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- [Lecturer] We live our lives knowing
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that many satellites orbit
our planet every day,
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and that they are helping
us in several ways.
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You might be surprised to know
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that there are almost 4900
satellites orbiting the Earth.
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The most obvious questions
that come to mind are,
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why are these satellites in
totally different orbits?
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How does the satellite carry
out all of its functions?
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And what are the components inside them,
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which help them to accomplish
all of their allotted tasks?
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Let's explore the answers to
all these questions in detail.
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It's a well known fact that
a satellite stays in orbit
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because of the balance
between gravitational pull
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and centrifugal force.
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The angular velocity of the satellite
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is decided by the force balance equation
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that balances the gravitational
and centrifugal forces.
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When the satellite is deployed,
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it is given sufficient speed
to balance these two forces.
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A satellite near to
Earth requires more speed
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to resist the gravitational pull
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than the ones located
further from the earth.
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Due to the negligible resistance in space,
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satellites never lose speed.
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This means satellites will
continue their circular motion
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around the earth without
any external energy source.
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Satellites are placed
either in Low Earth Orbit,
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Medium Earth Orbit or
Geosynchronous Earth Orbit.
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These three orbits are illustrated here.
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We will get into more
details of them later.
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There is an interesting
region in space called
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the Van Allen belt.
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A region full of highly
energetic charged particles,
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which could seriously damage
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the electronics section of a satellite.
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Generally, it is preferred
not to park satellites
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in the Van Allen belt.
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The decision on what orbit is to be chosen
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for placing the satellite
depends on the application
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and purpose of the satellite.
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If the satellite is built
for Earth observation,
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weather forecasts,
geographic area surveying,
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satellite phone calls, et cetera,
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then orbits closer to
the earth are chosen.
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LEO is the closest to
the earth at an altitude
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of between 160 and 2000 kilometers,
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and its orbital period is
approximately 1.5 hours.
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But these types of satellite
cover less area of the earth
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so many satellites are required
to obtain global coverage.
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That's why in the case of broadcasting
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a high orbit such as GEO is chosen.
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Satellites in geosynchronous orbit
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are at a height of 35,786 kilometers
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and rotate at the same
angular speed as the earth.
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It means the satellite takes
exactly 23 hours 56 minutes
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and four seconds to complete one rotation.
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Within the geosynchronous orbit,
there is a special category
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of orbit called geostationary orbit,
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which is concentric to
the equator of the earth.
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These satellites remain stationary
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with respect to the earth.
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Due to this, geostationary
satellites are the ideal choice
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for television broadcasting
since it means you do not have
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to adjust the angle of your
satellite dish again and again.
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This is the reason why
the geostationary belt
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is so crowded with
satellites, and it is managed
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by an international
organization called ITU.
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Geosynchronous orbits are occupied
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by a few navigation satellites also.
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GEO satellites can cover one
third of the Earth's surface,
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so three satellites are sufficient
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to cover the entire Earth.
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For navigation applications such as GPS,
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MEO is the wise option.
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Even though the LEO is
closest to the earth,
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satellites in this orbit
revolve at a very high speed.
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Due to this, receivers on
earth fail to carry out
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the navigation calculations accurately.
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Moreover, LEO needs a lot more satellites
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to cover the entire Earth,
thus, GPS satellites use MEO.
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In a typical GPS system,
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24 satellites can cover the entire earth
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and the orbital period 12 hours.
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Now let's look at the main components
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of a communication satellite
along with their functions.
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At the heart of communication satellites
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are the transponders.
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The main task of a transponder
is to change the frequency
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of the received signal,
remove any signal noise
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and amplify the signal power.
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On KU band satellites,
the transponder converts
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from 14 gigahertz to 12 gigahertz
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and the satellite can have
20 or more transponders.
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It is obvious that transponders require
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a great deal of electrical power
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to handle all of these functions.
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For power supply, a
satellite has the options
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of batteries and solar panels.
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The solar panel is used to
power the electronic equipment
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but during an eclipse time
the batteries are used.
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You can see a sun sensor on the satellite.
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This sun sensor helps to
angle the solar panels
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in the right direction,
so that the maximum power
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can be extracted from the sun.
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Now let's see how the transponder receives
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the input signal from the antenna.
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The most common antenna fixed
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to satellites are reflector antenna.
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A satellite is supposed to
follow its intended smooth orbit.
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However, the gravitational field
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around the satellite is not uniform
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due to the unequal mass
distribution of the earth
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and the presence of the moon and the sun.
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Because of this, sometimes
the satellite gets displaced
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from its intended orbital position.
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This is a dangerous situation,
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since it will lead to a
complete loss of signal.
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To avoid such a situation,
satellites make use of thrusters.
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The thrusters are fired
and keep the satellite
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in the right position.
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These also help satellites
to avoid space junk.
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The fuel needed for the thrusters
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is saved in tanks in the satellite body.
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The position of the satellite
and control of the thrusters
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are continuously monitored
from an earth station.
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Apart from the position
controls, the earth station
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also monitors the
satellite health and speed.
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This is done through tracking,
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telemetry and control systems.
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The systems continuously send the signal
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to the earth station
and maintain the contact
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between Earth and the satellite.
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Generally, these signals are exchanged
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at different frequencies to distinguish
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from other communication signals.
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Have you ever thought what
happens to a satellite
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when it is no longer functional,
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or its lifespan is nearing the end.
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These satellites could harm
other operational satellites
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or spacecraft.
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To deal with this situation,
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inactive satellites are
transferred to the graveyard orbit
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by activating the thrusters.
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Just by increasing the rotational
speed of the satellite,
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we will be able to transfer
it to a higher radius orbit.
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This operation is made
clear in this animation.
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The graveyard orbit is
a few hundred kilometers
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above the geostationary orbit.
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For this operation, the thrusters consume
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the same amount of fuel
as a satellite needs
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for about three months of station keeping.
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The satellites we have discussed so far
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are communication satellites.
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For GPS satellites, the
most important components
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are an atomic clock and the antenna.
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The L band navigation antennas
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used in these kinds of satellites
are also illustrated here.
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The Earth observation satellites
which are mostly in LEO,
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carry various types of
sensors, imagers, et cetera,
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depending on their mission.
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Now, for some interesting information.
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In the visuals of the
satellite in this video,
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you might have observed
that they were covered
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with a gold colored foil.
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What is the purpose of this foil?
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In fact, it is not foil as it
appears to be at first sight.
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If you take a cross section of it,
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you can see it has a
multi layered structure.
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Satellites face huge
temperature variations in space
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where the temperature
is varies from minus 150
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to 200 degrees Celsius.
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Moreover, satellites face the issue
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of heavy solar radiation from the sun.
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This material actually acts as a shield,
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which protects the satellite components
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from the heavy temperature variations
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and from solar radiation.
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We hope that you have
gained a good insight
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into different types of satellites
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and how they work from this video.
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