1. Using the GAVRT Radio Telescope: The SETI Project
Lesson 2: Masking Interference
Questions? Please contact Carolyn

2. 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.
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3. How is data represented on a waterfall plot?
If a radio telescope looks at one point in the sky that is emitting at 4MHz (frequency) for a quick instant, then the waterfall plot would have a point at 4MHz and almost zero seconds.
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4. How is data represented on a waterfall plot?
If a radio telescope looks at one point in the sky that is emitting at 4MHz (frequency) for one second, then the waterfall plot would have a line at 4MHz from 0 seconds to 1 second.
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5. How is data represented on a waterfall plot?
What would the waterfall plot look like if the telescope looked at the 4MHz source for ten seconds?
Think about it, then click to see if you were right.
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6. How is data represented on a waterfall plot?
If the source was emitting at two frequencies (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.
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7. How is data represented on a waterfall plot?
If the source was emitting at two frequencies (4 and 8 MHz), and
the radio telescope was observing at all frequencies between 1 and 10 MHz, and
the telescope looked at the source for 5 seconds and then looked away from the source for 5 seconds
then the waterfall plot would look like this.
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8. How is data represented on a waterfall plot?
If the source was emitting at two frequencies (4 and 8 MHz), and
the radio telescope was observing at all frequencies between 1 and 10 MHz, and
the telescope looked at the source for 2 seconds and then looked away from the source for 2 seconds (repeating that pattern 3 times)
then the waterfall plot would look like this.
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9. How is data represented on a waterfall plot?
Same scenario BUT
there is a source of Radio Frequency Interference (RFI) (at 2, 5, and 7 MHz) that comes from every direction,
then the waterfall plot would look like this.
RFI is often caused by media broadcasting (radio and television) from commercial satellites in geostationary orbit and cell phone towers.
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10. How does a waterfall plot help us do a search for extraterrestrial intelligence?
If there are any alien civilizations trying to communicate with us, we have to separate their signals from RFI.
Using the waterfall plot, we can isolate a real signal (that only lasts a short time) from RFI (that is always present).
The telescope needs to scan a large area of sky and verifying signals by looking at the same piece of sky repeatedly.
We need to eliminate the frequencies dominated by RFIs from our scan.
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11. How to isolate a real signal?
The telescope scans the sky in this pattern:
left to right (which takes ten minutes),
then it goes up and then right to left (which takes ten minutes),
then it goes down (not all the way back to the beginning) and goes left to right again.
This pattern allows the telescope to cross a potential source at least two times.
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12. How to isolate a real signal?
This scan and signal pattern would look like this (if source was 120 MHz):
*This scan has NO RFI.
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13. How to isolate a real signal?
This would be a typical scan, with several RFI.
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14. Generating a Waterfall Plot
Go to gi-bin/seti/seti.cgi
You should see a page like this (you may have to scroll to see everything):
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15. Generating a Waterfall Plot
If “Display markers at strongest hits” is “on”, circles on the waterfall plot appear where candidate signals are. You do not need this information at this time; you may turn markers off for now and turn them back on when you analyze the plot. (But you can leave them on, if you prefer.)
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16. Generating a Waterfall Plot
At the bottom of the page, there is a dropdown menu to choose a data source.
Find data source spc00223_ _133934
(When this PowerPoint was made, that was the second to last source in the list.)
If that one is not available, find one similar (that has bright vertical lines).
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17. Eliminating RFI
In this example, the first vertical line is about 50 MHz.
Let’s “zoom in” on that line to get the exact frequency.
Enter a frequency range of 40 to 60 MHz, then click on Update Plot (at the bottom of the page). 40 60
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18. Eliminating RFI Your scan should look something like this.
Enter a frequency range of 49 to 51 MHz, then click on Update Plot (at the bottom of the page).
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19. Eliminating RFI Your scan should look something like this.
Rather than one RFI, we really have four. We are going to work on ONE AT A TIME!
Enter a frequency range of 50 to 51 MHz, then click on Update Plot (at the bottom of the page).
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20. Eliminating RFI Your scan should look something like this.
Enter a frequency range of 50 to 50.5 MHz, then click on Update Plot (at the bottom of the page).
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21. Eliminating RFI Your scan should look something like this.
We are going to focus on the strongest RFI first (the brightest one).
Enter a frequency range of to 50.1 MHz, then click on Update Plot (at the bottom of the page).
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22. Eliminating RFI Your scan should look something like this.
Enter a frequency range of to MHz, then click on Update Plot (at the bottom of the page).
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23. Eliminating RFI Your scan should look something like this.
Enter a frequency range of to MHz, then click on Update Plot (at the bottom of the page).
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24. Eliminating RFI Your scan should look something like this.
Enter a frequency range of to MHz, then click on Update Plot (at the bottom of the page).
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25. Eliminating RFI Your scan should look something like this.
Enter a frequency range of to MHz, then click on Update Plot (at the bottom of the page).
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26. Eliminating RFI Your scan should look something like this.
When you cannot get the RFI frequency range any more precise, we need to mask the frequency.
Enter the start and end frequency and the start and end time and choose to “set masked data to minimum color scale value”, then update plot. *
3500
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27. Eliminating RFI Your scan should look something like this.
Going to end time didn’t quite get the entire line…
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28. Eliminating RFI Your scan should look something like this.
Going to end time didn’t quite get the entire line…
…so I masked a second time, using the same frequencies, but doing time of 3500 to 3600.
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29. Eliminating RFI Now my scan looks like this! One RFI is masked.
Changing your frequency to 0 to 200 MHz gets you back to the full scan.
Enter 0 to 200 for your frequency and click update plot.
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30. Eliminating RFI
Remember the RFI at 50 MHz was really four RFIs… that is why you still see the line at 50 MHz.
Now, mask another RFI!
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