10 research outputs found
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Refractive aiming corrections for satellite observation of stars
Standard references describe how apparent zenith angles differ from true zenith angles for observers on the Earth. In fact, correction formulae are available for aiming Earth-based sensors at stars; some corrections give variations as a function of observer altitude. Such corrections have not been available for observers in space. This report develops formulae appropriate for proper aiming from space-based sensors toward the relatively few stars that are near the Earth`s limb at any given time. These formulae correct for refractive effects and may be critical for steerable space-borne sensors with fields of view less than one degree, tasked to observe starlight passing near the Earth`s surface. Ray tracing in the U.S. Standard Atmosphere, 1976 including H{sub 2}O effects, is used to determine relations between the refracted tangent height, the apparent tangent height resulting from observation at the sensor, and the angle through which the detected rays have deviated. Analytic fits of the ray deviation as a function of apparent tangent height allows quick determination of corrections needed for a space-borne sensor. Using those results that apply in the plane of incidence and using the necessary coordinate rotations, alterations in the star`s apparent right ascension and declination are evaluated to improve the aim. Examples illustrate that alterations can be larger than one degree, with effects lasting up to a few minutes
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Analytic fit of deviation caused by atmospheric refraction of starlight as a function of space-based sensor position
A simple prescription describes how space-borne sensors with fields of view less than one degree can be properly aimed at starlight that passes near the Earth`s surface and is therefore refracted by the Earth`s atmosphere. Atmospheric refraction effects cause deviations up to about one degree that distort the light path and can cause the target to be missed. Deviations are contrasted with those experienced for a target on the Earth. Such refractive corrections have long been available for Earth-based sensors looking through the atmosphere. The corrections have not been available for sensors in space. The prescription is illustrated by example
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HELIOS: a computational model for solar concentrators
The HELIOS computer code calculates the power concentrated by a field of individually guided heliostats and the resulting flux density (watts/cm/sup 2/) falling upon an arbitrary target grid. The problem has individual subroutines for each task in order to incorporate options for a variety of facet shapes, heliostat designs, field layouts, and tower-receiver apertures, and to facilitate additions and code improvements. HELIOS has been used extensively to analyze questions on safety, performance, design trade-offs, and tower protection engineering. Comparisons of HELIOS results with measurements have given good agreement. HELIOS calculates the ''sun position'' and uses it to establish alignment geometries. Atmospheric attenuation effects are included. Measured angular-distributions of incoming photons (sunshapes) and effects of aureole scattering are incorporated. Nondeterministic factors such as sun-tracking errors and facet-surface errors are described statistically and combined with the sunshape by numerical convolution. Shadowing and blocking are included. Several output choices are available, including graphical display of flux density distributions, of shadowing and blocking and of sunshape. Some of the modeling in HELIOS and samples of results are described