Detecting Galactic Rotation With Software Defined Radio

Last summer in the heyday of software defined radio via USB TV tuners we asked hackaday readers a question: Is anyone using everyone’s favorite method of SDR for radio astronomy? It took nearly a year, but finally there’s an awesome project to turn a USB TV tuner into a radio telescope. It’s from the fruitful mind of [Marcus Leech] (PDF warning), and is good enough to detect the rotation of the galaxy with a three-foot satellite dish.

News of [Marcus]’ work comes to us from [Carl] over at who has been keeping tabs on the advances of building a radio telescope in a backyard. He’s been collecting a lot of interesting tidbits including this gif showing an arm of the galaxy entering and leaving [Marcus]’ telescope’s field of view over the course of a few hours.

Not only can [Marcus]’ telescope record continium measurements – basically, a single-pixel camera sensitive to only one frequency – it can also produce spectral plots of the sky. Combine the ability to measure multiple frequencies at the same time with the Doppler effect, and [Marcus] can measure the rotation of the galaxy with a USB TV tuner. That’s just awesome in our humble opinion.

If you already have an RTL-SDR TV tuner and a largish satellite dish, [Marcus]’ project should be fairly inexpensive to replicate; the feed assembly is made out of a coffee can, the amplifiers are repurposed satellite television equipment, and all the software – [Marcus]’ own simple_ra tool for GNU Radio – is open source. Of course with a 3 foot diameter dish, it will be impossible to replicate the data from huge radio telescopes. Still, it’s an impressive piece of work that leaves us searching craigslist for an old C-band dish.

27 thoughts on “Detecting Galactic Rotation With Software Defined Radio

    1. Now add a second dimension to it (an equatorial mount could scan a certain piece of the sky), represent the shift with false color mapping, and it will become “amazing”!

  1. Because it’s important to know the galaxy is still rotating properly. I assume it will Tweet you if it should suddenly speed up, slow down, or (the horror!) stop. ;)

    Seriously though, it’s pretty darn amazing this can even be done.

  2. Note, the galaxy arm did not ROTATE, the earth rotated. They galaxy rotation is so slow relative to us it would have to be measured night after night and then the motion of our planet rotation and the movement around the sun removed from the measurements.

    Still cool to see amateur radio astronomy becoming cheaper.

      1. Sure, has all the answers. Come on back when you have any clue at all to physics and astrophysics.

        Oh and learn what relativity is all about as well while you are there.

        1. You: This is totally wrong
          Me: Uh, did you even read the thing?
          You: You are totally wrong
          Me: …

          And you’re saying relativity changes the speed of light? wtf.

          1. Fartface is partially correct. Just because you observe a Doppler shift does not mean the galaxy is rotating, it just means a group of objects are moving relative to the observer. To actually measure the rotation you would have to make many measurements in 3 dimensions of multiple targets. Then there is the fact that the sun is part of the rotation – that would be the relative part.

  3. I thought about using the SDR to build a ghetto radar range finder, I thought about just turning it up to space and seeing what I could find, but sadly, the E4000 radios are getting harder and harder to find, I had to settle on a Nooelec brand R820T DVB-T stick.

  4. It is totally amazing to me when some people get on here and make a most definite statement of their, almost knowledge, of our place in time and of the galaxy and how exactly sure they think they are. Contrary to their wanna-be education, the milky way galaxy is rotating far faster that our planet. They should take into consideration that the spiral arms of our galaxy is just a tad bigger than out solar system. Though it may appear to be spinning slower, stop and think about it. Our entire solar system is measure in a mear few million miles. Whereas the Milky Way is measured in light years. Approximately 100,000 light years from edge to edge. But then again, consider the source and their name. I guess it takes all kinds

      1. there is definitely some confusion on here as to the difference between rotational (angular) velocity and and absolute velocity. Don’s point is clearly that, while the angular velocity of the milky way (360 degrees per 225 million years, you say) is less than the angular velocity of the earth, the absolute velocity of the arm is much greater as it is traveling around a much larger radius.

      1. It would be fairly difficult to do this kind of work with rtlsdr dongles. They have high frequency error, high phase noise, and no metadata on the samples which are sometimes dropped silently.

        If you’re going to do any kind of cross correlation you’ll need to do it in analog in front of the rtlsdr dongle. Not in digital afterward.

        1. If we had enough of them, couldn’t we eliminate most of those errors with statistical models?

          Not saying it is easy to get enough of those online (and i think we’d need insanely many to make it work)

    1. The major problem with using multiple dongles is frequency error.

      But if the two dongles are side-by-side it’s possible to connect them together (kids, don’t try this at home) so one dongle provides the frequency standard for both units. Then connect each dongle to a separate satellite dish separated by the width of your property and you’ve got an interferometer. It’ll have the apparent resolution of a dish the same diameter as the distance between the two dishes, but of course only the collecting area of the actual dishes.

      Professional radio astronomers are doing amazing things with VBLI (Very long Baseline Interferometry) (think of the cluster of satellite dishes in “Contact” or “2010: The Year We Make Contact”) and with baselines even greater than the diameter of the Earth! (By using a satellite in orbit as one of the receiving dishes.

      1. The issue is random phase error between the two halves. They’ll never cross-correlate to anything larger than a remnant of a gnats fart, which is why you want both clocks to be in synch with each other, all the time. If the two halves of any given correlated pair are wandering randomly with respect to each other, they won’t correlate to any useful degree.

        The lack of metadata is a pain that can be got around with some clever calibration pulses. But you have to start with both receivers in a pair sharing the same clocking hierarchy both to drive the synthesizers, and the ADCs.

  5. This took a year because of a lot of false starts on other bands. The fact that I was able to do this on 21cm from my semi-urban back yard is a kind of small miracle, particularly given the small dish.

    I also spend a bunch of my hacking time on this project here:

    And the back-yard ‘scope was rather a proof-of-concept for using the RTLSDRs in large numbers at the bigger Shirley’s Bay dish. If anyone has any ideas how we can raise funds to keep the SBRAC project moving forward, I’m all ears. Can’t use kickstarter, because we don’t have a ‘product’ we’re trying to produce, etc.


  6. Move the RTL to the feedpoint or very close, use a Raspberrypi or Android TV stick as a SDR server and the losses are minimized.
    This will be my set up in a few months, parts on order, and I have both E4000 and R820 and I prfer the R820.

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