What is a satellite?
A satellite doesn’t necessarily have to be a tin can spinning through space. The word “satellite” is more general than that: it means a smaller, space-based object moving in a loop (an orbit) around a larger object. The Moon is a natural satellite of Earth, for example, because gravity locks it in orbit around our planet. The tin cans we think of as satellites are actually artificial (human-built) satellites that move in precisely calculated paths, circular or elliptical (oval), at various distances from Earth, usually well outside its atmosphere.
Who invented satellites?
The idea of using a satellite as a mirror in space—to bounce signals from one side of Earth to the other—was “launched” in 1945 by science fiction author Arthur C. Clarke (1917–2008), who wrote two hugely influential articles setting out his plan in detail (one was unpublished, the other published as “Extra-Terrestrial Relays: Can Rocket Stations Give World-Wide Radio Coverage?” in Wireless World, October 1945). His proposal was to place three satellites in a geosynchronous orbit 35,000km (23,000 miles) above Earth, spaced out evenly to cover about a third of the planet each: one would cover Africa and Europe, a second would cover China and Asia, and a third would be dedicated to the Americas. Although Clarke didn’t patent the geostationary communications satellite, he is generally credited with its invention, even though other space pioneers (notably German wartime pioneer Herman Oberth) had proposed similar ideas years before.
It took another decade for Clarke’s bold plan to move toward reality. First, satellites themselves had to be proved viable; that happened with the launch of the Russian Sputnik 1 in October 1957. Three years later, when the Echo communications satellite was launched, engineers successfully demonstrated that radio telecommunications signals could be relayed into space and back, just as Clarke had predicted. Telstar, the first communications satellite, was launched in July 1962 and immediately revolutionized transatlantic telecommunications. During the mid-1960s, 11 nations came together to form INTELSAT (International Telecommunications Satellite Consortium), which launched the world’s first commercial communications satellite INTELSAT 1 (“Early Bird”), in geosychronous orbit, in April 1965. This modest little space machine was a tiny electronic miracle: weighing just 35kg (76 lb), it could transmit 240 telephone simultaneous calls or a single black-and-white TV channel.
What do satellites do for us?
We tend to group satellites either according to the jobs they do or the orbits they follow. These two things are, however, very closely related, because the job a satellite does usually determines both how far away from Earth it needs to be, how fast it has to move, and the orbit it has to follow. The three main uses of satellites are for communications; photography, imaging, and scientific surveying; and navigation.
Uplinks and downlinks
If you want to send something like a TV broadcast from one side of Earth to the other, there are three stages involved. First, there’s the uplink, where data is beamed up to the satellite from a ground station on Earth. Next, the satellite processes the data using a number of onboard transponders (radio receivers, amplifiers, and transmitters). These boost the incoming signals and change their frequency, so incoming signals don’t get confused with outgoing ones. Different transponders in the same satellite are used to handle different TV stations carried on different frequencies. Finally, there’s the downlink, where data is sent back down to another ground station elsewhere on Earth. Although there’s usually just a single uplink, there may be millions of downlinks, for example, if many people are receiving the same satellite TV signal at once. While a communications satellite might relay a signal between one sender and receiver (fired up into space and back down again, with one uplink and one downlink), satellite broadcasts typically involve one or more uplinks (for one or more TV channels) and multiple downlinks (to ground stations or individual satellite TV subscribers).
One of the most surprising things about satellites is the very different paths they follow at very different heights above Earth. Left to its own devices, a satellite fired into space might fall back to Earth just like a stone tossed into the air. To stop that happening, satellites have to keep moving all the time so, even though the force of gravity is pulling on them, they never actually crash back to Earth. Some turn at the same rotational rate as Earth so they’re effectively fixed in one position above our heads; others go much faster. Although there are many different types of satellite orbits, they come in three basic varieties, low, medium, and high—which are short, medium, and long distances above Earth, respectively.
Scientific satellites tend to be quite close to Earth—often just a few hundred kilometers up—and follow an almost circular path called a low-Earth orbit (LEO). Since they have to be moving very fast to overcome Earth’s gravity, and they have a relatively small orbit (because they’re so close), they cover large areas of the planet quite quickly and never stay over one part of Earth for more than a few minutes. Some follow what’s called a polar orbit, passing over both the North and South poles in a “loop” taking just over an hour and a half to complete.
The higher up a satellite is, the longer it spends over any one part of Earth. It’s just the same as jet planes flying over your head: the slower they move through the sky, the higher up they are. A medium-Earth orbit (MEO) is about 10 times higher up than a LEO. GPS navstar satellites are in MEO orbits roughly 20,000 km (12,000 miles) above our heads and take 12 hours to “loop” the planet. Their orbits are semi-synchronous, which means that, while they’re not always exactly in the same place above our heads, they pass above the same points on the equator at the same times each day.
Many satellites have orbits at a carefully chosen distance of about 36,000 km (22,000 miles) from the surface. This “magic” position ensures they take exactly one day to orbit Earth and always return to the same position above it, at the same time of day. A high-Earth orbit like this is called geosynchronous (because it’s synchronized with Earth’s rotation) or geostationary (if the satellite stays over the same point on Earth all the time). Communications satellites—our “space mirrors”—are usually parked in geostationary orbits so their signals always reach the satellite dishes pointing up at them. Weather satellites often use geostationary orbits because they need to keep gathering cloud or rainfall images from the same broad part of Earth from hour to hour and day to day (unlike LEO scientific satellites, which gather data from many different places over a relatively short period of time, geostationary weather satellites gather their data from a smaller area over a longer period of time).