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