3. Orbits - Low Earth Orbit

Equally important to the design of a satellite is its orbit. This is the path it takes around the Earth.

There are two very popular paths. Polar orbit - where the satellite travels over the North and South Poles and the other is 'Equatorial' orbit where the satellite moves around the equator. A satellite in polar orbit will be able to scan the entire planet, as the Earth rotates beneath it.

LEO - Low Earth Orbit

Many satellites are only a few hundred kilometers above the planet. These are called 'Low Earth Orbit' (LEO) satellites.

From the ground they appear to move rapidly from one horizon to the other. They have some advantages over satellites further away.

• Being close to the ground, the on-board sensors see the Earth in great detail. This is why many weather satellites are in LEO orbit. Many scientific satellites use this orbit.
• They need little power to transmit data as they as so close to the Earth. So they can be small and thus cheaper to launch.
• There is very little time delay between a signal being sent and received. This low delay makes 'real-time' communication much easier. For instance many military satellites take advantage of the low delay to communicate with soldiers in the field. A signal to a satellite in geostationary orbit takes over a quarter of a second to travel there and back.

The one issue with LEO satellites is that you need a lot of them if you want constant coverage over the same spot. As one disappears then another needs to come up above the horizon. In many cases though, this does not matter.