With the Earth rotating once every 24 hours, a GPS satellite catches up to a point above the Earth approximately once a day. Relative to the center of the Earth, the satellite orbits twice in the time it takes a point on the Earth's surface to rotate once.
This can be compared to a more down-to-earth analogy of two horses on a racetrack. Horse A runs twice as fast as Horse B. They start at the same time and same position. It will take Horse A two laps to catch Horse B, which will have just completed its first lap at the time of being caught. Many telecommunications satellites are geostationary, enabling time-continuity of coverage above a chosen area, such as service to one country.
More specifically, they enable the pointing of an antenna in a fixed direction. If GPS satellites were confined to equatorial orbits, as in geostationary orbits, coverage would be greatly reduced. Furthermore, the GPS system does not use fixed antennae, so deviation from a stationary point, and therefore from an equatorial orbit, is not disadvantageous.
This is similar to someone throwing a cricket ball or baseball. The harder the ball is thrown, the further it will travel before it reaches the ground. If a player could throw the ball hard enough so that it reaches the necessary velocity, the ball would go into orbit.
It would never fall back to Earth. Throw it harder still, so that it reaches a speed of These satellites orbit Earth from west to east over the equator. Their orbital period is the same as Earth's rotation period : 24 hours. Because they move in the same direction and speed as Earth is spinning, they are always above the same location, so from the ground they don't appear to be moving. In the picture below, the yellow areas show what part of Earth each kind of satellite sees during its orbit.
Satellites are used for many purposes. Even though they are hundreds or thousands of miles out in space, satellites are part of our everyday life on Earth. They make us safer, broadcast entertainment, and make life more convenient. Without satellites, some of us couldn't watch television, or figure out how to travel from one place to another. Some of us might be endangered by bad weather that we didn't know was coming, or we might be unable to make a long-distance phone call. Satellites often affect our lives without our even realizing it.
Let's take a look at some of the jobs satellites do. Communication satellites allow television, radio, internet and telephone signals to be sent live anywhere in the world. Before satellites, long-distance transmissions were difficult and had many barriers. But with satellites, signals can be beamed from one location upward to a satellite and almost instantly be redirected down to many locations anywhere on the earth. Today, much of what you see on TV is transmitted by satellite signal.
Communication satellites allow video conferencing for businesses and classrooms. Where people live far from cities, communication satellites provide access to education and medical help that would otherwise not reach them.
Satellites even allow your parents to use a bank card to make purchases at stores or gas stations. Learn more about communication satellites. In the past, people used the positions of the sun and the stars, maps, and compasses to find their way. Today, satellite navigation systems such as GPS Global Positioning System allow people to figure out exactly where they are and how to get to where they want to go, making it almost impossible to get lost.
A system of 30 satellites circling the earth make up the Global Positioning System. The satellites constantly send out signals, your GPS receiver picks up those signals, and the distance from those satellites is calculated to determine your exact location.
Often GPS receivers are built into cars and cell phones. Other uses of GPS systems include use by military submarines for navigating under the sea, use of GPS for the treasure-hunt game of geocaching , and use of a handheld navigator that helps guide blind people to their destination. Learn more about GPS navigation satellites.
First responders rely on satellites as they help people in trouble. Satellite systems provide vital information and connect emergency response teams. Search and rescue satellites are designed to provide a way for ships at sea and planes in the air to communicate from remote areas.
These satellites locate distress beacons sent by ships, aircraft, or individuals in remote or dangerous regions. Radio distress beacons directly linked to a search and rescue satellite can lead rescuers quickly to an emergency location.
Learn more about search and rescue satellites. Earth observation satellites do lots of jobs! Because of their global view, they are able to observe large areas of Earth at one time. Satellites facing the earth enable us to better understand environmental changes on Earth. Satellite images can survey the earth and provide pictures and data about almost any region in the world - even your own house!
Without satellites, such measurements would require huge amounts of land-based inspection. Satellites provide information about oceans, land, clouds and ice. They can detect underground water and mineral sources. They provide information that helps farmers know which crops to plant, and helps city planners know how to improve traffic flow.
They monitor wildfires, volcanoes, smoke pollution, hurricanes, and oil spills so that emergency workers can respond to disasters. Weather satellites observe the earth from space to help meteorologists forecast weather patterns and track storms. Satellites are some of the best sources of data for climate change research.
They monitor ocean temperatures, sea levels, the sizes of glaciers, changes in tree cover, and gases in the atmosphere, such as ozone and carbon dioxide. Scientists can use this information to create models of climate change. Learn more about NASA's earth science missions. Satellites help scientists study space. They can provide much more data than instruments on Earth can. Satellites can see into space better than telescopes on Earth's surface because Earth's atmosphere doesn't interfere with the view from satellites.
Many of these satellites are operated by NASA and other countries' scientific organizations. Satellites that face toward space have many different jobs. Some watch for dangerous rays coming from the sun. Some explore asteroids and comets. Some fly close to or orbit other planets, looking for evidence of water on Mars or capturing pictures of Saturn's rings.
The Hubble Space Telescope orbits around Earth once every 95 minutes, at about 5 miles per second. It takes pictures of planets, stars, galaxies and black holes.
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