GAVRT/SETI Activity What Is a Waterfall Plot? (Lesson One

GAVRT/SETI Activity What Is a Waterfall Plot? (Lesson One
What Is a Waterfall Plot? (Lesson One)
GAVRT/SETI uses waterfall plots which are not exactly a line graph, bar, pie, or
They are a graph that shows an x-y plot with the frequency of light plotted on the
x-axis and time plotted on the y-axis. Data from the radio telescope’s scans are
complied into these graphs which in turn help us separate signals from interference.
A real signal will only last for a short period, while the longer signal, which is
interference, will always be there. The signals that may be of extraterrestrial origin
will disappear when radio telescope points in another direction.
The purpose of this lesson is for the students to be able to work with waterfall plots
to determine if the graph includes a true signal.
Required Background Knowledge:
 Read the document entitled, “What is a Waterfall Plot?”.
Watch the GAVRT/SETI Video.
Investigate waterfall plots using “SETI DSS28 Spectrometer Data” site
Students will be able to:
 Use the interactive waterfall site to eliminate false signals and isolate
possible true signals.
From the National Science Education Standards:
All students should realize:
 As a result of this activity in grades 5-8 and 9-12, all students should develop
an understanding of science as a human endeavor, nature of science, abilities
of technological design, and understanding about science and technology.
 Scientists formulate and test their explanations of nature using observation,
experiments, and theoretical and mathematical models. Although all scientific
ideas are tentative and subject to change and improvement in principle, for
most major ideas in science, there is much experimental and observational
confirmation. Those ideas are not likely to change greatly in the future.
Scientists do and have changed their ideas about nature when they
encounter new experimental evidence that does not match their existing
 In areas where active research is being pursued and in which there is not a
great deal of experimental or observational evidence and understanding, it is
normal for scientists to differ with one another about the interpretation of the
evidence or theory being considered. Different scientists might publish
conflicting experimental results or might draw different conclusions from the
same data. Ideally, scientists acknowledge such conflict and work towards
finding evidence that will resolve their disagreement.
From the Common Core State Standards for Mathematics
Eight grade should be able to use functions to model relationships between
 Construct a function to model a linear relationship between two
quantities. Determine the rate of change and initial value of the
function from a description of a relationship or from two (x, y) values,
including reading these from a table or from a graph. Interpret the rate
of change and initial value of a linear function in terms of the situation
it models, and in terms of its graph or a table of values.
 Describe qualitatively the functional relationship between two
quantities by analyzing a graph (e.g., where the function is increasing
or decreasing, linear or nonlinear). Sketch a graph that exhibits the
qualitative features of a function that has been described verbally.
Web Links for Further Investigation These should open.
What is a waterfall plot?
Waterfall plots interactive site (SETI DSS28 Spectrometer Data site)
Resource/Materials Needed:
 Computer or downloaded video, “GAVRT/SETI Video.”
 Computer or printed copies of “What Is a Waterfall Plot?”
 Computer so that groups of students are able to interact with the “SETI
DSS28 Spectrometer Data Interactive Site”
One-Computer Classroom:
If there is only one computer in the classroom, it is recommended that teachers use
an overhead, LCD, white board, or television screen to project images from the
computer onto a classroom screen. Have your students work through the SETI
DSS28 Spectrometer DATA site. You can do this by having individual students
come forth and manipulate the page or you can have small teams of students work
together to eliminate noise and discover if a signal is present. If the class has
trouble watching others operate the activity, provide other work for the class to do.
Teacher notes:
Please view the GAVRT/SETI Video, read through the document entitled “What is a
Waterfall Plot”, and investigate the interactive site before doing this activity with
your students. It is very important that your students also view the video
and read “What is a Waterfall Plot” before attempting either of the lessons.
Student Activity: Before attempting these lessons please read “What is a
Waterfall Plot” and view the GAVRT/SETI video.
Lesson 1for and selecting “Data” from the side menu.
At the top of the image you can see a representation of the data. The x-axis
represents the frequencies observed by GAVRT ranging from 1 to 200 MHz. The y –
axis shows the time axis, in this case in seconds ranging from 0 to 340 seconds.
Like the figures before you can see that as the telescope scanned the sky, the noise
sources were always constant at their various frequencies. It is your job to isolate
then and eliminate them from the plot.
To do this you will use the Minimum and Maximum on the Frequency Range to find
exactly where the noise sources are.
Looking at the image above you can see that there are some very bright streaks
around half way across the x-axis. Since the x-axis goes to 200 MHz then half of
that is 100 MHz. So let’s zoom into the region around 100 MHz by putting a
minimum frequency of 100 MHz and Maximum frequency of 120 MHz.
The web page should now look like the following figure:
You can see the very bright line towards the left side of the plot. But now having
zoomed into the image you can see that there are several other vertical lines
indicating lower levels of noise also present.
Still the first task is to eliminate the obvious noise before we go to the fainter ones
so we will estimate the range of frequencies where that bright line may be and we
will enter them into the Minimum and Maximum of the Frequencies until we have
isolated the line.
The next figure shows the range after having tried several different numbers for the
limits of the frequencies:
Here you can see that we have settled on a frequency range between 103 and 104
MHz where the noise is. So we will now mark that range as noise so that we do not
think that the signal there is coming from a real extraterrestrial source.
Now continue to do this for all the obvious noise sources until there are no
continuous vertical line/noise left.
Now go back to the original image range, frequency range between 1- 200 MHz.
You can also isolate a limited range in time as well as frequency. See that there are
a couple of short signals in the 235 to 245 second timeframe. Enter that range into
the “Time Range.”
In the above image you may notice that there may be two vertical lines. To make
the dimmer line more obvious, change the “Scale Max” to 45.
Now notice that the two lines are now very obvious because of the change in the
So by choosing different values for “Frequency Range,” “Time Range,” and the
“Scale Min” and “Max” you can find the noise and eliminate them.
Continue making adjustments with this page until you are confident that you
understand how to eliminate noise from the waterfall plot.
Additional Information:
This material should be read or covered before doing the lessons, either
completing the online activity or the matching activity.
What is a waterfall plot?
A waterfall plot is an x-y plot with frequency of light plotted on the x-axis and time
plotted on the y-axis. Follow the text below to see what a waterfall plot tells us
about observing the sky.
If a radio telescope looks at one point in the sky at one frequency, let’s say 4MHz,
for just a quick instant, and there is a source emitting radio waves at that location
then in a waterfall plot we would put a point at 4MHz and almost zero seconds.
Now what would the waterfall plot look like if the telescope stared at a 4MHz source
for 1 second? There would be a line from 0 seconds to one second at 4MHz:
Now what would the waterfall plot look like if the telescope stared at a 4MHz source
for 10 seconds? There would be a line from 0 seconds to ten seconds at the 4MHz:
But if the source was emitting at two frequencies, like 4 AND 8 MHz and the radio
telescope was observing at all frequencies between 1 and 10 MHz then the waterfall
plot would look like this:
Now let’s have the telescope look for 5 seconds at the emitting source and for 5
seconds away from the emitting source:
Notice that the lines in the graph show that the signal is detected only while the
telescope is looking at it. So there is a line lasting 5 seconds corresponding to the
time the telescope is looking at the object, and no signal while the telescope is
looking away for 5 seconds.
For the next example the telescope will look at the object for 2 seconds, and then
off of it for 2 seconds, and repeat this pattern for 3 times. Notice how that shows
up on the waterfall plot.
Now what would happen if we did the exact same thing, except this time there was
a source of Radio Frequency Interference (RFI) that came from every direction? In
this case the radio noise is at 2, 5, and 7 MHz.
So for radio emission that comes from every direction which could be from space or
from local sources like cell phone towers then the waterfall plot shows a straight
line during the whole time you are observing. This is because in the affected
channels there is always a signal.
But if the telescope is moving then only when the telescope is pointing at a radio
emitting object in the sky will there be a signal and every time the telescope
returns to that location in the sky then the signal should still be there. If the
telescope points in another direction then that signal should go away. That is why
we have the broken lines in the above waterfall plot. Those represent real radio
emitters at 4 and 8 MHz while the straight lines indicate RFI coming from all
So how does a waterfall plot help us do a Search for Extraterrestrial
Intelligence (SETI)?
If there are any alien civilizations out there and they are trying to communicate to
us using radio signals, then we have to separate their signals from RFI. The
waterfall plot shows us how we can separate RFI from real signals because if a
radio telescope is scanning across the sky then a real signal will only last a short
while but a signal that is RFI will always be there.
So what is necessary is to isolate the real signals? There are two parts to isolating
real signals. First the telescope has to scan the sky not only to cover a large area of
space but also to verify signals by looking repeatedly at the same piece of sky so
that if a signal is real, it can be double checked (that is when the telescope comes
back to the same sky location the signal would be detected again). The second part
of isolating a real signal is getting rid of frequencies dominated by RFI.
To accomplish both these goals the telescope takes the following pattern to look at
the sky:
The telescope starts at a point in the sky. Then it slowly scans a part of the sky. In
this example it takes 10 minutes to go from left to right. Then it goes up and then
tracks back for 10 minutes. And it makes a racetrack pattern on the sky. Notice
that the separation is such that if there is a signal then the telescope will cross it at
least twice (In this case 3 times). The waterfall plot for the above scan and signal
pattern would look something like this if the signal was coming at 120 MHz:
Notice that in the absence of RFI that the first three 10 minute tracks had no signal
so there is nothing . Then the telescope scanned across a signal which lasted two
minutes (red line covering 2 minutes starting at 32 minutes on the y axis) which
was emitting at 120 MHz. Then the telescope crossed the signal two more times for
a total of three.
But of course this is for a perfect case. There will always be RFI so the waterfall plot
is more likely to look like this:
But now let’s look at some real data. On the following figure you can see the
representation of data taken by the GAVRT telescope on the web page you will be
using to analyze the data and to eliminate the noise.
Notice that one of the main problems with the noise is that it is very bright while we
expect any signal that may be coming from extraterrestrials to be very weak. So to
be able to pick out the weak signals we need to eliminate the strong noisy ones.
Questions: As with all investigations, one question may give rise to more. Keep a
journal of your questions as you complete your investigation. If these questions
have not been answered when you have finished your GAVRT/SETI scan, you may
need to do further investigations using NASA websites. You are on the cutting edge
of scientific research. Your journey may need to continue after your project is
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