# DIY Quasar Interferometry

CJ Trowbridge

2020-12-02

ASTR 11

Lab N13 Report

I’m writing about Radio Interferometry. This is a topic that I am very interested in. As a licensed amateur radio technician, I have tried for several semesters to do an honors project on this topic but haven’t yet found a professor who was as interested as I am in radio interferometry.

Let’s start with the definition from Britannica. “[an] apparatus consisting of two or more separate antennas that receive radio waves from the same astronomical object and are joined to the same receiver.”

The National Radio Astronomy Observatory explains its function, “a radio telescope interferometer can combine measurements from each of the pairs of antennas in an array simultaneously, it can make a very high resolution measurement of a specific point in the focal plane of the radio telescope.  By combining many pairs of antennas one can create an image of a specific point in the sky.”

The type of radio interferometry I am most interested in is Quasar Interferometry. According to Wikipedia, Quasar Interferometry is also called Very-long-baseline interferometry or VLBI. For the purposes of this paper, there is no functional difference between Pulsars and Quasars since both have known locations and emit similar radio signals. In reality (According to HowItWorksDaily.com) Quasars are galaxies while Pulsars are merely stars, but again for our purposes what matters is that they both have known locations and emit predictable radio signals.

According to Space.com, a quasar is a distant object powered by a black hole which transmits a predictable radio signal out into space. These signals reach us as a regular sine wave on a certain frequency. We know about many quasars and we know where they are and what their signal should sound like.

One type of antenna for Radio Interferometry is called a Phased Array. According to radartutorial.eu, a phased array is “an array antenna whose single radiators can be fed with different phase shifts. As a result, the common antenna pattern can be steered electronically.” This is very exciting for two reasons. First, because with several small antennae, you can listen and measure the microscopic differences in timing in order to precisely determine the direction from which a signal originated.

Secondly, phased arrays are interesting because when you record overlapping signals coming from different directions, you can also isolate noise propagating across the array in other directions, and remove it from the signal you are looking for. In this way, a phased array is to radio interferometry as lasers are to adaptive optics.

There are several interesting potential observations that could be made with a phased radio interferometer. In my own proposed honors project, I would be using software defined radio to listen to the predictable transmissions of various quasars.

The first interesting observation would be simply using a phased array. This is because I would theoretically be able to pinpoint the precise direction to each of the quasars. Since these directions are known, I could triangulate the position of the array. This would essentially function as a much more precise alternative to GPS which would not require the satellites. Some kind of similar technology will be necessary once humans begin to explore space and move beyond the range of the GPS satellites.

The second interesting observation would be measuring small variances in the sine waves we receive from each quasar. The signals sent out by Quasars do not change in frequency, but the signal we receive from them should. This is because of the propagation of gravity waves across the interstellar medium. Theoretically, measuring these small fluctuations in frequency would mean we can pinpoint the point of origin for gravity waves as they move across the various Quasar beams we are listening to.

This would make a phased radio interferometer function as both a better alternative to GPS and a powerful gravity wave sensor. We do have existing GPS infrastructure, but gravity wave detection is still bleeding edge science, and our current sensors are limited by their human-made terrestrial size. If we used the enormous distance between earth and the many Quasars as a gravity sensor, then we could expand our gravity sensor to the size of that distance between us and the Quasars. Imagine the increase in resolution and precision, and all from a few SDR dongles that can fit in your pocket.

There are some technical challenges to using off the shelf products to conduct these kinds of observations. According to Caltech, Quasars emit radio waves between 10mhz and 100ghz. According to the manufacturer of RTL-SDR, this is well within the range of their cheap \$20 dongle. This means for just \$20 you can easily pick up the signals from any Quasar; of course you would need at least a few of them to build a phased array. Perhaps the biggest challenge would be writing the special software you would need to record and compare the signals from each phase in your antenna array. Harder still would be the software tools you would need to analyze the data and find the results.

In researching for this paper, I have learned that since my original honors project proposal, NASA has actually completed a proof of concept for this idea using a phased array of 52 radio telescopes. So apparently I have been scooped, but in the words of NASA scientist Keith Gendreau, “It could probably be done with a single telescope. ” So maybe there is some work left to do in radio interferometry yet!

Works Cited

Caltech. “Fanaroff-Riley Classification.” Accessed 2020-12-02. https://ned.ipac.caltech.edu/level5/Glossary/Essay_fanaroff.html

HowItWorksDaily.com. “What is the difference between a quasar and a pulsar?” Accessed 2020-12-02. https://www.howitworksdaily.com/what-is-the-difference-between-a-quasar-and-a-pulsar/

NASA. “NASA test proves pulsars can function as a celestial GPS.” Accessed 2020-12-02. https://www.nature.com/articles/d41586-018-00478-8