We live our lives knowing that many satellites orbit our planet everyday, 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.
The era of the satellite technology began with the launch of the satellite named Sputnik. It was a 23 inch diameter polished metal sphere, carried mainly thermometer, battery and 4 external antenna as shown in Fig:1. It was launched by the Soviet Union on 4 october, 1957 and had a life of 22 days. Today’s satellites have an average life of 10-15 years.
It’s a well-known fact that a satellite stays in orbit because of the balance between gravitational pull and centrifugal force as shown in Fig:2. 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(LEO), Medium Earth Orbit or Geosynchronous Earth Orbit. These three orbits are illustrated here in Fig:3A . We will get it 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 as shown in Fig:3B.
The decision on what orbit is to be chosen for placing the satellite depends on the application and purpose of the satellite.
Key Features of LEO
1. Altitude- 160km-2000km
2. Orbital period- 1.5 hour
3. Velocity- 7.8 km/s
4. Application- Data communication, Earth observation, military purposes
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 kms and its orbital period is approximately 1.5 hours (Fig:4). But these types of satellite cover less area of the earth; so many satellites are required to obtain global coverage.
Key Features of GEO
1. Altitude- 35786 km
2. Orbital period- 24 hours
3. Velocity- 3.07 km/s
4. Application- Television broadcasting
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 kms and rotate at the same angular speed as the earth (Fig:5A). 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. GEO satellites can cover one third of the earth's surface, so three satellites are sufficient to cover the entire earth (Fig:5B).
Key Features of MEO
1. Altitude- 2000km-35700km
2. Orbital period- 12 hour (GPS)
3. Velocity- 7.3 km/s (GPS)
4. Application- Navigation
For navigation applications such as GPS, MEO is the wise option (Fig:6A). 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 (Fig:6B).
Now, let's look at the main components of a communication satellite, along with their functions.
At the heart of communication 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 as shown in Fig:7A. If the signals being sent and received on the same frequency, can cause interference. On Ku band satellites, the transponder converts from 14 GHz to 12 GHz, and a satellite can have 20 or more transponders as shown in Fig:7B.
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 this is illustrated in Fig:8. A sun sensor is used 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 fixed to satellites are reflector antenna (Fig:9).There are two main functions of the satellite antennas. The first one is to provide services such as; television broadcasting to its users and the second one is to maintain the contact between the earth station and the satellite.
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 (Fig:10A). 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 (Fig:10B); these also help satellites to avoid space junk. The fuel needed for the thrusters is saved in tanks in the satellite body. The fuel used in most of satellites are combination of monomethylhydrazine (MMH) and nitrogen-tetroxide. The life span of the satellite depends on the amount of the fuel.
The position of the satellite and control of the thrusters are continuously monitored from an earth station (Fig:11). 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 life span 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 (Fig:12). 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 in Fig:13A. The earth observation satellites, which are mostly in LEO, carry various types of sensors, imagers etc. depending on their mission (Fig:13B).
Now for some interesting information, in the visuals of the satellite in this article you might have observed that they were covered with a gold colored foil (Fig:14A). 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 multilayered structure(Fig:14B). This multilayered structure is made of polyimide or polyester coated with aluminium. Satellites face huge temperature variations in space, where the temperature varies from -150 to 200 degree 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.