RTL-SDR for Budget Radio Astronomy

With the right additional hardware, the rtl-sdr software defined radio can be used as a super cheap radio telescope for radio astronomy experiments.

Marcus Leech of Science Radio Laboratories, Inc has released a tutorial document titled “A Budget-Conscious Radio Telescope for 21cm“, (doc version) (pdf here) where he shows

Two slightly-different designs for a simple, small, effective, radio telescope capable of observing the Sun, and the galactic plane in both continuum and spectral modes, easily able to show the hydrogen line in various parts of the galactic plane.

He uses the rtl-sdr as the receiving radio with an LNA and a couple of line amps, a 93cm x 85cm offset satellite dish (potential dish for sale here, and here), and GNU Radio with the simple_ra application. In his results he was able to observe the spectrum of the Galactic Plane, and the Hydrogen Line. Some more information about this project can be found on this Reddit thread.

Here is a link to an interesting gif Marcus made with his rtl-sdr, showing a timelapse of recorded hydrogen emissions over 24 hours. Reddit user patchvonbraun (A.K.A Marcus Leech) writes on this thread an explanation of what is going on in the gif.

Interstellar space is “full” of neutral hydrogen, which occasionally emits at photon at a wavelength of 21cm–1420.4058Mhz.

If you setup a small dish antenna, and point at a fixed declination in the sky, as that part of the sky moves through your beam, you can see the change in spectral signature as different regions, with different doppler velocities move through your beam.

This GIF animation shows 24 hours of those observations packed into a few 10s of seconds.

 Marcus’ setup is shown below.

RTL-SDR Radio Telescope Setup

And here is just one of his many resulting graphs shown in the document showing the Hydrogen line.

RTL-SDR Radio Telescope Hydrogen Line

A similar radio astronomy project has previously been done with the Funcube. More information about that project can be found in this pdf file. In that project they used the Funcube, a 3 meter satellite dish and the Radio Eyes software.

However, in this Reddit post patchvonbraun explains that the Funcube’s much smaller bandwidth is problematic, and so the rtl-sdr may actually be better suited for radio astronomy.

This image is from the Funcube project document.

Funcube Radio Telescope Project

Another related project is the Itty Bitty Telescope (IBT), which does not use SDR, but may be of interest.

You may also be interested in Marcus Leech’s other tutorial where he uses the rtl-sdr to detect forward meteor scatter. (doc here) (pdf here)

The Hobbyists Guide to RTL-SDR: Really Cheap Software Defined RadioIf you enjoyed this tutorial you may like our ebook available on Amazon.

The Hobbyist’s Guide to the RTL-SDR: Really Cheap Software Defined radio.

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