This article describes an indoor, game-type demonstration, incorporating physics (EM wave modulation), math (binary codes), space technology, and music to show how spacecraft put information into the radio signals they send back to Earth. Beat out rhythms on drums or desks and send messages using the same principles used in space exploration. This article was originally written for and published by the International Technology Education Association in its journal 'The Technology Teacher.' It is now archived on The Space Place Web site. Educational levels: Middle school

Nancy Leon

English

Digital Library for Earth System Education

1Originally published in The Technology Teacher, April 2001, by the International Technology Education Association
The tiny spacecraft we have sent to explore our
solar system “phone home” across millions of miles of
space using only about as much electricity as the light
bulb in your refrigerator!  How do they do it?
This question has been one of the biggest that
space scientists and engineers have had to answer.
The spacecraft are very tiny, from about the size of a
washing machine to the size of a delivery truck.  (New
ones being planned are even smaller.)  Most of them
use solar panels to generate electricity from the sun.
However, none of them makes enough electricity to
operate a big, powerful transmitter that could beam a
strong radio signal back to Earth.
One part of the answer is to focus the weak signal
into a very narrow beam.  Another part of the answer
is pointing that tiny beam very accurately toward
Earth.  And another big part of the answer is the Deep
Space Network (DSN for short) of giant receiving
antennas here on Earth.
The Power of Good Listening
The DSN has three groups of antennas, spaced
more or less evenly around Earth.  One set is in Cali-
fornia, one set is in Spain, and one set is in Australia.
As the Earth turns, at least one set of antennas is
“visible” to a far-away spacecraft at all times.
DSN’s huge dish-type antennas are especially
designed to detect radio waves, and very faint ones at
that.  By the time the spacecraft signal reaches Earth, it
is so weak, you would have to collect and save the
energy from the signal for 40,000 years to have
enough energy to light a Christmas tree bulb for 1
millionth of a second!
Radio waves are like light waves, but much
longer.  Our eyes cannot see them.  Radio waves rain
down on Earth all the time from stars, galaxies, and
even some planets (Jupiter is a good example).  A little
more than 100 years ago, humans learned
how to make radio waves too.  Now we
use them to carry the signals for our
radio, TV, and other types of communi-
cation on Earth, as well as to communi-
cate with our spacecraft, whether in orbit
around Earth or way out in “deep space.”
Radio and TV stations use powerful
transmitters mounted on giant towers to
broadcast their program signals in all directions, so
that anyone within a certain distance who has a radio
or TV receiver can tune in to their programs.
Now, the next question might be, how do the
radio or TV programs here on Earth, or the pictures
and other information collected by a spacecraft in
space, get carried by the radio waves being transmit-
ted?  How can a TV show be carried invisibly (and
without a sound) through the air?  How can a picture
of Saturn or Jupiter be carried by very weak waves of
radio energy passing through millions of miles of cold,
empty space?
Speaking in Phases
This 70-meter antenna
in Madrid, Spain, is
part of the Deep Space
Network.
2Originally published in The Technology Teacher, April 2001, by the International Technology Education Association
Waves of all Shapes and Sizes
First, let’s look at what we really mean by a
wave.  A wave appears on the surface of  the ocean
when a burst of energy passes through the water.  Lots
of bursts of energy in a short time make short, choppy
waves.  Fewer bursts of energy in the same time make
longer, drawn out, more graceful waves—great for
surfing.
Light waves and radio waves are both electro-
magnetic energy.  The only difference between them is
the size of the waves.  Light waves are kind of like
tiny ripples on the surface of the ocean.  Radio waves
are more like the long, slow ocean waves that move a
whole boat up and down.  Electromagnetic energy
comes in every wavelength in between these two, as
well as waves much shorter than light waves and much
longer than the radio waves we use for communica-
tion.  As a matter of fact, we don’t know how long or
how short electromagnetic waves can be.  We know
only the ones we have eyes or instruments to detect.
We could pick any wavelength to carry informa-
tion as long as we could build an instrument that could
transmit energy through the air or through space at that
wavelength.  But it turns out that light waves and
shorter wavelengths, besides taking more energy to
transmit, get scattered and absorbed easily by Earth’s
atmosphere.  Some short wavelengths (like x-rays and
gamma rays) can’t penetrate air at all.  That’s lucky for
us, since these highly energetic waves would bombard
Earth’s surface from space and eventually kill off
every living thing!
Wave to the Winning Wave
Radio waves, however, pass through Earth’s
atmosphere very nicely, without distortion.  This
property makes them ideal for sending signals.  They
also don’t require as much energy to produce as
shorter wavelength electromagnetic waves.  And, by
the way, they don’t hurt anybody.
Frequency is another way to describe wavelength.
Frequency refers to the time it takes for two crests
(highest part of the wave) or troughs (lowest part of
the wave) in a row to pass the same point in space.
The longer the wave, the lower (or slower) the fre-
quency (because it takes longer for the wave to pass a
point).  The shorter the wave, the higher (or faster) the
frequency.
In the above drawing, more B waves than A
waves will pass by a certain point in a certain time. So
the shorter the wavelength, the higher the frequency.
TV stations and some radio stations put their
program information on the wave by adjusting the
frequency.  FM (as in FM radio stations) means fre-
quency modulation.  Modulation means changing a
radio signal so that it carries information.  Some radio
stations use amplitude modulation (AM).  Amplitude is
the height of the wave from crest to trough.
3Originally published in The Technology Teacher, April 2001, by the International Technology Education Association
Going through a Phase
Spacecraft do not use either of these modulation
methods, however.  Spacecraft use phase modulation.
Phase refers to the part of the wave passing a particu-
lar point at a particular instant.  If two waves of the
same frequency (that is, wavelength) are in phase, the
exact crest or trough of both is passing the same point
at the same time.
Two waves of the same frequency are out of
phase if different parts of their wave forms are passing
the same point at the same time.
We measure in degrees how much out of phase
two waves are.  The above two waves are 180 degrees
out of phase.  They are as far out of phase as they can
get!  Waves can be out of phase by lesser amounts.
The waves below are about 90 degrees out of phase.
So, how does a spacecraft modulate phase in
order to put information into the signal?
Simple as a Light Switch
Well, one thing you need to know is that space-
craft and computers understand only two ideas:  on
and off.  Or, put another way, one and zero.  From
many sets of ones and zeroes, all other information is
constructed, stored, and communicated.
So, to communicate all these ones and zeroes, the
spacecraft’s transmitter starts off sending its normal
wavelength or, as more commonly called, its fre-
quency.  The DSN antenna receives the signal from the
spacecraft and “locks onto” it, so that it “knows”
exactly what phase of the spacecraft’s normal wave-
length would be coming in at any given time if the
spacecraft kept transmitting only the normal frequency
with no modulation.
Then the spacecraft begins sending information
by shifting the wave just a little bit out of phase for a
certain number of wavelengths.  If, for example, the
phase of the signal wave lags 90 degrees behind its
normal beat, it might mean “1.”  If the phase of the
signal wave is 90 degrees ahead of where it would
normally be, it might mean “0.”  A “1” or a “0” is
called a bit (as in a bit of information).
We can demonstrate how this works using
sounds.  We can even send secret messages using
drumbeats!  Perhaps the original Native Americans
were the inspiration for this space-age technology.
Let’s Get Rhythmic!
For this experience, you will need some way to
make rhythmic sounds (without echoes or much
reverberation).  Perhaps your school’s music teacher
can help with this activity.  You need to be able to
make two different sounds, perhaps one low-pitched
and one higher-pitched.  Here are some example ways
to do this:
Recommended option:  For the reference fre-
quency, use a metronome or an electronic key-
board that can automatically maintain a steady
beat.  For the signal frequency, use a drum, piano
key, ruler tapping on a desk, spoon tapping on a
glass of water, or anything that can sound out a
beat.
4Originally published in The Technology Teacher, April 2001, by the International Technology Education Association
Alternative option:  If you don’t have a metro-
nome or electronic keyboard, then find a way to
make two different beat sounds, one lower and
one higher, or one louder and one softer.
The only other materials you will need are paper
and pencils, plus the table of codes in this article so
you can translate back and forth from letters or num-
bers to symbols and bits (1 or 0).
Getting in the Groove
Here’s the general idea:  The metronome or
electronic keyboard, or one of the “instruments”
(preferably the lower or louder one) is used to create
an imaginary reference frequency (or beat, in this
case).  Then, the other instrument will represent the
signal.  We need to maintain the reference beat so that
we humans can make sense of the signal beat.  If the
two instruments are beating together—that is, are in
phase—there is no information being transmitted.  If
the signal beat lags an instant behind the reference
beat for four beats, that means 1.  If the signal beat is
just an instant ahead of the reference beat for four
beats, that means 0.  For every bit (1 or 0) being
“transmitted” there must be four “in phase” beats to
signal the break between bits.
So try this:
The metronome, electronic keyboard, or the
person keeping the reference frequency (or beat) we
will call “Reff.”  The person transmitting the signal
frequency we will call “Sig.”
Reff starts a slow, steady beat—about 50-60
beats per minute.  The important thing is that it be very
regular.  If you don’t have a metronome, hope there’s a
good drummer in the class!
Now, Sig comes in on the beat for four beats.
Now, Sig beats four beats just a little behind Reff.
So, using “BUMP” to mean the reference beat and
“bump” to mean the signal beat, it sounds sort of like
BUMP-bump (pause) BUMP-bump (pause)
BUMP-bump (pause) BUMP-bump (pause)
Get used to this sound.  In our game, four of these
beats means “1.”  In the spacecraft business, each beat
(or phase-modulated wavelength) is called a symbol.
Now, after these four beats, Sig again gets back in
phase with Reff for four beats, so mostly what you’ll
hear is
BUMP (pause) BUMP (pause)
BUMP (pause) BUMP (pause)
Note that after Sig’s last beat in the “1” sequence,
he or she will have to rush the next beat to be “in
sync” with Reff.
Now, without missing a beat, Sig rushes ahead
and beats just a little before Reff for four beats, like
this:
bump-BUMP (pause) bump-BUMP (pause)
bump-BUMP (pause) bump-BUMP (pause)
Get used to this sound, too, because these four
beats mean “0.”
Now, again Sig beats in phase with Reff  for four
beats.  Note that after the last beat of the “0” sequence,
Sig has to pause a bit to get back in sync with Reff.
Altogether, this sequence will sound like
BUMP (pause)BUMP (pause)
BUMP (pause) BUMP (pause)
[1] BUMP-bump (pause) BUMP-bump (pause)
BUMP-bump (pause) BUMP-bump (pause)
BUMP (pause)BUMP (pause)
BUMP (pause) BUMP (pause)
[0] bump-BUMP (pause)  bump-BUMP (pause)
bump-BUMP (pause)  bump-BUMP (pause)
You might have to practice for a while to get the
hang of these rhythm sequences.
The Beginnings of
Communication
Now, let’s transmit information!  Here is a table
showing the standard sequences of bits used by com-
puters to mean each of the letters of the alphabet
(capitals only here) and numbers 0-9.  We already
know it takes four symbols (off-phase beats) to make a
bit.  Now, in the table, notice that it takes eight bits
(called a byte) to make a letter or number.  Since we
are using four beats (like wavelengths for a spacecraft)
5Originally published in The Technology Teacher, April 2001, by the International Technology Education Association
in our game to make a bit (plus four beats between
bits), it is going to take 64 beats to make a single
letter!  This is just how it’s done by a spacecraft.  It
might take 64 symbols to make a byte of information.
Everybody needs a copy of the conversion table.
Sig picks one letter or number, but keeps it a
secret from everyone else.
Then, with the metronome or reference beat
going, Sig tries to beat out the correct sequence of
symbols that will make the 1s and 0s that will commu-
nicate the number or letter he or she picked.
Others in the class must listen very carefully, with
pencil and paper at hand.  If you are a listener, each
time you hear four beats out of phase, write down
whether its a 1 (BUMP-bump) or a 0 (bump-BUMP).
When you have 8 bits (1s and 0s), find that sequence
on the table and you will know the letter or number
Sig was transmitting!
From 1/0 to Shakespeare!
Now, when the drummer(s) gets very good at this,
they can put together whole words, even sentences.
To signal the break between letters, you can add
four more “in phase” beats, for a total of eight.
Binary      Letter or
Code           Digit
0011 0000 0
0011 0001 1
0011 0010 2
0011 0011 3
0011 0100 4
0011 0101 5
0011 0110 6
0011 0111 7
0011 1000 8
0011 1001 9
0100 0001 A
0100 0010 B
0100 0011 C
0100 0100 D
0100 0101 E
0100 0110 F
0100 0111 G
0100 1000 H
Binary      Letter or
Code           Digit
0100 1001 I
0100 1010 J
0100 1011 K
0100 1100 L
0100 1101 M
0100 1110 N
0100 1111 O
0101 0000 P
0101 0001 Q
0101 0010 R
0101 0011 S
0101 0100 T
0101 0101 U
0101 0110 V
0101 0111 W
0101 1000 X
0101 1001 Y
0101 1010 Z
To signal a break between words, you can add
another four or eight “in phase” beats, for a total of 12
or 16.
How fast can you transmit and receive informa-
tion before there are lots of errors?
This game just begins to give you an idea how
any kind and any amount of information can be com-
municated using only two states:  1/0, on/off, yes/no.
This system is called binary (meaning 2) notation.
This is the only language computers or spacecraft
understand.
For more on binary notation and how it is used in
the computer and space business, see http://
spaceplace.jpl.nasa.gov/vgr_fact1.htm .  Also, learn
more about the Deep Space Network at http://
deepspace.jpl.nasa.gov/dsn/.  For a fun activity related
to how the DSN antennas “hear” the tiny voices of far
distant spacecraft, go to http://spaceplace.jpl.nasa.gov/
tmodact.htm .  For more on spacecraft telecommunica-
tions, see the Basics of Space Flight on-line tutorial at
http://www.jpl.nasa.gov/basics.
This article was contributed by the Jet Propul-
sion Laboratory, California Institute of Technology,
reflecting research carried out under a contract with
the National Aeronautics and space Administration.  It
was written by Diane Fisher, who is also the writer,
designer, and developer of The Space Place, a website
with fun and educational space-related activities at
http://spaceplace.jpl.nasa.gov.  Thanks to David
Doody (author of Basics of Space Flight) for brain-
storming and technical help and Alex Novati for
illustrations.


Technology Teacher: Speaking in Phases

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Technology Teacher: Speaking in Phases

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