Analysis of Opportunities for Intercalibration Between Two Spacecraft
2018
There is currently a strong interest in obtaining highly accurate measurements of solar radiation reflected by Earth. For example, the
Traceable
RadiometryUnderpinning Terrestrial- and
Helio- Studies (TRUTHS) satellite mission has been under consideration in Europe for several years, and planning is now under way for the Climate Absolute Radiance and Refractivity Observatory (
CLARREO)
spacecraftin the United States. Such
spacecraftwill provide measurements whose high accuracy is
traceableto SI standards; these measurements will be useful as a reference for calibrating similar instruments on board other
spacecraft. Hence, analysis of opportunities for intercalibration between two
spacecraftplays an important role in the planning of future missions. In order for intercalibration to take place, the measurements obtained from two
spacecraftmust have similar viewing geometry and be taken within a few minutes of one another. Viewing geometry is characterized in terms of viewing
zenithangle,
solar zenith angle, and relative azimuth angle. Opportunities for intercalibration are greater in number and longer in duration if the sensor with high accuracy can be aimed at points on the surface of the Earth other than the nadir or sub-satellite point. Analysis of intercalibration over long periods is rendered tractable by making several simplifying assumptions regarding
orbital motionsof the two
spacecraftabout Earth, as well as
Earths
orbitabout the Sun. The shape of the Earth is also considered. A geometric construction called a tent is introduced to facilitate analysis. It is helpful to think of an intercalibration opportunity as the passage of one
spacecraftthrough a tent that has a fixed shape and moves with the
spacecraftwhose measurements are to be calibrated. Selection of points on Earth s surface as targets for measurement is discussed, as is aiming the boresight of a steerable instrument. Analysis results for a pair of
spacecraftin typical low
Earth orbitsare provided.
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