Satellites come in a variety of shapes and sizes and perform many different functions, but they all have several things in common.
- All of them have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch
- All of them have a source of power (usually solar cells) and batteries for storage. Arrays of solar cells provide power to charge rechargeable batteries. Newer designs include fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets. Power systems are constantly monitored, and data on power and all other onboard systems is sent to Earth stations in the form of telemetry signals.
- All of them have an onboard computer to control and monitor the different systems.
- All have a radio system and antenna. At the very least, most satellites have a radio transmitter/receiver so that the ground-control crew can request status information from the satellite and monitor its health. Many satellites can be controlled in various ways from the ground to do anything from change the orbit to reprogram the computer system.
- All of them have an attitude control system. The ACS keeps the satellite pointed in the right direction.
As you might expect, putting all of these systems together isn’t easy. It can take years. Everything begins with a mission objective. Defining the parameters of the mission enables engineers to specify the instruments needed and how they'll be arranged. Once these specifications (and their budget) are approved, satellite construction can begin. This typically takes place in a clean room, a sterile environment that makes it possible to maintain a constant temperature and humidity and protect the satellite during its development, construction and testing.
Artificial satellites generally aren’t mass-produced; they’re custom-built to perform their intended functions. With that said, some companies have designed their satellites to be modular, making it possible to start with a primary structure that can be customized as needed. For example, Boeing's 601 satellites have two basic modules -- a chassis for carrying the propulsion subsystem, bus electronics and battery packs; and a set of honeycomb shelves to hold arrays of equipment. This modularity enables engineers to assemble purpose-built satellites without starting from scratch. And, of course, some satellites, such as those in GPS and the Iridium system, work together in a coordinated network. Using a repeatable design makes it easier to set up and integrate the various components of the system.