Satellite TV Signals

Photo courtesy DirecTV Small-dish satellite system

Large-dish satellite antennas pick off unencoded or encoded signals being beamed to Earth by satellites. First, you point the dish to a particular satellite, and then you select a particular channel it is transmitting. The set-top box receives the signal, decodes it if necessary and then sends it to channel 3 or 4.Small-dish satellite systems are digital. The TV programs are encoded in MPEG-2 format and transmitted to Earth. The set-top box does a lot of work to decode MPEG-2, then converts it to a standard analog TV signal and sends it to your TV on channel 3 or 4. See How Satellite TV Works to learn more.

How do satellites orbit the earth?

Satellites are, to some degree, “mysterious” objects. They travel in space, which feels like an exotic place because most of us have never been there. They are so far away that we cannot see them. They usually cost millions or billions of dollars, which means none of us will ever own one personally. And so on…Orbital mechanics can also be mysterious because there is no easy way for us to experience orbital mechanics personally. However, with a little imagination, you can understand the basic idea behind orbital mechanics very easily. It turns out that we play with orbital mechanics all the time!

Think about what happens when you throw a ball. Imagine that you are standing in a big field and throw a baseball as hard as you can — like a pitcher. The ball might go 100 feet (30 meters) and then hit the ground. You put the ball in orbit — It’s just that a ball’s orbit is very short!

Now imagine that you shot a rifle straight and level instead of throwing a ball. The bullet might travel a mile (1.6 km) before succumbing to gravity and hitting the ground.

Now imagine that you shoot a very large cannon that is able to give its shell an extremely high initial velocity. Also imagine that our world is completely covered in water to remove any worries about hills, and that the cannon is shot straight and level. Its path might look like this:

One thing that gums these examples up is air resistance, so imagine that you took this cannon to the moon and mounted it on top of the highest mountain. The moon has no atmosphere and is completely surrounded by the vacuum of space. If you adjusted the speed of the shell just right and shot the cannon, the shell would follow the curve of the moon perfectly. It would fall at exactly the same rate that the curve of the moon falls away from it, so it would never hit the ground. Eventually it would curve all the way around the moon and ram right into the back of the cannon! On the moon you could actually have satellites in extremely low orbits like that — just a mile or two off the ground to avoid the mountains. And satellites could conceivably be launched from cannons.

On earth, it’s not so easy because satellites have to get up above the atmosphere and into the vacuum of space to orbit for any length of time. 200 miles (320 km) up is about the minimum to avoid atmospheric interference. The Hubble space telescope orbits at an altitude of 380 miles (600 km) or so. But the principle is exactly the same. The speed of the satellite is adjusted so that it falls to earth at the same rate that the curve of the earth falls away from the satellite. The satellite is perpetually falling, but it never hits the ground!

Our ancestors had to go to pretty extreme measures to keep from getting lost. They erected monumental landmarks, laboriously drafted detailed maps and learned to read the stars in the night sky.

Things are much, much easier today. For less than $100, you can get a pocket-sized gadget that will tell you exactly where you are on Earth at any moment. As long as you have a GPS receiver and a clear view of the sky, you’ll never be lost again.

In this article, we’ll find out how these handy guides pull off this amazing trick. As we’ll see, the Global Positioning System is vast, expensive and involves a lot of technical ingenuity, but the fundamental concepts at work are quite simple and intuitive.

What Causes Space Junk?

Debris in orbit can come from many sources:

• Exploding rockets - This leaves behind the most debris in space.
• The slip of an astronaut’s hand - Suppose an astronaut doing repair in space drops a wrench — it’s gone forever. The wrench then goes into orbit, probably at a speed of something like 6 miles per second. If the wrench hits any vehicle carrying a human crew, the results could be disastrous. Larger objects like a space station make a larger target for space junk, and so are at greater risk.
• Jettisoned items - Parts of launch canisters, camera lens caps, etc.

Items initially placed into high orbits stay in space the longest.The European Space Agency tracks more than 7,500 orbiting items with a width of 4 inches (10 cm) or more. Space debris may also be a reason why space shuttles typically orbit with their windows to the rear. This protects the astronauts onboard, at least to some degree.

A special NASA satellite called Long Duration Exposure Facility (LDEF) was put in orbit to study the long-term effects of collisions with space junk. The LDEF was later brought back to Earth via a space shuttle for analysis.

For more information on satellites and related topics, check out the links on the next page.

What is AMSAT?

AMSAT is a non-profit organization of ham radio operators worldwide that uses its own membership-supported satellites. The official name for AMSAT is the Radio Amateur Satellite Corporation. Hams that belong to AMSAT participate in:

• The actual development and assembly of over 40 satellites to date
• Ground control after the satellite is in orbit
• Conversations using the satellite and listening to others using the satellite as a radio relay link

Photo courtesy AMSAT Electronic modules in equipment bay of the Phase 3D AMSAT satellite: At top left is camera equipment; along the bottom row are various receivers. (Launch date: late July 2000, on Ariane 507)

AMSAT satellites can often be heard by use of a short-wave receiver or a radio scanner. Ham operators make use of the satellites during natural disasters when terrestrial links and cell phone systems may be down or overloaded.

The AMSAT-built satellites “hitch” a rocket launch on a “payload-space-available” basis. The first AMSAT satellite orbited in 1961 and was called OSCAR (Orbiting Satellite Carrying Amateur Radio). Tracking software is available for personal computers. Various AMSAT satellites have a combination of data, image and voice capabilities.

How Can I See an Overhead Satellite?

This satellite tracking Web site shows how you can see a satellite overhead, thanks to the German Space Operations Center. You will need your coordinates for longitude and latitude, available from the USGS Mapping Information Web site or at the website Topozone.

• Satellite-tracking software is available for predicting orbit passes. Note the exact times.
• Use binoculars on a clear night when there is not a bright moon.
• Ensure that your watch is set to exactly match a known time standard.
• A north-south orbit often indicates a spy satellite!

How Much Do Satellites Cost?

Satellite launches don’t always go well, as shown by this story on failed launches in 1999. There is a great deal at stake. For example, this hurricane-watch satellite mission cost $290 million. This missile-warning satellite cost $682 million.Another important factor with satellites is the cost of the launch. According to this report, a satellite launch can cost anywhere between $50 million and $400 million. A shuttle mission pushes toward half a billion dollars (a shuttle mission could easily carry several satellites into orbit). You can see that building a satellite, getting it into orbit and then maintaining it from the ground control facility is a major financial endeavor!

Satellite Altitudes

Looking up from Earth, satellites are orbiting overhead in various bands of altitude. It’s interesting to think of satellites in terms of how near or far they are from us. Proceeding roughly from the nearest to the farthest, here are the types of satellites whizzing around Earth:80 to 1,200 miles - Asynchronous Orbits

Photo courtesy USGS The island of Manhattan in New York City (Central Park at the top)

Observation satellites, typically orbiting at altitudes from 300 to 600 miles (480 to 970 km), are used for tasks like photography. Observation satellites such as the Landsat 7 perform tasks such as:

• Mapping
• Ice and sand movement
• Locating environmental situations (such as disappearing rainforests)
• Locating mineral deposits
• Finding crop problems

Search-and-rescue satellites act as relay stations to rebroadcast emergency radio-beacon signals from a downed aircraft or ship in trouble.The Space Shuttle is the familiar manned satellite, usually with a fixed duration and number of orbits. Manned missions often have the task of repairing existing expensive satellites or building future space stations.

Photo courtesy NASA The “glass cockpit” on Space Shuttle Atlantis, March 1, 2000

Teledesic, with the financial backing of Bill Gates, promises broadband (high-speed) communications using many planned low Earth orbiting (LEO) satellites.

3,000 to 6,000 miles - Asynchronous Orbits
Science satellites are sometimes in altitudes of 3,000 to 6,000 miles (4,800 to 9,700 km). They send their research data to Earth via radio telemetry signals. Scientific satellite applications include:

• Researching plants and animals
• Earth science, such as monitoring volcanoes
• Tracking wildlife
• Astronomy, using the Infrared Astronomy Satellite
• Physics, by NASA’s future study of microgravity and the current Ulysses Mission studying solar physics

6,000 to 12,000 miles - Asynchronous Orbits
For navigation, the U.S. Department of Defense built the Global Positioning System, or GPS. The GPS uses satellites at altitudes of 6,000 to 12,000 miles to determine the exact location of the receiver. The GPS receiver may be located:

• In a ship at sea
• In another spacecraft
• In an airplane
• In an automobile
• In your pocket

As consumer prices for GPS receivers come down, the familiar paper map may face tough competition. No more getting lost leaving the rental car agency at an unfamiliar airport!

• The U.S. military and the forces of allied nations used more than 9,000 GPS receivers during Operation Desert Storm.
• The National Oceanic and Atmospheric Administration (NOAA) used GPS to measure the exact height of the Washington Monument.

Photo courtesy NASA Taken while the Clementine spacecraft was orbiting the moon

Photo courtesy NASA Advanced Communications Technology Satellite, launched in 1993, used multiple antennas for narrow-beam transmissions.

22,223 Miles - Geostationary Orbits
Weather forecasts visually bombard us each day with images from weather satellites, typically 22,223 miles over the equator. You can directly receive many of the actual satellite images using radio receivers and special personal-computer software. Many countries use weather satellites for their weather forecasting and storm observations.

Data, television, image and some telephone transmissions are routinely received and rebroadcast by communications satellites. Typical satellite telephone links have 550 to 650 milliseconds of round-trip delay that contribute to consumer dissatisfaction with this type of long-distance carrier. It takes the voice communications that long to travel all the way up to the satellite and back to Earth. The round-trip delay forces many to use telephone conversations via satellite only when no other links exist. Currently, voice over the Internet is experiencing a similar delay problem, but in this case due to digital compression and bandwidth limitations rather than distance.

Communications satellites are essentially radio relay stations in space. Satellite dishes get smaller as satellites get more powerful transmitters with focused radio “footprints” and gain-type antennas. Subcarriers on these same satellites carry:

• Press agency news feeds
• Stock market, business and other financial information
• International radio broadcasters moving from short-wave to (or supplementing their short-wave broadcasts with) satellite feeds using microwave uplink feeds
• Global television, such as CNN and the BBC
• Digital radio for CD-quality audio

What Are the Types of Satellite Orbits?

There are three basic kinds of orbits, depending on the satellite’s position relative to Earth’s surface:

• Geostationary orbits (also called geosynchronous or synchronous) are orbits in which the satellite is always positioned over the same spot on Earth. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The “satellite parking strip” area over the equator is becoming congested with several hundred television, weather and communication satellites! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite’s signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.
• The scheduled Space Shuttles use a much lower, asynchronous orbit, which means they pass overhead at different times of the day. Other satellites in asynchronous orbits average about 400 miles (644 km) in altitude.
• In a polar orbit, the satellite generally flies at a low altitude and passes over the planet’s poles on each revolution. The polar orbit remains fixed in space as Earth rotates inside the orbit. As a result, much of Earth passes under a satellite in a polar orbit. Because polar orbits achieve excellent coverage of the planet, they are often used for satellites that do mapping and photography.

How Are Satellite Orbits Predicted?
Special satellite software, available for personal computers, predicts satellite orbits. The software uses Keplerian data to forecast each orbit and shows when a satellite will be overhead. The latest “Keps” are available on the Internet for amateur radio satellites, too.

Satellites use a variety of light-sensitive sensors to determine their position. The satellite transmits its position to the ground station.

What is Inside a Typical Satellite?

Satellites come in all shapes and sizes and play a variety of roles. For example:

• Weather satellites help meteorologists predict the weather or see what’s happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth’s weather, either from fixed geostationary positions or from polar orbits.
• Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder — a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.
• Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
• Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.
• Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.
• Rescue satellites respond to radio distress signals (read this page for details).
• Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.
• Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. Applications may include:
  • Relaying encrypted communications
  • Nuclear monitoring
  • Observing enemy movements
  • Early warning of missile launches
  • Eavesdropping on terrestrial radio links
  • Radar imaging
  • Photography (using what are essentially large telescopes that take pictures of militarily interesting areas)

Despite the significant differences between all of these satellites, they have several things in common. For example:

• 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 the use of fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets (read this page for details). 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 of them 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.The Hubble Space Telescope has a very elaborate control system so that the telescope can point at the same position in space for hours or days at a time (despite the fact that the telescope travels at 17,000 mph/27,359 kph!). The system contains gyroscopes, accelerometers, a reaction wheel stabilization system, thrusters and a set of sensors that watch guide stars to determine position.