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described in Sec. 7.3). Thus the arrays are only partially in sunshine at any given time, which places a limitation on power. Higher powers can be achieved with solar panels arranged in the form of rectangular solar sails. Solar sails must be folded during the launch phase and extended when in geostationary orbit. Figure 7.2 shows the HS 601 satellite manufactured by Hughes Space and Communications Company. As shown, the solar sails are folded up on each side, and when fully extended, they stretch to 67 ft (20.42 m) from tip to tip. The full complement of solar cells is exposed to the sunlight, and the sails are arranged to rotate to track the sun, so they are capable of greater power output than cylindrical arrays having a comparable number of cells. The HS 601 can be designed to provide dc power from 2 to 6 kW. In comparing the power
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Aussat B1 (renamed Optus B), Hughes first HS 601 communications satellite is prepared for environmental testing. (Courtesy of Hughes Aircraft Company Space and Communications Group.)
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Figure 7.3 Satellite eclipse time as a function of the current day of the year. (Courtesy of Spilker, 1977. Reprinted by permission of Prentice-Hall, Englewood Cliffs, NJ.)
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capacity of cylindrical and solar-sail satellites, the cross-over point is estimated to be about 2 kW, where the solar-sail type is more economical than the cylindrical type (Hyndman, 1991). As discussed in Sec. 3.6, the earth will eclipse a geostationary satellite twice a year, during the spring and autumnal equinoxes. Daily eclipses start approximately 23 days before and end approximately 23 days after the equinox for both the spring and autumnal equinoxes and can last up to 72 min at the actual equinox days. Figure 7.3 shows the graph relating eclipse period to the day of year. In order to maintain service during an eclipse, storage batteries must be provided. Ni-Cd batteries continue to be used, as shown in the Hughes HS 376 satellite, but developments in nickel-hydrogen (Ni-H2) batteries offer significant improvement in power-weight ratio. Ni-H2 batteries are used in the Hughes HS 601 (e.g., the SATMEX-5 and Anik-F2 satellites, see Secs. 7.9 and 7.10) and were introduced into the Intelsat series with INTELSAT VI (Pilcher, 1982) and INTELSAT VII (Lilly, 1990) satellites. 7.3 Attitude Control The attitude of a satellite refers to its orientation in space. Much of the equipment carried aboard a satellite is there for the purpose of controlling its attitude. Attitude control is necessary, for example, to ensure that
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directional antennas point in the proper directions. In the case of earth environmental satellites, the earth-sensing instruments must cover the required regions of the earth, which also requires attitude control. A number of forces, referred to as disturbance torques, can alter the attitude, some examples being the gravitational fields of the earth and the moon, solar radiation, and meteorite impacts. Attitude control must not be confused with station keeping, which is the term used for maintaining a satellite in its correct orbital position, although the two are closely related. To exercise attitude control, there must be available some measure of a satellite s orientation in space and of any tendency for this to shift. In one method, infrared sensors, referred to as horizon detectors, are used to detect the rim of the earth against the background of space. With the use of four such sensors, one for each quadrant, the center of the earth can be readily established as a reference point. Any shift in orientation is detected by one or other of the sensors, and a corresponding control signal is generated which activates a restoring torque. Usually, the attitude-control process takes place aboard the satellite, but it is also possible for control signals to be transmitted from earth, based on attitude data obtained from the satellite. Also, where a shift in attitude is desired, an attitude maneuver is executed. The control signals needed to achieve this maneuver may be transmitted from an earth station. Controlling torques may be generated in a number of ways. Passive attitude control refers to the use of mechanisms which stabilize the satellite without putting a drain on the satellite s energy supplies; at most, infrequent use is made of these supplies, for example, when thruster jets are impulsed to provide corrective torque. Examples of passive attitude control are spin stabilization and gravity gradient stabilization. The latter depends on the interaction of the satellite with the gravitational field of the central body and has been used, for example, with the Radio Astronomy Explorer-2 satellite, which was placed in orbit around the moon (Wertz, 1984). For communications satellites, spin stabilization is often used, and this is described in detail in Sec. 7.3.1. The other form of attitude control is active control. With active attitude control, there is no overall stabilizing torque present to resist the disturbance torques. Instead, corrective torques are applied as required in response to disturbance torques. Methods used to generate active control torques include momentum wheels, electromagnetic coils, and mass expulsion devices, such as gas jets and ion thrusters. The electromagnetic coil works on the principle that the earth s magnetic field exerts a torque on a current-carrying coil and that this torque can be controlled through control of the current. However, the method is of use only for satellites relatively close to the earth. The use of momentum wheels is described in more detail in Sec. 7.3.2.
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