Satellites
Since the 1950's thousands of satellites have been launched and most remain in orbit to the present day. This means that the space around the Earth is a complex tangle of working or broken satellites and debris. The Image below shows a representation of the satellites and debris over 10 cm in orbit as of 2013:-
Types of Orbit
There are Two main types of Orbit an artificial satellite can be placed in:-
1. Low-Earth Orbit - Orbital Period ~ 90 minutes
2. Geostationary Orbit - Orbital Period = 1 day, Orbital Altitude = 36,000 km
Low Earth Orbit
Low Earth Orbit is the location of most satellites in orbit. Most Earth-observation (weather etc.) satellites and spy satellites are found in this region, as being close to the Earth allows them to observe the Earth to a very high resolution. The International Space Station is also found in Low-Earth orbit (~400 km altitude).
Geostationary Orbit
Although most satellites are found in Low-Earth orbit, there is a large group of satellites are a much further distance from Earth. At a distance of 35,786 km from the Earth's surface, these satellites form a ring around the Earth's equator as can be seen in the satellite tracking image below:-
Geostationary orbit is useful due to the period of the satellite's orbit at this altitude. It takes exactly 1 day for the satellite to orbit the Earth.
This means that for an observer on the Earth, a Geostationary satellite will appear stationary in the sky.
Direct (Line of Sight) Communication
In order to send signals over short distances, it is possible to send a direct signal. Over larger distances, however, the curve of the Earth can prevent 'Line of Sight' communication.
The effect of the Earth's curvature can be seen when ship appear to 'fall off the edge' over the horizon :
In the above photo, no ship is sinking or falling off an edge, they are simply at different distances from the viewer, as with the wind turbine example above.
Sattelite Communication
In order to send a signal to anywhere on Earth, Satellites can be used. A group of satellites positioned correctly around the Earth can recieved and send signals to the entire planet :
