tailieunhanh - Advances in Spacecraft Technologies Part 4

Tham khảo tài liệu 'advances in spacecraft technologies part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 110 Advances in Spacecraft Technologies Fig. 11. ESA s SILEX project. Credits ESA multimedia gallery. over km were reached with up to 50 Mbps binary rates Fletcher Hicks Laurent 1991 . Other significant projects never went beyond the design table such as the OCDHRLF project which in 2002 intended to load a Gbps optical communication terminal on board the International Space Station using commercial off-the-shelf components Ortiz et al. 1999 . Or the EXPRESS project in which a link was designed to download data from the space shuttle with a speed of up to 10 Gbps Ceniceros Sandusky Hemmati 1999 . Or the most ambitious NASA s MLCD project which in 2009 intended to prove a link of up to 100 Mbps link from Mars by using a small low-power 5W of average power terminal on board the MTO Mars Telecom Orbiter which was not launched after all due to budget pressures Edwards et al. 2003 . Diffraction limit of a telescope and beam divergence In fact a telescope s primary mirror or lens can be considered a circular opening because it produces light inside a circle described by its primary mirror. If the opening s diameter is D and the wave length is Ấ the angular variation of intensity of radiation is given by the Eq. 6 Hecht 2002 I 0 2 1 nDsln 101 0 in 0 2 6 where J t x is the Bessel function of first order of x. The first zero refers to nD Ấ sin 6 . Using the approach sin 6 6 we get a telescope s diffraction limit which is given by the equation 7 0 1 221 I rad 7 IDI This limit determines the lowest diffraction angle and consequently the minimum of beam divergence with an increase in distance Fig. 12 . Here the diffraction limit formula has been calculated according to the criterion of the first zero in the Bessel function. If a different criterion were used the multiplying factor of Ấ D Development of Optoelectronic Sensors and Transceivers for Spacecraft Applications 111 would be different. For example Franz 2000 if one were to take the point where the .

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