-Last week, we discovered the world of satellites. Their genesis, services they provide, and all the business around. This week, we will have a closer look at the way they operate. In order to correctly understand how a satellite operates, it is mandatory to also correctly understand how the whole system it is made of operates. The satellite itself as well as the different ground equipments. This sequence aims at examining the different elements forming the system. There will be some vocabulary. Then, at the end of the sequence, we will describe a few major physics laws which drive communications satellites' performance. You will see that these laws are extremely important. According to the divide and rule policy, we will slice a communications satellite system in several segments. Of course, there will be the space segment which includes satellites. Let us now see what these segments are made of. Honor where honor is due once again, there is the space segment. The space segment includes one or several satellites. Then, there is the ground segment. It is globally made of two equipment categories. On one side user terminals which provide users with services. On the other side, ground stations also called gateways. These gateways connect the space world and the Earth networks. Finally, there is the control segment. The role of this segment is to handle the other segments but also the service provided to users. This definition and the words I use for the different segments can be debatable. I personally follow the literature, not Albert Camus of course, but I refer to a book that I consider as being a reference, entitled "Satellite Communications Systems". It was written by Gérard Maral and Michel Bousquet. The definitions I just gave come from this book. To understand how all this works and how these different segments interact, let us see the example of satellite Internet. At the level of the terminal located in the user's or subscriber's home, if I get back to last week's common thread example, there is an outside part with the satellite dish and its head, and there is an inside part, the satellite box. This satellite box will allow communicating with the satellite, through the modem, and includes network and management equipments. This is the terminal. At the level of the ground segment, there is also the gateway. It is located in the operator's or service provider's premises. This gateway allows me to indirectly connect, via the satellite, to Internet. Then there is the control segment. The control segment handles the satellite's health but also makes sure that the service is correctly provided. It makes sure that I remain within the scope of my subscription by checking, for example, that the authorized volume is not exceeded. Indeed, remember that subscriptions for individuals for satellite Internet often depend on a monthly data volume flat rate. Let us see what happens when I request a web page on the Internet. The web page request leaves my terminal, and goes through the satellite to reach the gateway. The latter will send a request to the control segment to check if I am allowed to request this web page. Actually, it checks if I do not exceed my monthly data flat rate. The control segment gives me the green light. In that case the gateway acts as a relay at the level of Internet, and the returned web page follows exactly the same return path. It will go through the gateway and the satellite to eventually reach my equipment. This is how it works for satellite Internet access. You clearly see the role of each segment. Let us now see what happens for another application such as satellite TV broadcasting. It is much simpler. First reason, it is a one-way service if you remember. It is quite simple. The movie, or the TV stream, I request is located in the service provider's system. The service provider pushes this stream towards the satellite and it will eventually reach my terminal. You see that this is also a very simple operation. Let us now conclude with telephony. For telephony, I added a special feature. You can notice that at the level of the space segment there are several satellites. Indeed, I took as an example the Iridium satellite constellation. We talked about it during week 1, it is made of several satellites. There is another specificity. This satellite constellation has radiofrequency links between each satellite. Thus they are not forced to go through the ground to talk to each other. Let us imagine that with my terminal, illustrated on the far left, an Iridium satellite phone, I want to give a call to another subscriber who has a classic mobile phone. What happens is that my voice will go up to the first satellite, the one I can see since it is a low-orbit satellite constellation. Then, to reach the gateway, I will jump to another satellite. Now that I can see the gateway, I get down to it. The gateway will check with the control segment if my subscription is valid and if I did not exceed my flate rate. The control segment gives me a green light, and the gateway forwards my phone call through the core telephony networks to eventually reach the subscriber on the other side. When the subscriber talks to me, it goes the other way. I go through the core network again, Ïto eventually reach the gateway, go up to the satellite again, and jump from one satellite to another. Those could be different satellites depending on the elapsed time and their movement in relation to my terminal. Eventually, it goes down to the terminal. As you can see, different systems, different services, but always several segments that each play a role. The ground segment for ground stations and terminals. The control segment to handle services and other segments. Finally, the space segment which includes the different satellites.
