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Paper: The Effects of the Atmosphere
Volume: 278, NAIC-NRAO School on Single-dish Radio Astronomy: Techniques and Applications
Page: 413
Authors: Olmi, Luca
Abstract: In this lecture I will discuss the effects of the neutral atmosphere and, to a lesser extent of the ionosphere, on radio astronomical observations carried out from single-dish telescopes on the surface of the Earth. We are concerned with three types of effects: large-scale refractive effects, absorption and scattering by the turbulent structure in the media. The phenomenon of scattering results in ``seeing'', and creates refractive index variations which limit the resolution and sensitivity of observations of astronomical sources. In the troposphere, water vapor plays a fundamental role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in near-infrared or optical regimes. As a consequence, phase fluctuations at frequencies higher than about 1 GHz are predominantly caused by fluctuations in the distribution of water vapor, and I will thus concentrate on tropospheric-induced radio seeing. On filled-aperture telescopes radio seeing shows up as an anomalous refraction (AR), i.e. an apparent displacement of a radio source from its true position. The magnitude of this effect, as a fraction of the beam width, is bigger on larger telescopes, and thus its impact on the pointing is likely to become critically important in the next generation of electrically large (D/<~mbda>104), filled-aperture radio telescopes. I will thus present the results of recent, systematic AR measurements and discuss a model study of AR effects, obtained producing numerical simulations of two-dimensional phase screens. I will finally discuss the basic concept and requirements of a tip-tilt compensation system at millimeter wavelengths, and will also present a proposed design based on a scanning microwave radiometer as a wave front sensing device.
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