Search results for: astronomy

Radio Astronomy using a Differential Radiometer and Interferometer with an RTL-SDR

Amateur radio astronomer Marcus Leech often makes use of RTL-SDR dongles for his amateur radio astronomy experiments. Recently Marcus wrote a technical paper discussing a modern SDR implementation of a Dicke Radiometer, which is a type of radio telescope that is designed to significantly reduce the effects of receiver noise. Marcus has also developed an RTL-SDR approach to another similar system called the Phase-Switched Interferometer.

Using his new SDR based approach together with GNU Radio, a 10ft satellite dish and two RTL-SDR dongles he was able to plot a transit of the Milky Way Galaxy as shown below.

Milky Way Galaxy Transit
Milky Way Galaxy Transit

Radio Astronomy with RTL Bridge and Radio-Sky Spectrograph

Amateur radio astronomy hobbyist Jim Sky has written on his blog about his new program called RTL Bridge with allows the RTL-SDR to directly connect to his other radio astronomy programs Radio-SkyPipe and Radio-Sky Spectrograph. Jim describes his two existing program as follows.

Radio-Sky Spectrograph displays a waterfall spectrum. It is not so different from other programs that produce these displays except that it saves the spectra at a manageable data rate and provides channel widths that are consistent with many natural radio signal bandwidths. For terrestrial , solar flare, Jupiter decametric, or emission/absorption observations you might want to use RSS.

Radio-SkyPipe is a souped-up strip chart program which plots signal strength over time. When getting its data from RTL Bridge, RSP is plotting the total power in the spectrum covered by the RTL receiver centered around its set frequency. While the raw values are proportional to power, you will have to apply a function via the RSP Equations feature to apply a calibration if you want absolute values. For signals that do not have significant spectral structure of interest, this would be the preferred way to plot the data.

RTL Bridge for Radio Astronomy
RTL Bridge for Radio Astronomy

Radio Astronomy with a 0.2dB Noise Figure LNA

Over on our Facebook page member Александр has let us know about a Russian amateur astronomer, Alex who has been using the RTL-SDR for radio astronomy. Alex uses an Elonics E4000 RTL-SDR combined with a 3.7m mesh parabola dish with 1420 MHz waveguide.

At the center of his system is an LNA with 40dB gain and a very low noise figure of 0.2dB. This LNA appears to be based on G4DDK’s VLNA, but modified to work with the 1420 MHz frequency used for radio astronomy. It seems the LNA can be ordered for 140 USD from the above link.

Note: The above Russian links are machine translated with Google to English.

0.2dB Noise Figure Low Noise Amplifier
0.2dB Noise Figure Low Noise Amplifier
Radio Astronomy Results
Radio Astronomy Results

Horn Antenna for RTL-SDR Radio Astronomy

Rishi Patel has written on his blog about his radio astronomy project which involves an RTL-SDR and an impressive homemade horn antenna.

The horn antenna is constructed from cardboard foam and aluminium foil and is mounted on a telescopic mount. The horn then connects to a waveguide feed which is constructed out of a large metal can. The antennas waveguide then connects to a microstrip filter and LNA before finally connecting to an RTL-SDR. Rishi then uses a simple python program to gather the IQ samples from the RTL-SDR, and then draw the power spectral density plots.

Horn Antenna
Homemade Horn Antenna for Radio Astronomy

With this setup Rishi was able to detect the Hydrogen line. Rishi also used a second even larger horn with a different design to plot the spectrum shown below of the Cygnus (red), Cassiopeia (green), and Cepheus (blue) constellations.

largeHornSpectra
Spectral Plots of Cygnus (red), Cassiopeia (green), and Cepheus (blue) constellations.

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 such as Hydrogen line detection, meteor scatter and Pulsar observing.

Hydrogen Line

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 (low noise amplifier) 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.

Meteor Scatter

Meteor scatter works by receiving a distant but powerful transmitter via reflections off the trails of ionized air that meteors leave behind when they enter the atmosphere. Normally the transmitter would be too far away to receive, but if its able to bounce off the ionized trail in the sky it can reach far over the horizon to your receiver. Typically powerful broadcast FM radio stations, analog TV, and radar signals at around 140 MHz are used. Some amateur radio enthusiasts also use this phenomena as a long range VHF communications tool with their own transmitted signals. See the website www.livemeteors.com for a livestream of a permanently set up RTL-SDR meteor detector.

In Europe typically the Graves radar station can be used for meteor scatter experiments. Graves is a space radar based in France which is designed to track spacecraft and orbital debris. If you are in Europe you can also make use of the Graves radar simply by tuning to its frequency of 143.050 MHz and listening for reflections of its signal bouncing off things like meteors, planes and spacecraft. Since Graves points its signal upwards, it’s unlikely that you’ll directly receive the signal straight from the antenna, instead you’ll only see the reflections from objects.

In other countries old and distant analogue TV stations can be used or FM transmitters can also be used.

To set meteor scatter up, simply use an outdoor antenna to tune to a distant transmitter. It should be far enough away so that you can not be receive the transmitter directly, or the signal should be weak. If you detect a meteor the signal will briefly show up strongly at your receiver. Performance can be enhanced by using a directional antenna like a Yagi to point upwards at the sky in the direction of the transmitter.

We have several post about meteor scatter available on the blog here. Read through them to get a better understanding of the ways in which it can be monitored. You may also be interested in Marcus Leech’s tutorial where he uses the RTL-SDR to detect forward meteor scatter. (doc here) (pdf here)

Pulsar Observing

A pulsar is a rotating neutron star that emits a beam of electromagnetic radiation. If this beam points towards the earth, it can then be observed with a large dish antenna and a radio, like the RTL-SDR. 

Pulsars create weakly detectable noise bursts across a wide frequency range. They create these noise bursts at precise intervals (milliseconds to seconds depending on the pulsar), so they can be detected from within the natural noise by performing some mathematical analysis on the data. Typically a few hours of data needs to be received to be able to analyze it, with more time needed for smaller dishes.

One problem is that pulsar signals can suffer from ‘dispersion’ due to many light years of travel through the interstellar medium. This simply means that higher frequencies of the noise burst tend to arrive before the lower frequencies. Mathematical de-dispersion techniques can be used to eliminate this problem enabling one to take advantage of wideband receivers like the RTL-SDR and other SDRs. The more bandwidth collected and de-dispersed, the smaller the dish required for detection.

Pulsar detection requires some pretty large antennas, and a good understanding of the techniques and math required for data processing so it is not for the beginner. See the previous Pulsar posts on this blog for more information.


If 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.

Slow Scan Moon Bounce Event for 50th Anniversary of the Moon landing

Fifty years ago Neil Armstrong became the first man to step foot on the moon. This weekend on June 20th and 21st 2019 Amateur Radio operators at the [PI9CAM] team have been transmitting Slow-Scan Television images in commemoration of this historic event at the Dwingeloo radio astronomy station in the Netherlands. This station is the oldest rotatable 25-meter radio telescope in the world. 

 Dwingeloo radio astronomy station
Dwingeloo radio astronomy station
An EME SSTV Image
An EME SSTV Image

Slow-Scan Television is a method often employed by ham radio operators to send photos over radio waves. You may be familiar with this from some of our previous articles on the SSTV event held by ARISS for the International Space Station.

Station [S1NDP] has previously sent slow-scan EME images between the PI9CAM team and himself. These images can potentially be heard by anyone within line-of-site with the moon during the operation of this event.

The team transmit in the 23cm band at a frequency of 1296.11 MHz, according to the ARRL even a 2.5 to 3meter dish should be enough for reception assuming you have a 23cm feed for your dish. It will be interesting to see what photos are heard by the end of this event.

PICTOR: An Open Source Low Cost Radio Telescope based on RTL-SDR

PICTOR is an open source and open hardware radio telescope that aims to promote radio astronomy on a budget. It consists of a 1.5 meter parabolic dish antenna, 1420 MHz feedhorn, a two stage low noise amplifier (LNA), high pass filter, and from what we gather, an RTL-SDR. Future designs may also use higher bandwidth SDRs. Currently there doesn't seem to be much information about the build and exact components used in their design, but we're hoping that those details will come in time.

The radio telescope allows a user to measure hydrogen line emissions from our galaxy. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). The emissions themselves are very rare, but since our galaxy is full of hydrogen atoms the aggregate effect is that a radio telescope can detect a power spike at 21cm. If the telescope points to within the plane of our galaxy (the milky way), the spike becomes significantly more powerful since our galaxy contains more hydrogen than the space between galaxies. Radio astronomers are able to use this information to determine the shape and rotational speed of our own galaxy.

PICTOR also has a very interesting web based interface which can be used to let users from anywhere in the world access the telescope and log an observation. The first PICTOR telescope is currently online and observations can be created simply by going to their website, and clicking on the "Observe" link. Users can then enter the frequency and other parameters for their observation, and the resulting graph will be emailed to you after the observation. The software source is available on their GitHub page, and is based on a GNU Radio flowgraph and Python plot script.

For more information about PICTOR, logging an observation, and radio astronomy in general, we recommend checking out their PDF guide. We test ran a short observation at the hydrogen line frequency, and we received a graph with the hydrogen line peak clearly visible (spliced in to the photo below). We note that the wavy shape is due the to shape of the filters they used.

PICTOR Radio Telescope
PICTOR Radio Telescope

Signals and Bits A New SDR Podcast by the President of the GNU Radio Project

[Ben Hilburn] the president of the [GNU Radio Project] has recently started a new podcast called [Signals and Bits]. If you were unaware, GNU Radio is the defacto open source framework for implementing digital signal processing code. Without it, many SDR programs that we take for granted may have never been developed as it is responsible for a lot of community DSP knowledge and algorithm development.

This podcast is scheduled for a new release every Wednesday and will be composed in an interview style focusing on a multitude of topics from Software Defined Radio to Spectrum Enforcement, Radio Astronomy and so much more.

In the first episode Ben interviews Harold Giddings AKA Corrosive of [Signals Everywhere] where they discuss the state of Software-Defined radio and how he got started with radio communications having come from an IT/Computer Networking background.

Ben has already pre-recorded several episodes which will ensure great content is always just around the corner. Ben would love it if you could also send feedback his way over on the [Signals and Bits Twitter] page.

Ben Hilburn President of GNU Radio (Left), Harold Giddings AKA Corrosive of Signals Everywhere (Right)