Satellites

Satellites are regularly launched into the earth’s orbit. Rockets are used to launch satellites for cell phone, television, and internet communiction services.

Very short wave radio signals (microwaves) can easily pass through the atmosphere out into space, ther are therefore used to communicate between the Earth and the satellites.

They are transmitted from a ground station to an orbiting satellite. This is usually in a geostationary orbit so that its geographical position above the Earth's surface does not alter. The satellite receives these and then, acting like a space mirror, redirects them at a receiving station on Earth. Being high in the sky, obstacles like mountain ranges do not get in the way of transmission of the signal by satellite.

Orbits

Satellites must not travel too slowly in space, or they will fall back to earth (decaying orbit!). If they travel too fast (at least 25,039 mph) they will escape earth’s orbit altogether and go out into the solar system and beyond. The velocity and altitude that allows the satellite to remain in a geostationary orbit has to be calculated - the satellite stays fixed in the same spot over the earth by orbiting once a day. A satellite in geostationary orbit appears to remain in the same spot in the sky all the time. Really, it is simply travelling at exactly the same speed as the Earth is rotating below it, but it looks like it is staying still regardless of the direction in which it travels, east or west. A satellite in geostationary orbit is very high up, therefore, they are also known as high orbits. This kind of orbit allows communications companies to know exactly where their satellites are in order to build a more efficient network.

It is not possible to orbit a satellite which is stationary over a point which is not on the equator. This limitation is not serious, however, since most of the earth's surface is visible from geostationary orbit. In fact, a single geostationary satellite can see 42 percent of the earth's surface and a constellation of geostationary satellites can see all of the earth's surface between 81° S and 81° N.The advantage of a satellite in a geostationary orbit is that it remains stationary relative to the earth's surface. This makes it an ideal orbit for communications since it will not be necessary to track the satellite to determine where to point an antenna. However, there are some disadvantages:

• the long distance between the satellite and the ground. With sufficient power or a large enough antenna, though, this limitation can be overcome.
• the fact that there is only one geostationary orbit presents a more serious limitation. There are only so many slots into which geostationary satellites can be placed.

A geosynchronous orbit - is not geostationary - it orbits at a slight incline around the Earth. A geosynchronous one has the period of one day (like the geostationary ones) but the shape of the orbit can be elliptical - geostationary ones have to be circular and stationed over the equator.Polar orbits

These are useful for spy satellites and weather satellites. On every pass around the Earth,they pass over both the north and south poles. Therefore, as the Earth rotates to the east underneath the satellite which is travelling north and south, it can cover the entire Earth's surface. A polar orbiting satellite typically covers the entire globe every 14 days.