15-11-2012, 06:14 PM
Different Types of Satellite Orbits
Different Types of Satellite Orbits.docx (Size: 204.78 KB / Downloads: 76)
The satellite’s orbit is a critical component of its function. Basically, there are 3 kinds of satellite orbits, which depend on its position relative to the surface of the Earth. These are:
Geostationary orbit
A satellite on a geostationary or geosynchronous orbit stays in the same spot relative to Earth. It flies at an altitude of about 35,790 km, orbiting with the planet in the same direction. This is why to people on the ground, it looks like the satellite is stationary.
There are many geostationary satellites located over the Equator, which causes congestion of the area. To prevent signal interference, precise positioning of each satellite is ensured prior to launching.
Geosynchronous satellites are often used to monitor weather events or transmit television and communications signals.
Polar orbits
Satellites on polar orbits circle the planet on a near-polar inclination, maintaining an altitude of at least 700 km. These satellites pass over the equator at the same solar time every single day, which allows it to collect data consistently. A satellite flying on a polar orbit is generally on a low elevation, passing over the poles each time it revolves. This type of satellite offers the best views of the planet, particularly of areas that are often difficult to cover. Polar orbiting satellites are generally used for photography and mapping.
Asynchronous orbit
A satellite on an asynchronous orbit flies at a lower altitude of approximately 644 km, passing over the planet during different times. They may be used for a variety of functions, such as photography, mapping, observing environmental changes and locating deposits of important minerals.
Predicting satellite orbits
Satellites utilize different light-sensitive sensors to calculate their positions. These calculations are then transmitted to a ground station. There is a type of software that may be used to predict and locate an orbiting satellite. Using Keplerian data, the software can forecast the location of a satellite at a particular location.
Satellite Orbits
There is only one main force acting on a satellite when it is in orbit, and that is the gravitational force exerted on the satellite by the Earth. This force is constantly pulling the satellite towards the centre of the Earth.
A satellite doesn't fall straight down to the Earth because of its velocity. Throughout a satellites orbit there is a perfect balance between the gravitational force due to the Earth, and the centripetal force necessary to maintain the orbit of the satellite.
The formula for centripetal force is: F = (mv2)/r
The formula for the gravitational force between two bodies of mass M and m is (GMm)/r2
The most common type of satellite orbit is the geostationary orbit. This is described in more detail below, but is a type of orbit where the satellite is over the same point of Earth always. It moves around the Earth at the same angular speed that the Earth rotates on its axis.
We can use our formulae above to work out characteristics of the orbit.
(mv2/r) = (GMm)/r2
=> v2/r = (GM)/r2
Now, v = (2πr)/T.
=> (((2πr)/T)2)/r = (GM)/r2
=> (4π2r)/T2 = (GM)/r2
=> r3 = (GMT2)/4π2
We know that T is one day, since this is the period of the Earth. This is 8.64 x 104 seconds.
We also know that M is the mass of the Earth, which is 6 x 1024 kg.
Lastly, we know that G (Newton's Gravitational Constant) is 6.67 x 10-11 m3/kg.s2
So we can work out r.
r3 = 7.57 x 1022
Therefore, r = 4.23 x 107 = 42,300 km.
So the orbital radius required for a geostationary, or geosynchronous orbit is 42,300km. Since the radius of the Earth is 6378 km the height of the geostationary orbit above the Earth's surface is ~36000 km.
There are many different types of orbits used for satellite telecommunications, the geostationary orbit described above is just one of them. Outlined below are the most commonly used satellite orbits. The orbits are sometimes described by their inclination - this is the angle between the orbital plane and the equatorial plane.
Geostationary Orbit
The most common orbit used for satellite communications is the geostationary orbit (GEO). This is the orbit described above – the rotational period is equal to that of the Earth. The orbit has zero inclination so is an equatorial orbit (located directly above the equator). The satellite and the Earth move together so a GEO satellite appears as a fixed point in the sky from the Earth.
The advantages of such an orbit are that no tracking is required from the ground station since the satellite appears at a fixed position in the sky. The satellite can also provide continuous operation in the area of visibility of the satellite. Many communications satellites travel in geostationary orbits, including those that relay TV signals into our homes.
However, due to their distance from Earth GEO satellites have a signal delay of around 0.24 seconds for the complete send and receive path. This can be a problem with telephony or data transmission. Also, since they are in an equatorial orbit, the angle of elevation decreases as the latitude or longitude difference increases between the satellite and earth station. Low elevation angles can be a particular problem to mobile communications.