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The first man made
signal from outer space.

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Sputnik was the first artificial
Earth satellite launched by

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the Soviet Union on October 4th, 1957.

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Its main spherical body was surrounded
by four external radio antennas,

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which transmitted a signal.

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This signal is thus the first man
made signal sent from outer space.

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Although, Sputnik was in orbit for
three months, its signal only lasted for

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22 days,
at which point it ran out of power.

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Amateur radio operators could detect
the signal all over the world.

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So, just imagine you're one of
these radio operators back in 1957.

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All excited to participate
in this historic moment.

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And let us listen to the received signal.

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[SOUND].

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The transmitted signal was
just a sequence of beeps,

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which is what we can in fact hear.

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We also perceive a large
amount of background noise,

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which is due both to the type
of equipment of the day.

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But also, because of the background noise
that is natural in such recordings.

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Let us have a look at the signal.

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This plot shows only two
seconds of the audio recording.

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We clearly see the beep,

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as well as the noise contained
in the signal between the beeps.

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In class, we also studied another tool,

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namely the Fourier transform, which
allows representing a signal in terms of

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the different frequency
components it contains.

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It is just a change of point of view, and

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the operation can be inverted to
recover exactly the original signal.

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We are plotting the magnitude
of the Fourier Transform of

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the signal transmitted by Sputnik.

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There is a large component
at omega is equal to 0,

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the origin, which corresponds to
the constant component of the signal.

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This is the so-called DC
component in electronics.

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We also observe two small peaks,

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which correspond to the frequency
of the transmitted beeps.

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Fourier presentations are defined on
the interval minus pi to plus pi, and

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are two pi periodic.

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However, it might not be clear how the
concept of discrete frequency relates to

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the more intuitive one
of continuous frequency.

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The latter are easy to understand
from our daily life and

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expressed in one over seconds, or hertz.

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The continuous frequency f and

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the discrete frequency omega
are related by the following formula.

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Where fs is a sampling
rate of the signal that is

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the frequency at which
we measure the signal.

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This link and the explanation of this
formula will be made clearer when we

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study the chapter on sampling and
interpolation.

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We see that the two peaks appear
now at plus minus 1653 hertz.

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Let us further study what happens when
moving from time to frequency domain.

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In the time domain let us
simply fire our signal and

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model as a product between a train
of square pulses and a sinusoid.

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So, the square pulse here turns on and
off,

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a sinusoid here,
by simply multiplying the two signals.

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Now let us study what happens
in the frequency domain.

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The train of pulses is a periodic
signal whose DFT consist of deltas

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placed at the multiples of the fundamental
frequency at which the pulse oscillates.

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So, the FT of the sinusoid consists of two

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deltas placed at the positive and negative
digital frequency of the sinusoid.

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Moreover, as seen in the lecture
when we studied the properties of

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the Fourier transform,

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the product in time domain correspond
to a convolution in frequency domain.

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Thus, we obtain a DFT that looks
like the figure at the bottom.

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So, we convulve this spectrum with this
one, and this replicates the little

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spectrum here at the two locations
of the xerox given by the sinusoid.

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How does this compare
with our original signal?

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First, notice that the frequency
of the sinusoid is

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much higher than the frequency
of the train of pulses.

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So, the axles of pulses
are going to be very small and

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masked by the overall noise.

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So, two main peaks at 16 63 hertz
correspond to the derack of the sinusoid.

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The transmitted signal did
not contain any particle or

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information, so what makes it such
an important scientific milestone?

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Its first impact lies in the fact that
it demonstrated the possibility of

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satellite communications.

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It would take another 10 years to become
reality, whereas it is ubiquitous today.

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The successful launch of Sputnik
also arose at the time of

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intense tension between the Soviet Union
and the USA, called the Cold War.

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With the successful launch of Sputnik,
the Soviet Union clearly demonstrated it's

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scientific advance, which led to a crisis
in the US called the Sputnik Crisis.

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So, US government started intensely
funding research and education programs in

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engineering and sciences to
catch up with the Soviet Union.

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This program culminated ten years
later with the Apollo program and

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the US being the first
country to land on the moon.