Proof of principle Testing that demonstrates that a particular principle or process works. Pyranometer An instrument for measuring total hemispherical solar (beam plus diffuse) radiation, usually on a horizontal surface. Pyrheliometer An instrument using a collimated detector for measuring solar radiation from a small portion of the sky including the sun (i.e., beam radiation at normal incidence). Radiation, beam That solar radiation received from the sun without change of direction. Radiation damage Damage produced by high energy particles that exist outside of the earth's atmosphere. Refers particularly to damage to photovoltaic cells or reflective surfaces. Radiation, diffuse That solar radiation received from the sun after its direction has been changed by reflection and scattering by the atmosphere. Ramjet A jet engine in which the air is continually compressed by being rammed into the open front end. Rectenna A coined term for a receiving antenna. Reference system The theoretical, projected satellite power system that has been devised for study purposes and is used as a yardstick for all calculations in the concept development and evaluation program. Reflectivity The ratio of radiant energy reflected by a body to that falling upon it.
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