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Communications satellite

Satellites

by Chris Woodford. Last updated: August 5, 2016.

Eyes in the sky, space mirrors bouncing phone calls round Earth, heavenly compasses helping us home—these are just three of the things that satellites do for us. When you gaze through the clouds on a brilliant blue day, you might catch sight of a plane or two leaving vapor trails in its wake. But you're unlikely to see all the thousands of meticulously engineered satellites, some as small as your hand, some as huge as trucks, spinning in orbits high above your head. "Out of sight, out of mind" is probably one of the reasons we take satellites for granted, even though they play a crucial part in everything from TV broadcasting and transcontinental telephone calls to weather forecasting and the Internet. What exactly is a satellite and how does it work? Let's take a closer look!

Photo: A typical communications satellite from the 1980s. The blue squares are solar panels that provide power. The white circles are the sending and receiving antennas. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

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.

The Space Shuttle launching a communications satellite from its payload bay.

Photo: The Space Shuttle launches a communications satellite from its payload bay in 1984 by spinning it gyroscopically. You can see Earth to the left. Picture courtesy of NASA Johnson Space Center (NASA-JSC).

We put satellites in space to overcome the various limitations of Earth's geography—it helps us step outside our Earth-bound lives. If you want to make a phone call from the North Pole, you can fire a signal into space and back down again, using a communications satellite as a mirror to bounce the signal back to Earth and its destination. If you want to survey crops or ocean temperatures, you could do it from a plane, but a satellite can capture more data more quickly because it's higher up and further away. Similarly, if you want to drive somewhere you've never been before, you could study maps or ask random strangers for directions, or you could use signals from satellites to guide you instead. Satellites, in short, help us live within Earth's limits precisely because they themselves sit outside them.

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.

Communications

Communications satellites are essentially used to relay radio waves from one place on Earth to another, catching signals that fire up to them from a ground station (an Earth-based satellite dish), amplifying them so they have enough strength to continue (and modifying them in other ways), and then bouncing them back down to a second ground station somewhere else. Those signals can carry anything radio signals can carry on the ground, from telephone calls and Internet data to radio and TV broadcasts. Communications satellites essentially overcome the problem of sending radio waves, which shoot in straight lines, around our curved planet—intercontinental signals, in other words. They're also useful for communicating to and from remote areas where ordinary wired or wireless communications can't reach. Calling with a traditional landline (wired phone), you need a very convoluted network of wires and exchanges to make a complete physical circuit all the way from the sender to the receiver; with a cellphone, you can communicate anywhere you can get a signal, but you and the receiver both still need to be within range of cellphone masts; however, with a satellite phone, you can be on top of Mount Everest or deep in the Amazon jungle. You're entirely free from any kind of telecommunications "infrastructure," which gives you geographic freedom and an instant ability to communicate (you don't have to wait for someone to string up telephone lines or set up cellphone masts).

The best known modern communications satellite systems are probably INMARSAT and INTELSAT. INMARSAT was originally a satellite system for ships, planes, and other travelers, though it now has many other uses as well. INTELSAT is an international consortium that owns and operates several dozen communications satellites that provide things like international broadcasting and satellite broadband Internet.

How do communications satellites work?

What do they do?

Communications satellites are "space mirrors" that can help us bounce radio, TV, Internet data, and other kinds of information from one side of Earth to the other.

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).

Satellite communication across Earth using an uplink and downlink

Artwork: Communications satellites bounce signals from one side of Earth to the other, a bit like giant mirrors in space. A ground-based satellite transmitter dish (red) beams a signal to the satellite's receiving dish (yellow). The satellite boosts the signal and sends it back down to Earth from its transmitter dish (red) to a receiving dish somewhere else on Earth (yellow). Since the whole process happens using radio waves, which travel at the speed of light, a "satellite relay" of this kind usually takes no more than a few seconds, at most. The various transmitters and receivers on the satellite and on Earth are examples of antennas.

Satellites are like any other vehicle inasmuch as they have two main parts: the generic vehicle itself and the specific thing it carries (the payload) to do its unique job. The "vehicle" part of a satellite is called the bus, and it includes the outer case, the solar panels and batteries that provide power, telemetry (a remote-controlled system that sends monitoring data from the satellite to Earth and operational commands back in the other direction), rocket thrusters to keep it in position, and reflective materials or other systems ("heat pipes") to protect it from solar radiation and dissipate heat. The payload might include transponders for a communications satellite, computers and atomic clocks to generate time signals for a navigation satellite, cameras and computers to images back to digital data for a photographic satellite, and so on.

What's inside a satellite?

Labeled parts of a typical communications satellite.

These are amazingly complex and expensive machines with tons of electronic bits and pieces jammed into them, but let's not get too bogged down in the details: the basic idea is very simple. In this outside view of a typical satellite, from a patent filed in 1968 by German engineer Hans Sass (US Patent: #3,559,919: Active communication satellite), you can see all the main bits and it's easy to figure out what they do.

I've left the original numbers on the diagram and I won't bother to label them all, since some are obvious and some are duplicates of others. The most interesting bits are the fold-out solar panels that power the satellite, the sending and receiving antennas that collect signals coming up from Earth and send them back down, and the motors and engines that keep the satellite in exactly the right position at all times:

4: Large parabolic dish antenna for sending/receiving signals. (Orange)

5: Small parabolic dish antenna for sending/receiving signals. (Orange)

6: Lower solar "battery" of four solar panels. (Red)

7: Upper solar "battery" of four more solar panels. (Red)

8: Supports fold out the lower solar panels once the satellite is in orbit. (Gray-brown)

9: Supports fold out the upper solar panels. (Gray-brown)

10: Main satellite rocket motor. (Light blue)

11, 12, 15, 17: Small control engines keep the satellite in its precision position, spin, and orbit. (Green)

Artwork from US Patent: #3,559,919: Active communication satellite, courtesy of US Patent and Trademark Office.

Photography, imaging, and scientific surveying

Landsat satellite photo of Havana, Cuba

Photo: Satellite photography has revolutionized map-making. This is Havana, Cuba photographed by the Landsat satellite. Picture courtesy of NASA Landsat program.

Not so many years ago, newspapers used to run scare stories about spy satellites high in space that could read newspapers over your shoulder. These days, we all have access to satellite photos, albeit not quite that detailed: they're built into search engines like Google and Bing, and they feature routinely on the news (giving us an instant visual impression of things like disappearing rainforests or tsunami destruction) and weather forecasts. Scientific satellites work in a similar way to photographic ones but, instead of capturing simple visual images, systematically gather other kinds of data over vast areas of the globe.

There have been many interesting scientific satellite missions over the last few decades. NASA's TOPEX/Poseidon and Jason satellites, for example, have routinely measured sea levels since the early 1990s. SeaWiFS (active until 2010) scanned the color of the ocean to measure plankton and nutritional activity in the sea. As its name suggests, a weather satellite called TRMM (Tropical Rainfall Measuring Mission) monitored rain near the equator from 1997 through 2015. As of 2016, NASA listed 25 ongoing satellite missions on its website, including CALIPSO (which studies how clouds and aerosols interact); Nimbus (a long-running scientific study of weather and climate using satellite data); and, the longest-running and perhaps best known scientific satellites of all-time, Landsat, a series of eight satellites that have been continuously mapping and monitoring changes in land use across Earth since 1972.

Navigation

Finally, most of us with GPS-enabled cellphones and "sat-nav" devices in our cars are familiar with the way satellites act like sky compasses; you'll find GPS, Glonass, and similar systems discussed in much more detail in our article about satellite navigation.

Navigating with GPS on an iPhone

Photo: Where am I, where am I going, and how will I get there? Most of us with smartphones take satellite navigation for granted.

Satellite orbits

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.

Low-Earth orbits

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.

Medium-earth orbits

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.

High-Earth orbits

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).

Who invented satellites?

The Echo communications satellite pictured at NASA's Langley Research Center, 1960.

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.

Photo: Echoes of history: Designed by NASA, the Echo communications satellite was a giant mylar balloon some 30m (100ft) in diameter designed to sit in space and bounce signals back like a mirror. You can see how big it is from the size of the car and people at the bottom, which I've colored red to help you pick them out. Picture courtesy of NASA on The Commons.

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For those of you interested in deeper technical detail, here are a few representative examples of patented satellite designs from the last few decades:

Satellite images of Earth

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Woodford, Chris. (2006/2016) Satellies. Retrieved from http://www.explainthatstuff.com/satellites.html. [Accessed (Insert date here)]

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