • H.H. Kieffer, T.C. Stone, R.A. Barnes, S.C. Bender, R.E. Eplee, J.A. Mendenhall and L. Ong, "On-orbit Radiometric Calibration Over Time and Between Spacecraft Using the Moon", Proc. SPIE 4881, 287-298 (2003). 
  • T.C. Stone and H.H. Kieffer, "Absolute Irradiance of the Moon for On-orbit Calibration", Proc. SPIE 4814, 211-221 (2002). 
  • T.C. Stone, H.H. Kieffer, and J.M. Anderson, "Status of Use of Lunar Irradiance for On-orbit Calibration", Proc. SPIE 4483, 165-175 (2002). 
  • J.M. Anderson, H. Kieffer, and K. Becker, "Modeling the brightness of the Moon over 350-2500 nm for spacecraft calibrations", Proc. SPIE 4169, 248-259 (2000).
  • H.H. Kieffer, J.M. Anderson, and K.J. Becker, "Radiometric Calibration of Spacecraft using Small Lunar Images", Proc. SPIE 3870, 193-205 (1999). 
  • J.M. Anderson and H.H. Kieffer, "Photometric Imaging of the Moon from the Robotic Lunar Observatory", in Workshop on New Views of the Moon II: Understanding the Moon Through the Integration of Diverse Datasets, 2-24, Sep. 1999. 
  • J.M. Anderson, K.J. Becker, H.H. Kieffer, and D.N. Dodd, "Real-Time Control of the Robotic Lunar Observatory Telescope", Pub. Astronomical Soc. Pacific 111, 737-749 (1999). 
  • H.H. Kieffer and J.M. Anderson, "Use of the Moon for spacecraft calibration over 350--2500 nm", Proc. SPIE 3498, 325-335 (1998). 
  • H.H. Kieffer, "Photometric Stability of the Lunar Surface", Icarus 130, 323-327 (1997). 
  • H.H. Kieffer and R.L. Wildey, "Establishing the Moon as a Spectral Radiance Standard", J. Atmospheric and Oceanic Technology 13, 2, 360-375 (1996). 
  • H. Kieffer and R. Wildey, "Use of the Moon for calibration", CEOS CAL/VAL newsletter 4 (1994). 
  • R.L. Wildey and H.H. Kieffer, "A new facility for absolute photometric imaging of the Moon", Bull. American Astronomical Society 25, 1089 (1993). 
  • H.H. Kieffer and R.L. Wildey, "Spectrophotometry of the Moon for calibration of space-borne imaging instruments", Proc. Lunar and Plan Sci. Conf. number 23, Abstracts, 687-688 (1992). 
  • H.H. Kieffer and R.L. Wildey, "Absolute Calibration of Landsat Instruments Using the Moon", Photogramm. Eng. Remote Sens. 51, 1391-1393 (1985). 

Related Lunar Calibration Papers 

  • R. E. Eplee Jr., S. W. Bailey, R. A. Barnes, H. H. Kieffer, and C. R. McClain, "Comparison of SeaWiFS On-orbit Lunar and Vicarious Calibrations," Proc. SPIE 6296 (2006). 
  • X. Wu, T. C. Stone, F. Yu, and D. Han, "Vicarious calibration of GOES Imager visible channel using the Moon", Proc. SPIE 6296 (2006). 
  • F. S. Patt, R. E. Eplee, R. A. Barnes, G. Meister, and J. J. Butler, "Use of the Moon as a calibration reference for NPP VIIRS," Proc. SPIE 5882 (2005). 
  • R. A. Barnes, R. E. Eplee Jr., F. S. Patt, H. H. Kieffer, T. C. Stone, G. Meister, and C. R. McClain, "Comparison of SeaWiFS measurements of the Moon with the U.S. Geological Survey lunar model", Appl. Optics 43, 5838-5854 (2004). 
  • J. Sun, X. Xiong, B. Guenther, and W. Barnes, "Radiometric stability monitoring of the MODIS reflective solar bands using the Moon", Metrologia 40, S85-S88 (2003). 
  • X. Xiong, J. Sun, K. Chiang, S. Xiong, and W.L. Barnes, "MODIS on-orbit characterization using the Moon", Proc. SPIE 4881, 299-307 (2003). 
  • H.H. Kieffer, P. Jarecke, and J. Pearlman, "Initial Lunar Calibration Observations by the EO-1 Hyperion Imaging Spectrometer", Proc. SPIE 4480, 247-258 (2002). 
  • I. F. Grant, H. H. Kieffer, T. C. Stone, and J. M. Anderson, "Lunar calibration of the GMS-5 visible band," Proceedings of the International Geophysics and Remote Sensing Symposium 2001 Vol 4, pg 2769-2771 (2001). 
  • R.A. Barnes, R.E. Eplee, W.D. Robinson, G.M. Schmidt, F.S. Patt, S.W. Bailey, M. Wang, and C.R. McClain, "The calibration of SeaWiFS", Proc. CalCon2000 (2000). 
  • R.A. Barnes and C.R. McClain, "The calibration of SeaWiFS after two years on-orbit", Proc. SPIE 3870, 214-227 (1999). 
  • R.A. Barnes, R.E. Eplee, F.S. Patt, and C.R. McClain, "Changes in the radiometric response of SeaWiFS determined from lunar and solar-based measurements", Applied Optics 38, 4649-4664 (1999). 
  • • R.A. Barnes, R.E. Eplee Jr., and F.S. Patt, "SeaWiFS Measurements of the Moon", Proc. SPIE 3498, 311-324 (1998). 
  • I. Grant, H.H. Kieffer, and J.M. Anderson, "Lunar calibration of geostationary visible-band images", Proc. SPIE 3498, 337-347 (1998). 
  • M.S. Maxwell and H.H. Kieffer, "Calibrating the GOES imager visible spectral band using the moon as a radiance standard", Proc. SPIE 3439 (1998). 
  • R.A. Barnes, A.W. Holmes, and W.E. Esaias, "Stray light in the SeaWiFS radiometer", NASA Tech. Mem. 104566 31, 1-76 (1995)
  • J.P. Antikidis and M. Reynolds, "In-orbit Calibration of Meteosat and its Possible Extension to Other Applications and Fields", ESA Journal 3, 2, 115-121 (1979). 
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