Basic Parts of a Satellite
What's Inside a Satellite ?
Satellites have a great deal of equipment packed inside them. Most satellites have seven subsystems, and each one has special work to do.
1. The propulsion subsystem includes the rocket motor that brings the spacecraft to its permanent position, as well as small thrusters (motors) that help to keep the satellite in its assigned place in orbit. Satellites drift out of position because of solar wind or gravitational or magnetic forces. When that happens, the thrusters are fired to move the satellite back into the right position in its orbit.
2. The power subsystem generates electricity from the solar panels on the outside of the spacecraft. The solar panels also store electricity in storage batteries, which can provide power at times when the sun isn't shining on the panels. The power is used to operate the communications subsystem. The entire communications subsystem can be operated with about the same amount of power as would be used by 10 light bulbs.
3. The communications subsystem handles all the transmit and receive functions. It receives signals from the Earth, amplifies them, and transmits (sends) them to another satellite or to a ground station.
4. The structures subsystem helps provide a stable framework so that the satellite can be kept pointed at the right place on the Earth's surface. Satellites can't be allowed to jiggle or wander, because if a satellite is not exactly where it belongs, pointed at exactly the right place on the Earth, the television program or the telephone call it transmits to you will be interrupted.
5. The thermal control subsystem keeps the active parts of the satellite cool enough to work properly. It does this by directing the heat that is generated by satellite operations out into space, where it won't interfere with the satellite.
6. The attitude control subsystem points the spacecraft precisely to maintain the communications "footprints" in the correct location. When the satellite gets out of position, the attitude control system tells the propulsion system to fire a thruster that will move the satellite back where it belongs.
7. Operators at the ground station need to be able to transmit commands to the satellite and to monitor its health. The telemetry and command system provides a way for people at the ground stations to communicate with the satellite.
How Big Is a Satellite ?
Different kinds of satellites are used in different situations, for different purposes. To talk about the sizes of satellites, we'll use two examples, the HS 376, which is used mostly for network and cable TV, and the HS 601, which is used mostly for direct broadcast TV and business communication networks.
The HS 376 is a small, barrel-shaped structure with an antenna reflector that looks like a lid on the barrel. When the HS 376 is first launched, its antenna reflector and solar panels are stowed that is, put away so it can fit inside a launch vehicle. After launch, the satellite travels through space until it reaches its assigned orbital position. Then its reflector and solar panels are deployed that is, unpacked and put in the right position for doing their work.
A typical HS 376 is 2.16 meters (7 feet 1 inch) in diameter, and 2.82 meters (9 feet 3 inches) high, in its stowed position. When it is deployed, its diameter is the same, but it is much taller: 6.57 meters (21 feet 7 inches) tall. The height of the deployed satellite is more than twice its height when stowed.
The satellite body is made like a telescope; when it is deployed, an outer cylinder is driven down by tiny electric motors to reveal the inner cylinder and locks into place. All of the outside of the satellite body is made of solar cells, which take the sun's energy and convert it to electricity. That means that when the outside is in its full telescope position, more solar cells are exposed to the sun, and the satellite can generate more power. The deployed HS 376 can generate more than twice as much power as the stowed HS 376.
Satellites weigh more at the beginning of life in orbit than at the end. This is because they carry rocket fuel for the thruster engines that will keep them in place in their orbits. As the fuel is used up, the satellite gets lighter. The HS 376 weighs 634 kg (or 1395 pounds) at the beginning of its life in orbit.
The HS 601 is a larger and more powerful satellite. When it is stowed for launch, it looks like a big box, 3.8 meters (12.6 feet) high. The HS 601 carries its solar panels on long wing-like structures. When the satellite is deployed, the solar panels are extended to a width of 26 meters (86 feet), and the antenna reflectors make the middle of the satellite 7.1 meters (23.3 feet) wide. The HS 601 weighs 1727 kg (3800 pounds) at the beginning of its life in orbit.
The Anatomy of a Satellite
Satellites have only a few basic parts: a satellite housing, a power system, an antenna system, a command and control system, a station keeping system, and transponders.
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 bellow:
The materials used in the construction of satellite housings are typically very expensive. In newer satellites, lightweight and extremely durable epoxy-graphite composite materials are often used.
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.