Abstract
PEOPLE engaged in radio tracking of space vehicles are well aware of the fact that the Earth's atmosphere may cause serious refractive errors in the elevation angle determination. It is also generally accepted that refractive errors rapidly decrease with an increase in the elevation angle, and become virtually negligible above 10 or 15 degrees. In the case of radio astronomy, this is quite true. However, in the case of space-vehicles, which travel in the immediate vicinity of the Earth, this is not the case. The refractive errors due to the troposphere rapidly decrease with the elevation angle, while those due to the ionosphere initially increase with the elevation angle, and then gradually fall off. This behaviour of ionospheric refraction is a necessary consequence of the spherical geometry. The value of the elevation angle at which the maximum ionospheric refractive error occurs lies typically between 100 and 200 milliradians. The exact expressions are rather involved, but it can be shown that the value of this angle is roughly proportional to the square root of the height of the layer. The maximum value of the ionospheric refractive error is about 10–15 per cent higher than its value for a tangentially departing ray.
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Evans, J. B., Proc. Phys. Soc., B, 69, 953 (1956).
Weisbrod, S., and Anderson, L. J., Proc. Inst. Rad. Eng. (in the press).
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WEISBROD, S., COLIN, L. Refraction of Very High Frequency Radio Signals at Ionospheric Heights. Nature 184, 119 (1959). https://doi.org/10.1038/184119a0
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DOI: https://doi.org/10.1038/184119a0
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