How do Satellites work？ ｜ ICT #10

We live our lives knowing that many satellites orbit our planet every day, and that they are helping us in several ways. You might be surprised to know that there are almost 4,900 satellites orbiting the Earth. The most obvious questions that come to mind are, why are these satellites in totally different orbits? How does a satellite carry out all of its functions? And what are the components inside them which help them to accomplish all of their allotted tasks? Let's explore the answers to all these questions in detail. It's a well-known fact that a satellite stays in orbit because of the balance between gravitational pull and centrifugal force. The angular velocity of the satellite is decided by the force balance equation that balances the gravitational and centrifugal forces. When the satellite is deployed, it is given sufficient speed to balance these two forces. A satellite near to Earth requires more speed to resist the gravitational pull than the ones located further from the Earth. Due to the negligible resistance in space, satellites never lose speed. This means satellites will continue their circular motion around the Earth without any external energy source. Satellites are placed either in low-Earth orbit, medium-Earth orbit, or geosynchronous-Earth orbit. These three orbits are illustrated here. We will get into more details of them later. There is an interesting region in space called the Van Allen Belt, a region full of highly energetic charged particles, which could seriously damage the electronics section of a satellite. Generally, it is preferred not to park satellites in the Van Allen Belt. The decision on what orbit is to be chosen for placing the satellite depends on the application and purpose of the satellite. If the satellite is built for Earth observation, weather forecasting, geographic area surveying, satellite phone calls, etc., then orbits closer to the Earth are chosen. LEO is the closest to the Earth at an altitude of between 160 and 2,000 km, and its orbital period is approximately 1.5 hours, but these types of satellite cover less area of the Earth, so many satellites are required to obtain global coverage. That's why, in the case of broadcasting, a high orbit such as GEO is chosen. Satellites in geosynchronous orbit are at a height of 35,786 km and rotate at the same angular speed as the Earth. It means the satellite takes exactly 23 hours, 56 minutes, and 4 seconds to complete one rotation. Within the geosynchronous orbit, there is a special category of orbit called geostationary orbit, which is concentric to the equator of the Earth. These satellites remain stationary with respect to the Earth. Due to this, geostationary satellites are the ideal choice for television broadcasting since it means you do not have to adjust the angle of your satellite dish again and again. This is the reason why the geostationary belt is so crowded with satellites, and it is managed by an international organization called ITU. Geosynchronous orbits are occupied by a few navigation satellites also. Geosatellites can cover one-third of the Earth's surface, so three satellites are sufficient to cover the entire Earth. For navigation applications such as GPS, MEO is the wise option. Even though the LEO is closest to the Earth, satellites in this orbit revolve at a very high speed. Due to this, receivers on Earth fail to carry out the navigation calculations accurately. Moreover, LEO needs a lot more satellites to cover the entire Earth. Thus, GPS satellites use MEO. In a typical GPS system, 24 satellites can cover the entire Earth, and the orbital period is 12 hours. Now, let's look at the main components of a communications satellite, along with their functions. At the heart of communications satellites are the transponders. The main task of a transponder is to change the frequency of the received signal, remove any signal noise, and amplify the signal power. On KU band satellites, the transponder converts from 14 GHz to 12 GHz, and a satellite can have 20 or more transponders. It is obvious that transponders require a great deal of electrical power to handle all of these functions. For power supply, a satellite has the options of batteries and solar panels. The solar panel is used to power the electronic equipment, but during an eclipse time, the batteries are used. You can see a sun sensor on the satellite. This sun sensor helps to angle the solar panels in the right direction, so that the maximum power can be extracted from the sun. Now, let's see how the transponder receives the input signal from the antenna. The most common antenna fix to satellites are reflector antenna. A satellite is supposed to follow its intended smooth orbit. However, the gravitational field around a satellite is not uniform, due to the unequal mass distribution of the Earth, and the presence of the Moon and the Sun. Because of this, sometimes the satellite gets displaced from its intended orbital position. This is a dangerous situation, since it will lead to a complete loss of signal. To avoid such a situation, satellites make use of thrusters. The thrusters are fired and keep the satellite in the right position. These also help satellites to avoid space junk. The fuel needed for the thrusters is saved in tanks in the satellite body. The position of the satellite and control of the thrusters are continuously monitored from Apart from the position controls, the Earth station also monitors the satellite's health and speed. This is done through tracking, telemetry, and control systems. These systems continuously send the signal to the Earth station and maintain the contact between Earth and the satellite. Generally, these signals are exchanged at different frequencies to distinguish from other communication signals. Have you ever thought what happens to a satellite when it is no longer functional or its lifespan is nearing the end? These satellites could harm other operational satellites or spacecraft. To deal with this situation, inactive satellites are transferred to the graveyard orbit by activating the thrusters. Just by increasing the rotational speed of the satellite, we will be able to transfer it to a higher radius orbit. This operation is made clear in this animation. The graveyard orbit is a few hundred kilometers above the geostationary orbit. For this operation, the thrusters consume the same amount of fuel as a satellite needs for about three months of station keeping. The satellites we have discussed so far are communication satellites. For GPS satellites, the most important components are an atomic clock and the antenna. The L-band navigation antennas used in these kinds of satellites are also illustrated here. The Earth observation satellites, which are mostly in LEO, carry various types of sensors, imagers, etc., depending on their mission. Now for some interesting information. In the visuals of the satellite in this video, you might have observed that they were covered with a gold-colored foil. What is the purpose of this foil? In fact, it is not foil, as it appears to be at first sight. If you take a cross-section of it, you can see it has a multi-layered structure. Satellites face huge temperature variations in space, where the temperatures varies from minus 150 to 200 degrees Celsius. Moreover, satellites face the issue of heavy solar radiation from the sun. This material actually acts as a shield, which protects the satellite components from the heavy temperature variations and from solar radiation. We hope that you have gained a good insight into different types of satellites and how they work from this video. Please support our educational service by clicking the support button. Thank you!