- [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.
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