- [Lecturer] 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 4900
satellites orbiting the Earth. The most obvious questions
that come to mind are, why are these satellites in
totally different orbits? How does the 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 forecasts,
geographic area surveying, satellite phone calls, et cetera, then orbits closer to
the earth are chosen. LEO is the closest to
the earth at an altitude of between 160 and 2000 kilometers, 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 kilometers and rotate at the same
angular speed as the earth. It means the satellite takes
exactly 23 hours 56 minutes and four 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. 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 12 hours. 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. On KU band satellites,
the transponder converts from 14 gigahertz to 12 gigahertz and the 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 fixed to satellites are reflector antenna. A satellite is supposed to
follow its intended smooth orbit. However, the gravitational field around the 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 an earth station. Apart from the position
controls, the earth station also monitors the
satellite health and speed. This is done through tracking, telemetry and control systems. The 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, et cetera, 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 temperature
is 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.
