Geostationary satellite how many

The second Lagrange point is about the same distance from the Earth, but is located behind the Earth. Earth is always between the second Lagrange point and the Sun.

Since the Sun and Earth are in a single line, satellites at this location only need one heat shield to block heat and light from the Sun and Earth. The third Lagrange point is opposite the Earth on the other side of the Sun so that the Sun is always between it and Earth. A satellite in this position would not be able to communicate with Earth. Closer to the Earth, satellites in a medium Earth orbit move more quickly. Two medium Earth orbits are notable: the semi-synchronous orbit and the Molniya orbit.

The semi-synchronous orbit is a near-circular orbit low eccentricity 26, kilometers from the center of the Earth about 20, kilometers above the surface. A satellite at this height takes 12 hours to complete an orbit. As the satellite moves, the Earth rotates underneath it. In hours, the satellite crosses over the same two spots on the equator every day. This orbit is consistent and highly predictable. The second common medium Earth orbit is the Molniya orbit.

Invented by the Russians, the Molniya orbit works well for observing high latitudes. The Molniya orbit offers a useful alternative. The Molniya orbit is highly eccentric: the satellite moves in an extreme ellipse with the Earth close to one edge. As it moves away, its speed slows, so it spends more time at the top of its orbit farthest from the Earth. A satellite in a Molniya orbit takes 12 hours to complete its orbit, but it spends about two-thirds of that time over one hemisphere.

Like a semi-synchronous orbit, a satellite in the Molniya orbit passes over the same path every 24 hours. This type of orbit is useful for communications in the far north or south.

Most scientific satellites and many weather satellites are in a nearly circular, low Earth orbit. Therefore, it has a relatively low inclination 35 degrees , staying near the equator.

In this highly inclined orbit, the satellite moves around the Earth from pole to pole, taking about 99 minutes to complete an orbit. During one half of the orbit, the satellite views the daytime side of the Earth. This is a large benefit for the military. If, for example, the United States is concerned about activities in a certain region of the world — or it wants to see how its troops are doing — a geosynchronous orbit allows constant pictures and other surveillance of one particular region.

Joining a "constellation" of four other WGS satellites, it extends the military's communications system to provide blanket coverage over virtually the entire planet. The network serves troops, ships, drones and civilian leaders and is supposed to provide communications for ground personnel.

Communications for civilians also benefit from geosynchronous orbit. There are numerous companies that provide telephone, Internet, television and other services from satellites in that orbital slot. Because the satellite is constantly hovering over one spot on the ground, communications from that location are reliable as long as the satellite is well connected to the location you want to communicate with.

According to Satellite Signals , there are satellites in geosynchronous orbit. However, there are obvious space and technological limitations. The International Telecommunication Union assigns slots for geosynchronous orbit and settles disputes between countries about slots. Similarly, it is considered good practice to move almost-dead satellites into a "graveyard" orbit above geosynchronous orbit before they run out of fuel, to clear the way for the next generation. This means there are more available routes for satellites in LEO, which is one of the reasons why LEO is a very commonly used orbit.

It is the orbit most commonly used for satellite imaging, as being near the surface allows it to take images of higher resolution. It is also the orbit used for the International Space Station ISS , as it is easier for astronauts to travel to and from it at a shorter distance. Satellites in this orbit travel at a speed of around 7. However, individual LEO satellites are less useful for tasks such as telecommunication, because they move so fast across the sky and therefore require a lot of effort to track from ground stations.

Instead, communications satellites in LEO often work as part of a large combination or constellation, of multiple satellites to give constant coverage. This lets them cover large areas of Earth simultaneously by working together. It is similar to LEO in that it also does not need to take specific paths around Earth, and it is used by a variety of satellites with many different applications. It is very commonly used by navigation satellites, like the European Galileo system pictured.

Galileo powers navigation communications across Europe, and is used for many types of navigation, from tracking large jumbo jets to getting directions to your smartphone.

Galileo uses a constellation of multiple satellites to provide coverage across large parts of the world all at once. Satellites in polar orbits usually travel past Earth from north to south rather than from west to east, passing roughly over Earth's poles. Satellites in a polar orbit do not have to pass the North and South Pole precisely; even a deviation within 20 to 30 degrees is still classed as a polar orbit. Polar orbits are a type of low Earth orbit, as they are at low altitudes between to km.

Sun-synchronous orbit SSO is a particular kind of polar orbit. Satellites in SSO, travelling over the polar regions, are synchronous with the Sun. This means that the satellite always visits the same spot at the same local time — for example, passing the city of Paris every day at noon exactly. This means that the satellite will always observe a point on the Earth as if constantly at the same time of the day, which serves a number of applications; for example, it means that scientists and those who use the satellite images can compare how somewhere changes over time.

This is because, if you want to monitor an area by taking a series of images of a certain place across many days, weeks, months, or even years, then it would not be very helpful to compare somewhere at midnight and then at midday — you need to take each picture as similarly as the previous picture as possible.

Therefore, scientists use image series like these to investigate how weather patterns emerge, to help predict weather or storms; when monitoring emergencies like forest fires or flooding; or to accumulate data on long-term problems like deforestation or rising sea levels. Often, satellites in SSO are synchronised so that they are in constant dawn or dusk — this is because by constantly riding a sunset or sunrise, they will never have the Sun at an angle where the Earth shadows them.

A satellite in a Sun-synchronous orbit would usually be at an altitude of between to km. At km, it will be travelling at a speed of approximately 7. Transfer orbits are a special kind of orbit used to get from one orbit to another. When satellites are launched from Earth and carried to space with launch vehicles such as Ariane 5, the satellites are not always placed directly on their final orbit.

Often, the satellites are instead placed on a transfer orbit: an orbit where, by using relatively little energy from built-in motors, the satellite or spacecraft can move from one orbit to another. This allows a satellite to reach, for example, a high-altitude orbit like GEO without actually needing the launch vehicle to go all the way to this altitude, which would require more effort — this is like taking a shortcut.

Reaching GEO in this way is an example of one of the most common transfer orbits, called the geostationary transfer orbit GTO. Orbits have different eccentricities — a measure of how circular round or elliptical squashed an orbit is.