USGS  : Lunar Calibration, ROLO - Robotic Lunar Observatory : ROLO Facility Telescope
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Overview of Lunar Calibration

Introduction

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. For detecting signals of global climate change from space, the long-term stability requirement for solar-band instruments is 1% over a decade (NISTIR 7047 (PDF, 3.9mb) [http://physics.nist.gov/Divisions/Div844/publications/NISTIR7047/nistir7047.pdf]). Lunar calibration may be the only practical means for meeting this criterion.

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 effectively mandates the use of a lunar radiometric model to compare against spacecraft observations of the Moon. Such a model must be developed from a program 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. To date, SeaWiFS has made well over 100 lunar observations using a pure-pitch satellite maneuver. The ALI and Hyperion instruments on EO-1 have acquired monthly lunar views for several years. Of the NASA EOShttp://eospso.gsfc.nasa.gov flagship satellites, only Terra has captured the Moon in nadir-view, once.

Observational Database

The basis of the USGS lunar irradiance specification is an extensive database of radiance images acquired by the ground-based RObotic Lunar Observatory (ROLO[∗]), located at the USGS 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. [See further details of the ROLO instrumentation, phase and libration coverage, and data reduction and calibration.]

Operational Lunar Model

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. Further details of the irradiance model development, the model form, and the data fitting procedure are under the Lunar Modeling link.

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.

Spacecraft Instrument Team Interaction

The procedures for spacecraft instrument teams to participate in lunar calibration have been largely formalized; an overview of the information exchange between a Spacecraft Calibration Team (SCT) and the USGS Lunar Calibration Team (LCT) can be found at the Spacecraft Calibration link. At present, 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 provided assistance to some instrument teams with lunar image processing, generally the SCT is expected to provide their observations calibrated to irradiance.

Utility

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. This capability has been demonstrated for SeaWiFS[1], and represents attainment of the stability requirement for measuring global climate change from space for solar-band instruments (NISTIR 7047 (PDF, 3.9mb) [http://physics.nist.gov/Divisions/Div844/publications/NISTIR7047/nistir7047.pdf]).

[∗] The acronym ROLO was created by Bob Wildey, and is used in his memory

[1] Applied Optics 43, 5838-5854 (2004)

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