Francois Luus (IBM Research|Africa), Adam Cox (IBM Watson® Data Platform), Gerald Harp & Jon Richards (SETI Institute), Graham Mackintosh
The first interplanetary eavesdropping SETI observation between two TRAPPIST-1 planets (e & f) in conjunction with Earth was conducted on April 6, 2017. The hypothesis is that an advanced civilization could have established radio-frequency communications between planets e & f, and if these planets line up with Earth (conjunction) we could “eavesdrop” on that interplanetary transmission with a sensitive radio telescope.
Can we do fast cloud-based signal processing on a very large 5 TB dataset of fast-rate TRAPPIST-1 measurements?
In this post we are bringing SETI to the cloud by using IBM® Data Science Experience to compute signal spectrograms and autocorrelation plots to look for signs of possible ETI transmissions during a TRAPPIST-1 conjunction.
The TRAPPIST-1 System
The star in the TRAPPIST-1 solar system is an ultra-cool red dwarf that hosts at least 7 temperate planets, more than any other known planetary system, so its recent discovery enjoyed widespread media attention. The harmoniously resonant planetary orbits ensure endured orbital stability, so this is an especially interesting exoplanetary system for SETI research.
An interplanetary transmission hypothesis is considered between planets e & f during an e-f-Earth conjunction as shown in the side-figure. The Search for Extraterrestrial Intelligence (SETI) predominantly focuses on electromagnetic signal inspection and analysis. Exoplanets in the habitable or “Goldilocks Zone” that are neither too cold nor to hot make for prime SETI targets, which is doubly true for TRAPPIST-1 as interesting multi-planet hypotheses can be investigated for its many habitable planets. A pixel map radio image of TRAPPIST-1 at 2.84 GHz from the Allen Telescope Array is shown in the below figure.
An alignment simulation of TRAPPIST-1 during the measurement period is shown in the figure below, aiming for an alignment between Earth, planet f (orange) and planet e (green). The approximated alignment occurs at 15h53 UTC time, but the hypothetical interplanetary signal from planet f (orange) is likely occluded by planet e (green), so we measure for some time before and after the conjunction.
In reality we don’t necessarily know the exact moment when the transmitter associated with planet f is lined up with the receiver on planet e. The transmitter/receiver could be in orbit around those planets. Having said that, we can put some limits on a reasonable allowed angle range that may be interesting. Since we know the orbits of e and f, we should be able to work out the angle between a line between the planets and our observation direction.
Crudely, we would expect that this angle would be no more than a few degrees with reasonable assumptions. For example, say that you’re willing to accept signals as potentially interesting if the transmitter were in orbit with radius Rf from the center of planet f, and the receiver is within an orbital radius of Re around planet e. A reasonable upper limit for Re and Rf might be, say, the orbital radius of our moon w.r.t. Earth, Rf = Re = Rmoon.
For such a choice, can we work out the range of times when we might have seen an alignment? In other words, do we have to be within 1 second of the time of best alignment? Or would the duration of allowed times be an hour long? Probably something in between, though for the purposes of this analysis the centre of the conjunction is approximated as 15h53 UTC time.
The SETI Institute commandeered the Allen Telescope Array (ATA) at the Hat Creek Radio Observatory to eavesdrop on the hypothetical TRAPPIST-1 e/f interplanetary broadband transmission, simultaneously measuring at 2.84 GHz and 8.2 GHz centre frequencies from 15h34 UTC to 19h17 UTC on 6 April 2017. These are the frequencies we humans use for spacecraft communications, but they are unusual since SETI observations normally look at the 1 GHz so-called “waterhole”. Note the rich set of possible interplanetary transmissions between the TRAPPIST-1 planets, some of which could be present in the measurements, such as between the second inner-most planet c and planet f (orange).
The ATA backend has a 104 MHz bandwidth and two correlators are used, correlator 1 at 8.2 GHz with a 0.43° field of view (FWHM) and correlator 2 at 2.84 GHz with a complete field of view of 1.2°. Beam 1 is focused to a field of view of 0.012° at 8.2 GHz centre frequency with 0.1 GHz bandwidth, and beam 2 is focused to 0.035° at 2.84 GHz centre frequency with 0.1 GHz bandwidth, each beam producing 5 TB for 6.5 Hr measurements.
One missing piece is to give a sensitivity limit for the ATA or more likely, the minimum transmitter strength on planet f that we could possibly see. It is a straightforward calculation if you know the sensitivity of the ATA beam. A good rule of thumb is that in a single sample, the point-source flux density equivalent to the system noise is around 1000 Jy, or 10^-23 watts / meter squared / Hz. 1000 Jy sounds like a lot, but if we average a million points, then the noise level is reduced to 1 Jy (factor is sqrt(Nsamples)). For one of the 67 MB blocks of data, that number is probably even smaller.
Once we know the minimum detectable flux for our observations we can work backwards to the strength of the transmitter using just the inverse square law and how far away TRAPPIST-1 is from Earth. There are a number of subtleties to consider here, so this calculation is omitted in this article.
IBM Data Science Experience (DSX) features a deployment of Apache Spark on IBM Cloud, with an optimized high-speed SoftLayer Object Store interconnect to enable Big Data Analytics on the cloud. This Spark compute cluster is accessed through DSX’s Jupyter notebook interface to perform a variety of signal processing operations on the SETI measurements.
Editor’s note: DSX was recently rebranded to “IBM Watson Studio,” which adds APIs and more deep-learning goodness to the platform.
The two ATA beam measurements are approximately 2.5 TB each, and have been uploaded from the ATA backend directly to SoftLayer Object Store at faster-than TCP/IP speeds with Aspera® (an IBM company) transmission technology. Hadoop-style segmentation of these measurements into same-sized segments are ideal for signal processing computations, such as Fourier Transforms and autocorrelations, using Spark parallelization.
The Hadoop-style segment sizes are 67,043,328 bytes each fitting sequentially together to represent one continuous measurement. Each segment consists of separate 4,160 byte packets, each packet with a 64-bit header containing a timestamp. The packet body contains alternating 8-bit real and imaginary measurement/voltage components, which are subsequently stored in a complex numpy array together with the starting packet timestamp for the segment.
A Fast Fourier Transform (FFT) is the primary operation performed on these 67 MB segments, in order to directly inspect signal frequency content over time. Note that for a 104 MHz sampling bandwidth, the FFT width needs to at least be in the same magnitude order, which is chosen as 67 million FFT elements in this case. If the FFT width was 10,000 elements, for example, then the FFT resolution would be too coarse to adequately measure power at finer frequency intervals.
Average total power is calculated over the 67 MB segments, as well as spectrogram and auto-correlation waterfalls to obtain complementary views of signal activity. The spectrogram shows basic power distribution over frequency and time, whereas autocorrelation plots can indicate more specific signal-pattern activity.
Spark’s MapReduce roughly performed all three computations at 1 sec/segment for the 67 MB segments, so the computation for 38,000 segments (2.5 TB) in one beam took approximately 11 hours with an Enterprise Instance on IBM Data Science Experience.
The Power and the Waterfalls
The average total power of approximately 34 million complex voltage values per segment is calculated for all segments, which gives us the power of the radio signals as a function of time in the below graph. We’re looking for significant increases in radio signal power around the time of the conjunction (indicated by e-f-Earth), but there are no notable power fluctuations directly around the conjunction time.
Each of the approximately 38,000 segments of 67 MB size represents a line/row in the waterfall of signal content over frequency and time. Each line indicates a new timestep, with a sequential series of timesteps forming the waterfall, which depicts the signal power at different frequencies over time.
The pairwise waterfall plot below shows the signal frequency content of the two beams synchronized over the horizontal time axis. Since this is a long 3-hour waterfall, the time axis is split into multiple sections to allow for the visualization at a higher time resolution. Higher pixel values in a waterfall indicates that there was relatively more signal power at that specific frequency and time, and we have to inspect the waterfall during critical conjunction periods to look for evidence of such higher power signals.
The above waterfall plots have significant signal activity at 15h35–15h40 (beam 1), 17h02–17h17 (both beams), around 17h19–17h30 (both beams), around 17h47–17h51 (both beams), and around 18h20–18h23 (beam 2). During the actual approximate conjunction time there is signal activity at f=~25 MHz (0.24=f/104 MHz) in beam 1, with a pattern of activity around that frequency generally appearing from 15h46 to 16h27. All of these relatively high power signals are most likely signal activity of man-made origin, as it is in the same power range as other likely RFI activity.
Autocorrelation is the correspondence of a signal with a delayed version of itself as a function of the delay period, such that fixed-period signal components will become pronounced in the result. This could amplify more complex multipart signal components in a way that a normal FFT-based spectrogram can not, so the autocorrelation is complementary to the FFT.
As with the previous waterfall plot, the pairwise autocorrelation plot for the two synchronized beams is also given for an extended timeline broken into separate rows. Note that only the positive delays (y-axis) of the symmetric autocorrelation plot is used and the 8-bit values are scaled between 0 and 1 in the colormap. We are looking for thin vertical stripes that stand out from the background, which could indicate that there were fixed-period signal components at the specific timestep and thus a likely sign of a deliberate comms signal.
Some interesting auto-correlations occur at 16h35–16h46 (beam 1), 17h20–17h31 (beam 1), around 17h50 (beam 2), and around 18h21 (both beams). Note the periodic component autocorrelations in beam 2 around 17h50 in the side figure. Unfortunately, no obvious autocorrelation patterns are seen directly around the conjunction time of 15h53 for the specific plot calibration.
What if radio transmissions are made by an ETI between planets e & f in the habitable TRAPPIST-1 system, and the Allen Telescope Array is sensitive enough to eavesdrop on the transmission during an TRAPPIST-1 e-f-Earth conjunction? Maybe the ETI is resident on planet f and has a satellite orbiting planet e and conducts radio-spectrum electromagnetic communications, like we do from Earth with our satellites orbiting Mars or Jupiter.
This interplanetary eavesdropping hypothesis has been investigated for the 6 April 2017 TRAPPIST-1 e-f-Earth conjunction, conducted by the SETI Institute using its ATA radio telescope, with the subsequent signal analysis performed on IBM Data Science Experience. The Spark MapReduce capability of DSX was leveraged for expedient FFT-based signal processing to do an initial inspection of spectral/autocorrelation content in 5 TB of recordings.
While there does appear to be signal activity in parts of the measurement, there is nothing apparent at either 2.84 GHz or 8.2 GHz directly around the conjunction instance at 15h53 UTC on 6 April 2017. Since these are also the frequencies used for our comms with our own satellites, the signal activity that is seen is likely to be human-made and/or radio-frequency interference.
TRAPPIST-1 with its 7 temperate planets and stable/fast planetary orbits is a solar system rich with SETI hypothesis exploration, because of numerous other interplanetary conjunctions and planet/star occultations. So TRAPPIST-1 will be a focal point of SETI research for many years to come, but for now its inhabitants wish to remain hidden.