Dear friends,
The anticipation for the SETI course has been ramping up with Spring classes starting this week at UCLA. There are currently 15 students enrolled in the SETI course, which is a good size for the first offering of this course – large enough to allow for work in small teams, but small enough to provide individual attention to students.
Almost all of these students have an astrophysics, physics, or engineering background. Unfortunately, we are facing an unavoidable scheduling conflict with an astronomy course that some students need to take. I used a variety of scheduling tools with the students and with a university scheduler to identify other possible class times, but we reached an impasse because there are no other times that work for everyone. I will try to solve the scheduling conflict again when the students and I meet on Wednesday.
I have been having conversations with my colleagues at the Arecibo Observatory and at the Green Bank Telescope about data-taking modes. Radio telescopes typically have a number of receivers ("frontends") and data-recording devices ("backends"). Astronomers and engineers design a variety of backends to meet specific science goals. For instance, someone interested in identifying certain molecules in the interstellar medium would prefer to use a spectrometer that splits the incoming radiation into its constituent wavelengths. The data would then be examined for evidence of spectral lines that uniquely identify these molecules. In planetary radar work, we most often use a data-recording mode in which the high-frequency radio signal (typically 2.4 or 8.6 billions of cycles per second) is shifted in frequency towards lower frequencies, filtered, and sampled. This baseband recording mode yields a time series of samples that can then be processed with maximum flexibility.
I favor baseband recording for the SETI course because students will have to think about the processing steps required to convert from this stream of samples to the desired final data product. To do so, the students will learn how to implement Fast Fourier Transform algorithms, which are essential components of the signal processing toolbox. Another advantage of the baseband mode is that it will allow students to look for a variety of signals, either in the time domain or in the frequency domain.
During my first postdoctoral position at the Arecibo Observatory, I designed a data-taking system for baseband recording. We used it primarily to record radar echoes, but we also used it to study the ionosphere, spacecraft signals, and pulsars. To obtain the best possible spatial resolution in the radar images, we wanted to sample four separate signals 20 million times per second. With traditional 8-bit analog-to-digital converters, this requirement translated into a data rate of 4 bytes x 20 million/s = 80 million bytes per second (80 MB/s). There was no way at the time to record the data to disk at this rate. The solution was to use a field programmable gate array (FPGA), i.e., a programmable electronic chip, to repackage the samples. The fastest mode of the radar backend retains the two most significant bits of each sample, packs the four 2-bit samples into a single 8-bit quantity (i.e., a byte), and writes 20 million bytes to disk per second (20 MB/s). This solution works remarkably well. The overwhelming majority of high-resolution images of asteroids obtained at Arecibo and at the Goldstone Observatory in the last 16 years have been obtained with this device or one of its clones. Other teams have built much fancier and faster backends that use similar principles for pulsar observing. We are contemplating using one of these backends for the SETI course.
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The Field Programmable Gate Array used in the radar data-taking systems installed at Arecibo, Goldstone, and Green Bank
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Images of the ~1.4 mile near-Earth asteroid 2000 ET70 acquired with the radar data-taking system
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There have been a few hiccups with the telescopes at Arecibo and Green Bank. My contact in Green Bank delivered the bad news that the baseband data-taking mode that I was favoring would not be available. We are still trying to resolve this situation and are exploring other backends as backup options. At Arecibo, the system went down for major maintenance work, and it is unclear whether the telescope will be ready for our target observing date in mid-April. We may have to delay the Arecibo observations. One the bright side, our new 100 TB storage server is up and running very well. Installation was remarkably smooth.
Last week, I had the chance to speak with Andrew Siemion and Dan Werthimer who are on the Breakthrough Listen team. They were both supportive of the SETI course and shared expert advice about hardware and software. Perhaps some of the students who take the SETI course will go on to work with Andrew and Dan.
As I write this, I am on my way to Chicago to give a short presentation about the extraordinary range of space-related activities that originate or take place at UCLA. I counted 12 current spacecraft missions with either direct leadership (e.g., Dawn, WISE) or substantial involvement (e.g., MESSENGER, Europa mission) by UCLA faculty in the Division of Physical Sciences. I fly back this evening just in time for our first SETI class tomorrow! In my next newsletter, I will describe how the class is going.
Warm regards,
Jean-Luc Margot
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