-In the previous episode, we saw how to characterize a satellite's orbit. In this episode, I will describe two typical orbits. Do not worry, there are many others. But with those two, we cover a vast majority of communications systems. The first is the geostationary orbit. It is the king of orbits. This orbit is circular. Its plane is merged with the equatorial plane. The satellite is at an altitude of about 36 000 kilometers. The specificity of this orbit is that at this altitude, satellites revolve at exactly the same speed as the Earth. The very interesting consequence is that, for an observer on Earth, these satellites do not move and remain a fixed point in the sky. This is the favorite orbit of service providers who want to broadcast TV or radio because once the antenna is directed towards the satellite, it does not need to be moved anymore. During week 1, I also mentioned the geosynchronous orbit. The geosynchronous orbit is a geostationary orbit but with an angle different from 0, that is to say not merged with the equatorial plane. The second orbit category I will mention are the low orbits. Most of the time they are circular. Their altitude is between 350 and 800 kilometers. Below 350 kilometers, it is tricky because there are traces of atmosphere that generate friction. Over 800 kilometers, we might face the Van Allen radiation belts that were introduced during week 1. What is the benefit of low orbits? Remember one of the laws of satellite communication systems, the inverse-square law. We know that the smaller the distance, the easier it is to communicate. Secondarily, it is also easier to observe. Think about spy satellites, for example. At these altitudes, they can take quite precise pictures. However, the disadvantage of these low orbits is that these satellites are not fixed in the sky since they do not revolve at the same speed as the Earth. These satellites are thus sometimes called moving satellites. The angle is also one of the important parameters for this kind of orbit. It will have an impact on the service coverage. We will see this during the next episodes.