Tagged: radio astronomy

Creating a 21cm Galactic Sky Map with an Airspy and 1.8m Dish

Marcus Leech from ccera.ca is a pioneer in using low cost software defined radios for observing the sky with amateur radio telescopes.  In the past he's shown us how to receive things like the hydrogen line,  detect meteors and observe solar transits using an RTL-SDR. He's also given a good overview and introduction to amateur radio astronomy in this slide show.

His recent project has managed to create a full Hydrogen sky map of the northern Canadian sky. In his project memo PDF document Marcus explains what a sky map shows:

A [sky map] shows the brightness distribution over the sky for a given set of observing wavelengths. In the case of the 21cm hydrogen line wavelength, maps show the distribution of hydrogen over the sky. For amateur observers, such maps generally show the distribution within our own galaxy, since extra-galactic hydrogen is considerably more faint, and significantly red/blue shifted relative to the rest frequency of 1420.40575 MHz, due to relative motion between the observer and the target extra-galactic hydrogen.

He was able to make this observation using his radio telescope made from a 1.8m dish antenna, a NooElec 1420 MHz SAWBird LNA + Filter, a 15dB line amplifier, another filter and two Airspy R2 software defined radios locked to an external GPSDO. The system runs his custom odroid_ra software on an Odroid XU4 single board computer, which provides spectral data to an x86 host PC over an Ethernet connection. 

Over 5 months of observations have resulted in the Hydrogen sky map shown at the end of this post. Be sure to check out his project memo PDF file for more information on the project and how the image was produced. Marcus' blog post over on ccera.ca also notes that more data and different maps will be produced soon too.

Hydrogen Sky Map
Hydrogen Sky Map

Updates on the PICTOR Low Cost Open Source Radio Telescope Based on RTL-SDR

Back in July we posted about PICTOR, an open source and RTL-SDR based radio telescope project. The owner of the project recently wrote in and wanted to share some updates. His text is below:

A few months ago, PICTOR was launched. PICTOR is a free to use open source radio telescope that allows anyone to observe the sky in the 1300~1700 MHz range at any time via the easy-to-use online platform.

The goal of this effort is to introduce students, educators, astronomers and others to the majesty of the radio sky, promoting radio astronomy education, without the need of building a large and expensive radio telescope. 

Since the initial launch, PICTOR has gotten lots of updates and improvements, particularly in the software backend, providing more data to the users, using advanced techniques to increase the signal-to-noise ratio by calibrating spectra and mitigating radio frequency interference (RFI) (if present).

Here is an example observation with PICTOR, clearly showing the detection of 3 hydrogen-dense regions corresponding to 3 unique spiral arms in the Milky Way!

Graphs from the PICTOR RTL-SDR Radio Telescope showing the 3 unique spiral arms in the Milky Way.
Graphs from the PICTOR RTL-SDR Radio Telescope showing the 3 unique spiral arms in the Milky Way.

If you’re new to radio-astronomy, the developer of PICTOR has provided a PDF including some introductory radio astronomy information and instructions on how to observe the radio sky with PICTOR: https://www.pictortelescope.com/Observing_the_radio_sky_with_PICTOR.pdf

Building An Open Source SDR Based Hydrogen Line Radio Telescope

Over on Reddit we've seen a post by u/ArtichokeHeartAttack who has been working on a hydrogen line radio telescope, based on an RTL-SDR dongle and horn antenna designs by the DSPIRA program, and the Open Source Radio Telescopes website (site appears to be down, linked to the archive.org copy). [u/ArtichokeHeartAttack] has documented their radio telescope building journey, providing a comprehensive top-level document that is able to point interested people in the right direction towards understanding and building their own Hydrogen line radio telescope.

Briefly, their build consists of a horn antenna and reflector designed for the 1,420.4 MHz Hydrogen line frequency. The horn is built out of a few pieces of lumbar, metallic house wall insulation sheets and aluminum tape. The feed is made from a tin can and piece of wire. In terms of radio hardware, they used an Airspy SDR, GPIO labs Hydrogen Line Filter + LNA, and 2x Uputronics Wide band preamps, and a Minicircuits VBF-1445+ filter. For software processing, they used a GNU Radio flowgraph to integrate and record the spectrum.

The results show that they were able to achieve a good hydrogen line peak detection, and they were able to measure the galactic rotation curve doppler shift, and tangent points which prove that we do in fact live in a spiral galaxy.

The Finished Hydrogen Line SDR Based Horn Radio Telescope Antenna
The Finished Hydrogen Line SDR Based Horn Radio Telescope Antenna

New LNA + Filter for Radio Astronomy Hydrogen Line Observations Released by NooElec

NooElec have recently released a new LNA + filter combo called the "SAWbird+ H1 Barebones" which significantly lowers the entry bar for new amateur radio astronomers. It's designed to be used with RTL-SDR or other SDRs for radio astronomy, and in particular reception of the Hydrogen line.

The filter is centered at 1.42 GHz with a 70 MHz bandpass region. The LNA has a minimum gain of 40dB. For hydrogen line observations it is important that the LNA have very low noise figure, and this LNA fits the bill with a ~0.5dB to ~0.6dB noise figure. An additional feature on the PCB is an RF switch that is electrically controlled via expansion headers. This switch allows you to switch out the LNA for a 50 Ohm reference which is useful for calibration in more serious radio astronomy work.

This LNA draws 120mA of current meaning that it will work with the RTL-SDR V3 and Airspy's bias tee, but probably not with the SDRplay's bias tee which is limited to 100mA and seems to trip a fuse at higher current draws. For an SDRplay you could use external power instead, although you will need an additional DC blocking cap to prevent power from entering the SDR and destroying the ESD diodes.

If you don't know what the Hydrogen line is, we'll explain it here. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very rarely emit a photon, but space and the galaxy is filled with many hydrogen atoms so the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike. You can also measure the rotational speed of our galaxy by noting the frequency doppler shift.

Although this LNA lowers the entry bar, in order to receive the Hydrogen line with the SAWBird+ H1 you will still need a ~1m+ satellite dish and a feed tuned to 1.42 GHz or high gain Yagi, horn or helical antenna. Antennas and feeds like this are not yet available off the shelf, but if you search our blog for "hydrogen line" you'll see many project examples

The NooElec SAWBird+ H1. For Hydrogen Line Observations.
The NooElec SAWBird+ H1. For Hydrogen Line Observations.

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

Vela Pulsar Glitch Detected with RTL-SDR Based Radio Telescope

On February 1st 2019 the HawkRAO amateur radio telescope detected a "glitch" during it's observations of the Vela Pulsar. 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 or directional antenna and a radio, like the RTL-SDR. The Vela pulsar is the strongest one in our sky, making it one of the easiest for amateur radio astronomers to receive.

Pulsars are known to have very accurate rotational periods which can be measured by the radio pulse period. However, every now and then some pulsars can "glitch", resulting in the rotational period suddenly increasing. Glitches can't be predicted, but Vela is one of the most commonly observed glitching pulsars.

The HawkRAO amateur radio telescope run by Steve Olney is based in NSW, Australia and consists of a 2 x 2 array of 42-element cross Yagi antennas. The antennas feed into three LNAs and then an RTL-SDR radio receiver. He has been observing the Vela pulsar for 20 months.

His observations indicate that Vela glitched and spun up by 2.5PPM at 14:09 UTC on Feb 1, 2019. He claims that this glitch detection is a first for amateur radio astronomy as far as he is aware.

If you're interested in Pulsar detection, check out a few of our previous posts on the topic.

The HawkRAO Amateur Radio Telescope Vela Glitch Detection
The HawkRAO Amateur Radio Telescope Vela Glitch Detection (Blue graph on the right indicates the glitch detection)

More Talks from GNURadio Con 2018

Last week we posted about some videos of talks from the 2018 GNU Radio Conference which had been release on YouTube. This week a few more videos have been released and we display a small selection below. The full collection of videos can be found on their YouTube channel.

RF Ranging with LoRa Leveraging RTL-SDRs and GNU Radio

Wil Myrick discusses the use of RTL-SDRs and GNU Radio to create a low cost LoRa RF ranging prototype, to aid in the localization of IoT transmitters.

GRCon18 - RF Ranging with LoRa Leveraging RTL SDRs and GNU Radio

Using GNU Radio and Red Pitaya for Citizen Science

Robert W McGwier discusses the use of Red Pitaya SDRs and GNU Radio for use in citizen science ionosphere measurement experiments.

GRCon18 - Using GNU Radio and Red Pitaya for Citizen Science

SETI Breakthrough Listen

Steve Croft discusses the Search for Extraterrestrial Intelligence (SETI) project and how software defined radio is being used in the search.

GRCon18 - SETI Breakthrough Listen

Logging Meteor Scatter Observations Online

Thank you to Florent for submitting his website which contains a live log of his meteor scatter observations. Meteor scatter occurs when radio signals reflect off the ionized trail left behind by meteors when they enter the atmosphere. This trail is highly RF reflective, so it can allow distant radio stations to be briefly received.

His set up consists of an RTL-SDR dongle running on a Raspberry Pi 3. His antenna is a homemade 6 element Yagi. Florent is based in France and listens for reflections from the Graves radar at 143.05 MHz. His software captures 768 Hz worth of bandwidth every 0.5s, and then uploads and displays the spectrum plot on his website. When the Graves radar signal is visible on the spectrum, it is an indication of a meteor having entered the atmosphere (or possibly an aircraft).

If you are interested in other peoples live meteor scatter streams, then there is another site at livemeteors.com which displays a live video of an SDR# screen looking for meteor echoes.

Some Meteor Scatter Logs by Florent
Some Meteor Scatter Logs displayed on Florents website