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Aviation History
1962
1962 - 0107.PDF
/ !GHT International, 18 January 1962 107 r ,,vable paddles. The Earth may eclipse the satellite from sunlight f as long as 74min at a time; during such eclipses, power will be nided by the battery. The total power required from the solar iS for the satellite in orbit will be about 600 watts, including : watts for battery recharging. rhe ground stations will use 60ft parabolic antennas for both nsmission and reception and will transmit normally at a power \. elof2kW. The satellite will have to make automatic angular corrections i Keep its antenna aimed at the Earth within two degrees of the iical, and to keep its solar cells perpendicular to sunlight within t\,o degrees. The necessary corrections will be sensed by infra-red hi azon sensors mounted on the body of the satellite and by sun- sors mounted on the body and on the solar paddles. The m jhanism for making the corrections will consist of motor-driven inertia wheels and a cold nitrogen gas propulsion system having several nozzles distributed over the surface of the satellite, and capable of changing its speed with an accuracy of 0.005ft/sec. The satellite will also have a hot-gas propulsion system for injection into orbit. To enhance the reliability of the satellite and to keep its instru ments in proper operating condition, its mean internal temperature will be kept between 50 and 75 F. This will be done automatically by the movable shutters on the shadowed sides of the satellite. The solar cells will be protected from overheating in orbit by thin alass plates; during ascent to orbit they will be protected from over- cooling by a shroud that will be jettisoned before the satellite separates from the second stage. The antennas mounted on the satellites for telemetry, tracking and command will be crossed dipoles. Telemetry will provide data on all critical satellite control functions, such as temperature, eas pressure, solar-cell current, and the like, together with data on the condition of the repeater electronics. Tracking will be based on the azimuth and elevation angles of the ground antennas and on doppler measurements of the radial velocity of the satellite. The commands sent to the satellite will control its propulsion system, the telemetry channels, and the communication channels. The total weight of the satellite will be of the order of 1,0001b. Early flight-test information on some of the satellite's subsystems will be obtained in three low-altitude, near-circular, inclined orbital flights this year, using Atlas Agena B as the launch vehicle. The Atlas portion of the flight will be guided by the General Electric Vlk 3 radio guidance system, and the Agena B will use the standard Lockheed autopilot. The test satellites to be employed will include 451b of special environment instrumentation, four communication repeater channels and a hot-gas propulsion system. The next phase of flight testing will employ Atlas Centaur vehicles which will directly inject Advent satellites into stationary orbit at about 105: W. Two launches will use this direct-injection method, which is suitable for the particular longitude stated but which does not have sufficient flexibility for global use. The third and final phase, again using Atlas Centaur vehicles, will make use of an intermediate orbit known as a "walking" or •drift" orbit (see diagrams). This orbit has a period less than 24hr and is elliptical. The satellite carries a hot-gas propulsion system which can provide the acceleration to circular speed at the correct altitude when the satellite has "drifted" eastward to the desired longitude. This procedure enables the satellite to be placed at any longitude and, because the initial velocity needed to attain the intermediate orbit is less than that required for direct injection, a nigher payload is possible. The hot-gas system for injecting the satellite into its final stationary orbit will be commanded only from the ground, but the cold-gas system used for fine correction will be actuated either by ground command (for velocity corrections) or automatically by the satellite's attitude-sensing system (for attitude control). \lthough global communications can in principle be ensured b\ a system of three stationary satellites placed at 120° intervals long the equator, factors such as satellite lifetime and the location if ground stations in foreign countries make it impossible to predict lust how many Advent satellites will be needed in the eventual nerational system. The more modest aim of obtaining transatlantic - :id transpacific communication could be achieved much sooner, Mng two satellites over each ocean, spaced 10° apart (see lower ip). Advent ground terminals will be located at Fort Dix. New Jersey :id Camp Roberts, California, and aboard a ship in the South ' :ciftc. These, together with a station in Hawaii, will control the '-'ration of Advent satellites in orbit. Model Agena stages for Advent (upper) and Discoverer, shown by Lockheed Advent system manager H. W. Huntley Above, earth track of equatorial Advent satellite; below, hypothetical four-satellite Advent system for transatlantic and transpacific communica tion (ovals show coverage regions assuming limit of 7.5 deg above horizon)
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