Additional Information for Spacecraft Teams
Suggested Observing Strategy
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% 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 should extend at least one degree past the edge of the Moon to to allow adequate sampling of the space level beyond the extended point-spread function in all bands.
Lunar Maximum Radiance Prediction
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.
Considerations for Geostationary Instruments
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.
U.S. Department of the Interior | U.S. Geological Survey | U.S.G.S. Astrogeology
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