-In the previous episode, I introduced the engineering system notion. I also mentioned compromises. Precisely, when a satellite communications system is designed, there are a few very important physics relations that will bring the previously mentioned constraints. I will try to explain these relations in a simple way, without mathematics, in this episode. In order to give a sense of magnitude to this episode, I entitled it "The laws of satellite communications". There are three laws. Let us now see what they are. The first is called "communication rectangle". The communication rectangle refers to a digital communication. That is to say the transmission of 0s and 1s. This communication, this bit transmission of 0s and 1s, can be characterized by four intrinsically linked factors. The first factor is throughput. That is to say the number of 0s and 1s I can pass through per second. Of course, the higher the throughput, the better for us. The second factor is the energy dedicated to this communication. How much it costs in terms of energy. Instinctively, we suspect that the more we reduce energy, the better. The third factor is communication reliability. Reliability is sending a 0 in the air, and receiving and interpreting this as a 0. The same goes for 1s. Of course, the less errors I make, the more my communication is reliable, and the better it is. The last factor is the communication bandwidth. Let us imagine a FM station emitting between 100 and 100.1 MHz. It uses a 0.1-MHz bandwidth, thus 100 kHz. The bandwidth is the frequency range I am using. Again, the less bandwidth I use, the better. I said that these four factors were linked. Indeed. Let us take the example of energy dedicated to communications. Let us consider a reduction of this energy while keeping a fixed throughput. Automatically, the communication reliability will decrease. Once again. I reduce the energy while keeping my throughput fixed, reliability will decrease. Conversely, if I increase my bandwidth I can choose to increase the communication throughput or to increase the communication reliability. I can also mix both. This is the rectangle law. We will often refer to this law because it is extremely scaling in the case of satellite communications. First law, the rectangle law. Second law, the "square of the distance" law. It says that 1 km by foot is hard, but that 2 km by foot is four times harder. It means that when we establish a digital communication at a certain distance, it will require energy. If I double this distance, it will require four times more energy. In other words, if I were to write this as a mathematical formula, the distance factor would be squared to calculate the equivalent energy. This is the second law, the distance law. Let us get to the third and last law, "the antenna parabola". To understand this law, we will use an analogy, and compare a transmitting or receiving antenna to a spray. There are two kinds of antennas. The antenna or spray on the left has a low directivity. This means that when we send energy, or water as illustrated, it will cover a large zone with this energy, thus water. However, for each surface unit covered, we will receive less energy. If I now compare it to the spray on the right, which has a high directivity, it covers a small surface but the good news is that for each surface unit we receive more energy. Remember the previous rectangle law, receiving a lot of energy is promising such as getting a high throughput. This antenna parabola law is important for satellite communications. We see that we can choose between a highly directive parabolic antenna, but we will only be able to target a small zone with a lot of energy, or we want to cover a very large zone. Then we will need an antenna with very little directivity. In satellite communications, we often use satellite dishes. Here is a small tip. The bigger a dish, the more directive it is. Thus, having a large dish is very handy since it allows a lot of energy to be concentrated on a small surface. But the bad news is that if the zone we want to cover moves, then the dish will also need to follow the target.
