The configuration of the satellite housing is determined by the system employed to stabilize the attitude of the satellite in its orbital slot. Three-axis-stabilized satellites use internal gyroscopes rotating at 4,000 to 6,000 revolutions per minute (RPM). The housing is rectangular with external features as shown below in Figure 5.
Satellites must have a continuous source of electrical power--24 hours a day, 365 days a year. The two most common power sources are high performance batteries and solar cells. Solar cells are an excellent power source for satellites. They are lightweight, resilient, and over the years have been steadily improving their efficiency in converting solar energy into electricity. Currently the best gallium arsenide cells have a solar to electrical energy conversion efficiency of 15-20%. There is however, one large problem with using solar energy. Twice a year a satellite in geosynchronous orbit will go into a series of eclipses where the sun is screened by the earth. If solar energy were the only source of power for the satellite, the satellite would not operate during these periods. To solve this problem, batteries are used as a supplemental on-board energy source. Initially, Nickel-Cadmium batteries were utilized, but more recently Nickel-Hydrogen batteries have proven to provide higher power, greater durability, and the important capability of being charged and discharged many times over the lifetime of a satellite mission.
A satellite’s antennas have two basic missions. One is to receive and transmit the telecommunications signals to provide services to its users. The second is to provide Tracking, Telemetry, and Command (TT&C) functions to maintain the operation of the satellite in orbit. Of the two functions, TT&C must be considered the most vital. If telecommunications services are disrupted, users may experience a delay in services until the problem is repaired. However, if the TT&C function is disrupted, there is great danger that the satellite could be permanently lost--drifting out of control with no means of commanding it.
Command and Control System
This control system includes tracking, telemetry & control (TT&C) systems for monitoring all the vital operating parameters of the satellite, telemetry circuits for relaying this information to the earth station, a system for receiving and interpreting commands sent to the satellite, and a command system for controlling the operation of the satellite.
Although the forces on a satellite in orbit are in balance, there are minor disturbing forces that would cause a satellite to drift out of its orbital slot if left uncompensated. For example, the gravitational effect of the sun and moon exert enough significant force on the satellite to disturb its orbit. As well, the South American land mass tends to pull satellites southward.
Station keeping is the maintenance of a satellite in its assigned orbital slot and in its proper orientation. The physical mechanism for station keeping is the controlled ejection of hydrazine gas from thruster nozzles which portrude from the satellite housing. When a satellite is first deployed, it may have several hundred pounds of compressed hydrazine stored in propellent tanks. Typically, the useful life of a satellite ends when the hydrazine supply is exhausted--usually after ten years or so.
A transponder is an electronic component of a satellite that shifts the frequency of an uplink signal and amplifies it for retransmission to the earth in a downlink. Transponders have a typical output of 5 to 10 watts. Communications satellites typically have between 12 and 24 on-board transponders.
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