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SETI
- Allen Telescope Array
- Optical SETI
- Interstellar Message Construction
- Project Phoenix 1995-2004
- Background Science
- History of SETI
- Center for SETI Research Team
- Other people doing SETI
- Hollywood's View of SETI
Past Observing Campaigns
Parkes, Australia
Data Display
MCSA
The quantity and variety of data produced by the TSS presents a real challenge for graphical displays.
Spectra Data
The TSS produces more than 56 million channels of spectral data every 0.7 seconds. Network bandwidths limit the data available for display to about 4,000 channels, but we need to be able to view a set of channels anywhere in the spectrum. We need to be able to examine the characteristics of signals over time. At the same time we need to be able to view the entire 10 MHz bandwidth from each TSS unit at moderate resolution. And all of this in real time, i.e., as the data are collected.
Spectrum Baseline
The MCSA computes an average of the spectrum at a resolution of 643 Hz every 14.7 seconds. This "baseline" spectrum is used to scale the data that are sent to the signal detectors. It also provides a convenient way to view the entire 10 MHz bandwidth (the MCSA "superband"). The display shown below is smoothed by a factor of 54, but the magnification buttons allow the observer to zoom-in on any section of the spectrum.
The MCSA processes the radio signal from the telescope after it has been converted to "baseband", a range of frequencies from -5 MHz to +5 MHz. Thus, the center of the processed spectrum is at 0 Hz. The effect of this is that the frequencies in the spectrum run from -5 MHz to +5 MHz with respect to the center frequency of the observation. An additional effect is that any constant voltage offsets between the various components that translate the signal from radio frequency to baseband will appear as extra power in the 0 Hz channel. (Since these voltages are constant, they "oscillate" at a frequency of zero hertz.) The 0 Hz channel appears as a spike in the center of the spectrum below. The solid curves are an overlap of blue and white displays indicating the average power (blue) and the peak power (white) detected in the left circular polarization. The dotted green and yellow curves provide the same information for the right circular polarization.
Waterfall Plot
In order to see spectral data at the 1 Hz resolution, the observer must select a small portion of the 10 MHz bandwidth for display. Data transfer rates in the computer network and the resolution of computer monitors limits the number of channels for display to about 1000 per polarization for each 10 MHz TSS unit. A 10 MHz MCSA produces the high resolution data as 15,552 groups of 924 channels, each group covering 643 Hz. These groups, called "subbands", provide a convenient breakpoint in the spectrum for display.
The "waterfall" display provides a way of showing the history of the power detected in each of the channels in a subband. Frequency increases across the screen from left to right. The power in each channel is represented by a dot with brightness proportional to power. As time passes, each new spectrum is plotted at the top of the display and previous spectra scroll down one line. A channel with a signal will tend to appear brighter than its neighbors due to the added power of the signal. Over time, the channel with the signal will form a straight line on the screen. The display below shows a test signal that is injected into the receiver system to provide an end-to-end system test.
For an example of a real signal from beyond the Solar System, see the Pioneer 10 signal.
Test Signal on the Waterfall Display The power in 924 channels (643 Hz) across the screen as dots with brightness proportional to power; about 400 spectra over about five minutes are stacked vertically, with the newest spectrum at the top.
Signal Detection Data
At the end of an observation the detectors may report on anywhere from a few to a few thousand signals. Each signal report contains the frequency, frequency drift rate, power, and in the case of a pulsed signal, the pulse length and period. We need to be able to see the distribution of signals over the entire band and be able to see the relationship of signals in terms of both frequency and drift rate.
Signal Detection Display
The main Signal Detection display screen shows a histogram of the number of signal detection reports of various types, as a function of frequency across an entire 10 MHz band. Pulse signals are plotted in blue and CW signals in red. Sections of the spectrum that have a large number of signals are classified as "bad bands" and are plotted in green (pulses) or orange (CW). The plot below shows a 10 MHz band with a significant amount of interference.
10 MHz Signal Detection Display Histogram of signal reports vs. frequency (10 MHz)
The frequency and frequency drift characteristics of individual signals can be displayed by selecting and zooming-in on signals seen in the main display. The zoomed display changes from a histogram to a scatter plot with frequency on the horizontal axis and frequency drift rate on the vertical axis. Individual signal reports are plotted according to those parameters as a colored symbol, blue for pulses and red for CW signals.
The plot below shows the components of a complex signal at full resolution. The frequency span of the plot is 250 Hz and the drift rates range from +1 Hz/sec at the top to -1 Hz/sec at the bottom.
"Zoomed" Signal Detection Display Scatter-plot of signals in drift rate vs. frequency (250 Hz)
