What is the distance of the orbit of a geostationary satellite from earths surface what is its utility?

A geosynchronous orbit is a high Earth orbit that allows satellites to match Earth's rotation. Located at 22,236 miles (35,786 kilometers) above Earth's equator, this position is a valuable spot for monitoring weather, communications and surveillance.

“Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south,” NASA wrote on its Earth Observatory website.

Satellites are designed to orbit Earth in one of three basic orbits defined by their distance from the planet: low Earth orbit, medium Earth orbit or high Earth orbit. The higher a satellite is above Earth (or any other world for that matter), the slower it moves. This is because of the effect of Earth's gravity; it pulls more strongly at satellites that are closer to its center than satellites that are farther away.

So a satellite at low Earth orbit — such as the International Space Station, at roughly 250 miles (400 km) — will move over the surface, seeing different regions at different times of day. Those at medium Earth orbit (between about 2,000 and 35,780 km, or 1,242 and 22,232 miles) move more slowly, allowing for more detailed studies of a region. At geosynchronous orbit, however, the orbital period of the satellite matches the orbit of the Earth (roughly 24 hours), and the satellite appears virtually still over one spot; it stays at the same longitude, but its orbit may be tilted, or inclined, a few degrees north or south.

This image depicts the geostationary equatorial orbit in which most communications and weather satellites are located. (Image credit: Smithsonian National Air and Space Museum )

Benefits

A satellite in geosynchronous orbit can see one spot of the planet almost all of the time. For Earth observation, this allows the satellite to look at how much a region changes over months or years. The drawback is the satellite is limited to a small parcel of ground; if a natural disaster happens elsewhere, for example, the satellite won't be able to move there due to fuel requirements.

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. An example of this is the United States' Wideband Global SATCOM 5, which launched in 2013. 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.

Orbital competition

According to Satellite Signals, there are 402 satellites in geosynchronous orbit. At geosynchronous orbit, the “ring” around Earth can accommodate a number of satellites — 1,800 altogether, according to one analysis by Lawrence Roberts, published in the Berkeley Technology Law Review. However, there are obvious space and technological limitations.

Specifically, satellites must remain in a very confined area and not drift too far from their assigned “slot” above Earth; otherwise they may pose a threat to other satellites. 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.

The satellites must also be located far enough away from each other so their communications don't interfere with each other, which could mean a separation of anything between 1 and 3 degrees. As technology has improved, it's possible to pack more satellites into a smaller spot.

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Geostationary orbits of 36,000km from the Earth's equator are best known for the many satellites used for various forms of telecommunication, including television. Signals from these satellites can be sent all the way round the world. Telecommunication needs to "see" their satellite all time and hence it must remain stationary in the same positions relative to the Earth's surface.

Meteosat Second Generation has a geostationary orbit

A stationary satellite provides the advantage for remote sensing that it always views the Earth from the same perspective, which means that it can record the same image at brief intervals. This arrangement is particularly useful for observations of weather conditions. One disadvantage of geostationary orbits is the great distance to the Earth, which reduces the achievable spatial resolution.

Meteosat and other satellites in geostationary orbit

There are a number of weather satellites evenly distributed in geostationary orbit all around the world to provide a global view.

Orbital motion depends on the nature of orbit

An object’s orbital motion depends on its velocity, mass of object it is orbiting around and the radius of orbit.

Greater orbital velocity is required for smaller orbits and when orbiting planets of heavier mass. This implicates on greater energy need and consumption as greater velocity can only be achieved and maintained by greater fuel consumption. Ultimately, this means satellites in bigger orbit paths need to carry more fuel otherwise they are destined to have short orbit time.

Satellites often encounter orbital decay whereby they are unable to maintain the stable orbit and as such fall back down onto Earth’s surface. There are couple of reasons for this:

As indicated by Newton’s law of gravitation, smaller orbital radius also causes the distance between two masses (e.g. Earth and satellite) to become smaller. This causes the gravitational force and acceleration experienced by the satellite to increase. This is a problem for satellites with low altitude orbits as it contributes to orbital decay

In the real world, orbiting in space is heavily associated with air resistance due to the presence of atmospheric particles. As a result, during a satellite’s motion, friction significantly contributes to orbital decay. This is more of a concern for low-Earth orbits as the atmosphere is typically denser at low altitudes

Depletion of fuel leads to loss of velocity which eventually becomes lower than the required orbital velocity à loses stable orbit.

Types of Satellites

There are two main types of satellites: low-Earth orbit (LEO) and geostationary orbit (GEO) satellites. Geosynchronous orbit satellites are similar to geostationary with one key difference: Geostationary satellites orbit the equator whereas geosynchronous orbit at a different latitude. Geosynchronous orbit satellites are not examinable in the HSC Physics syllabus.

Comparison between types of satellites

	Low-Earth (LEO)	Geostationary (GEO)
Orbital Radius	Satellites with low attitude (<2000 km) and short orbital period.	Satellites with much higher altitude of ~35000 – 36000 km. ‘Orbits’ at the same velocity as Earth’s rotation. Thus, it remains at the exactly same spot above Earth’s surface throughout its orbit.
Total energy	Lower (gravitational potential energy is more negative)	Higher
Orbital period	Shorter	Longer. Orbital period is approximately 24 hours.
Orbital velocity	Faster	Slower
Advantage	· Closer to surface of Earth which enables higher resolution photographs and videos · Cheaper to establish due to low attitude and lower fuel requirement to reach there · Accurate signal in relation to communication	· Orbital period is in sync with that of Earth which enables several essential applications · Easier than low-Earth to maintain the orbit due to greater orbital radius (thus low orbital velocity) · Wide coverage
Disadvantage	· Harder to maintain a stable orbit due to low attitude (greater orbital velocity) · Limited coverage in relation to communication	· More expensive to establish due to high attitude, more fuel required to reach the orbit · Requires strong signal for communication · Vulnerable to sun outages (elevation in radiation, occurs twice a year)
Application & uses	· Cellular communication that only require small coverage e.g. Iridium phone systems · Spy satellites · International Space Station	· Television · Radio · Weather forecast · Cell phones