-As we saw, several orbit geometries are possible. What is the relation between orbit geometry and service coverage? This is precisely the topic of this episode. To illustrate this sequence, I used an open-source software called SaVi. It has been developed by Lloyd Wood. Thanks to SaVi, we can visualize a major characteristic of an orbit, service coverage. Let us start with geostationary orbits. On the illustration, we can see a satellite from the Inmarsat fleet which is at 54 degrees west, or 54 degrees west of the Greenwich meridian. Several observations can be made. First, the satellite does not fully cover Earth, neither in longitude nor in latitude. This is a purely geometrical consequence as illustrated on the right. The poles cannot be seen from the geostationary orbit because they are blocked since the Earth is round. So at some point, they are blocked by our skyline. Exactly the same phenomenon happens for longitude. However, with three geostationary satellites correctly positioned, we can fully cover Earth, at least between latitudes of 81.3 degrees north and 81.3 degrees south. This means a significant number of customers who, remember, thanks to the geostationary orbit characteristics, will only direct their antenna towards the satellite once and will not have to change its position anymore. The only problem with this orbit is the distance to the satellite, which equals 36 000 km. Remember the inverse-square law. We can already imagine that a lot of energy will be required for communications. Finally, do not forget that this 36 000-km distance to the satellite is only valid when I am directly standing below it. As soon as I get away from this point directly below the satellite, the distance increases. It can even reach 40 000 km at the border of the coverage. In the previous illustration that is back here, the coverage is said to be geometrical. Instead of geometrical, I could have said "optimistic". Let us see why. When I look at a satellite in the sky, two parameters characterize where I should look. The first is called the azimuth and can be compared to the direction on a compass. The second is called elevation and specifies the point to which the eyes must go up above the horizon. The coverage on the left assumes that the minimum elevation equals 0 degree. This means that even if the satellite is on the skyline, which happens when we are at the border of the coverage, communications can be established. In reality, it does not happen because there is always something blocking the line of sight, a building, a car, in short, an obstacle. For this reason, it is preferable to assume that the minimum elevation is higher. On the right-hand illustration, I represented the service coverage assuming a 10-degree minimum elevation. The impact is noticeable and the coverage decreased on the borders. For example, Spain is not in the service zone anymore. If you go on operators' websites such as SES, Eutelsat or Inmarsat, you can easily find maps such as this one. This is the coverage map of a satellite called Astra 1L located at 19 degrees east. First remark, the coverage is not regular at all curiously. In fact it is normal because satellite antennas were designed to only concentrate energy on interesting zones from a market point of view. Furthermore, the values you can see in the small white discs are the minimum antenna diameters, in centimeters, required to correctly receive the signal. Let us see low orbits now. As we saw, many parameters characterize an orbit. But we will focus on two parameters, inclination and altitude. We will also only consider circular orbits. On the left-hand illustration, we can see the instantaneous coverage of a satellite on an orbit with a 45-degree inclination at an altitude of 400 km. The longitude of the ascending node is 0 degree, or exactly on the vernal point. The argument of the perigee is 90 degrees and the required minimum elevation is 8.2 degrees. Those are the parameters I set. On the right-hand illustration, it is the same satellite at an altitude of 2 000 km. We clearly see that the covered zone increases. But I mentioned it on the illustration and I also said it, it is the instantaneous coverage. Do not forget that as opposed to geostationary satellites, low orbit satellites fly across the sky and their coverage zone sweeps the ground. From the point of view of the terminal, it means that, either the antennas are directive, and will have to follow the satellite across the sky, or they are not, and they will receive less energy from the satellite. Never forget the antenna parabola law. This is a video showing the ground sweeping of the service coverage. On the right-hand illustration, we show the traces of the satellite trajectory. You can notice a small shift between two passes of the satellite. This is simply due to the fact that, during the required time to reach the same point on the second pass, the Earth has revolved. Thus the satellite will never successively pass at the same place after two passes. After altitude, we will now look at the influence of inclination. On the left-hand illustration, I reused the previous figure where the orbit inclination was 45 degrees. We see that a part of the globe, as the traces show it, will never be covered. It is the zone over and below a latitude of 45 degrees. We increase the inclination up to 90 degrees to make this orbit polar. In that case the coverage is total. But of course we have to wait for the satellite to pass. However, it is mathematical, each terrestrial point will have to wait longer to see the satellite pass since the covered zone is more important while the satellite's speed has not changed. But in that case, what can we do if we wish to offer simultaneous coverage to any terrestrial point? The answer is to have several satellites. This is illustrated here with the Iridium system. Iridium is a satellite constellation. I already mentioned it. It is a satellite constellation providing services, especially telephony and small data message exchanges. It is made up of 66 satellites. The 66 satellites are organized on 6 orbital planes. As you can see on the illustration, the whole Earth is covered. Each terrestrial point sees at least one satellite, especially the yellow-colored parts, and some points on Earth, when the color is darker, see several satellites at the same time. This is what we call satellite diversity. I would like to take this opportunity to thank Lloyd once again for writing SaVi which is an extremely handy and nice program. I salute him.
