-We have now reached the fifth and penultimate week of this MOOC. We have talked a lot about space, about satellites, about the way the signal travels through space, now this week it is time to focus on what happens on Earth. Remember the division into segments that we saw during week 2. We talked about the ground segment, the space segment, and the control segment. This week, we will focus on the ground segment, and we will leave the control segment aside, because it is a bit too specific. About the ground segment, we will see two things in particular. We will talk about terminals, and we will talk about gateways. So this sequence focuses on terminals, we will see what a terminal is made up of and what the different categories of terminals are, and then next week we will focus on gateways. Several types of terminals exist, and we will see how to differentiate between these terminal types, and what they are made up of. To differentiate the terminal types, we will look at two factors. The first factor is called the form factor. This factor, basically the weight and volume of the terminal, determines several categories. The first one is the light terminal. The light terminal fits in one hand, and can be carried in a pocket. Then we have the portable terminal, it can also be carried with one hand, but its format is more like a suitcase or a big laptop. Finally we have the transportable terminal. Transporting this terminal requires more substantial means, such as several people or even a lifting truck. Terminals from these three categories can be transported from one point to another before being used, and we also have one last category, fixed terminals, which typically cannot be transported or moved. There is another category, which is not based on the form factor, but rather on when the terminal is being used, the mobile terminal. Indeed, the four terminals I have presented, the four categories of form factors, need to be transported before they can be used. The mobile terminal, however, can be used even during transport. This is it for the form factor aspects. The second characteristic that will be used to differentiate the terminal types is of course the communication capacity, that we have already discussed at length. We can imagine that there is a relationship between the form factor on the one hand, and the communication capacity on the other hand. Indeed, the illustration shows that a light terminal will have a rather limited communication capacity. This can be understood intuitively, remember when we were talking about antennas and their size. We know that a larger antenna will generally have a higher gain, we know that a high gain contributes to the link budget, and allows for a greater throughput and a large communication capacity. However, a light terminal is bound to have a much smaller antenna, and we can therefore imagine that its gain will be lower. This is it for the various relationships between the form factor and the communication capacity. Let us now see what a terminal is made up of. To discuss the general architecture of a terminal, we will come back to the communication chain. Generally speaking, there are two communication chains, one for transmission, and one for reception. Each chain is made up of several blocks. Take for example the satellite phone. We always start with the services block, used to dial a number with the keyboard, or convert the voice captured by the microphone into a digital signal. This data is then sent to the network block, which arranges all this into bit packets. These packets carry the user's voice, and control information such as the number to call, for example. As they reach the modulator block, the packets are transformed into symbols, depending on the wave shape that is used. These symbols are transformed into energy by the electronics of the transmitter, and this energy is then sent into the air by the antenna. In the other direction, the signal coming from the antenna, so everything that is received, follows the exact reverse path, through the receiver and through the demodulator. This is an example for a telephony service, but it is frequent for terminals to host several services, for example telephony and data exchange. And in this general architecture, the limit between the hardware and the software is more or less at the modulator-demodulator. A relatively recent technological innovation, called software-defined radio, tends to push this software-hardware limit toward the left-hand side of the illustration, thus greatly reducing the hardware part. This is made possible only by the increasing power of processors, which are able to perform, by means of software, tasks that up until now had to be implemented as hardware, that is to say as circuit boards, in order to be performed.
