Re: what are satellites use for

Hi Jonathan!

NASA maintain J-Track - a program which allows you to see the locations of several hundred real satellites in orbit. The 3-D model is here.

If you take a few moments to look at this model, you can see that satellites fall into three main categories. The most obvious of these is a band of satellites situated around the equator, approximately 36000km up. At this altitude these take 24 hours to complete one orbit (they are said to be geosynchronous), and if situated directly over the equator (with no "inclination" or tilt) they are known as geostationary satellites. The Earth rotates at the same rate and therefore from the ground, these geostationary satellites appear fixed in the sky.

Geostationary satellites are telecommunication and data transmission satellites. You can see that they tend to cluster at the equator at longitudes which match major population areas: the Americas, Europe and the Middle East, India, etc. As the receiver on the ground doesn't have to track the satellite (by moving their dish over time) much commercial television is now broadcast from orbit.

Below these geostationary satellites, and situated about 2/3rds of the way out from the Earth, is a group of Global Positioning System satellites. These are arranged in inclined orbits, in order to provide GPS users on the Earth with a number of satellites over them at any one time. These days much navigation and positional information is gained from GPS measurements.

The bulk of the Earth's satellites are situated in Low Earth Orbit (often known as LEO), just a few hundred kilometres over the Earth. Here manned missions, such as the ISS and space shuttle, operate.

In highly inclined orbits - ones that pass near the poles - there are surveillance and weather satellites. Other satellites monitor land use, climate change, ozone depletion and the like. These are usually referred to as Earth Resources satellites. The other class of LEO satellites include those that look outwards, not inwards. The Hubble Space Telescope is a prime example of one of these.

An excellent website giving details of how to spot many satellites from the ground is at Heavens Above.

Regards,

Andy Goddard