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.
GNU Radio is a very powerful open source platform for implementing various digital signal processing (DSP) algorithms. It is very commonly used with software defined radios like the RTL-SDR, as well as much higher end units. The community that uses GNU Radio is very large, and so every year they hold a conference that highlights some of the most interesting applications and developments related to GNU Radio. The 2018 GNU Radio conference was held in Las Vegas during September 2018. Recently they have uploaded the talks to YouTube, and below we're posting some of our favorites. The full list can be found on their YouTube channel.
Keynote Talk: SatNOGs
In this keynote talk Manolis Surligas discusses the SatNOGs project. SatNOGs is a non-profit organization creating an open source and volunteer based satellite ground station network.
GRCon18 - Keynote: SatNOGs
Open Source Radio Telescopes
John L. Makous discusses his work in creating low cost and home made horn antenna radio telescopes designed to receive the 21cm hydrogen line and other astronomical objects and phenomena. The idea is to provide a low cost solution and easy to build telescope to use in schools.
GRCon18 - Open Source Radio Telescopes
Enter the Electromagic Spectrum with the USRP
Nate Temple gives us an overview of several signals that have been decoded with GNU Radio flowgraphs.
GRCon18 - Enter the Electromagic Spectrum with the USRP
Software Defined Radar Remote Sensing and Space Physics
Juha Vierinen discusses using a USRP to measure propagation conditions with ionospheric chip sounders, and improvements to chirp sounders by using spread spectrum noise. He also discusses various other radar techniques and applications.
GRCon18 - Software Defined Radar Remote Sensing and Space Physics
A corner reflector antenna is basically a monopole antenna with a metallic 'corner' reflector placed behind it. The reflector helps the monopole collect signals over a wider aperture resulting in signals coming in stronger from the direction that the corner is pointing at. In past posts we've seen a homemade tinfoil corner reflector used to improve reception of the generic stock RTL-SDR monopole antenna, and a larger one was used in a radio astronomy experiment to detect a pulsar with an RTL-SDR.
Recently The Thought Emporium YouTube channel has uploaded a video showing how to build a large 2 meter 3D corner reflector out of readily available metal conduit pipes and chicken wire. While the antenna has not been tested yet, they hope to be able to use it to receive weather satellite images from GOES-16, to receive moon bounce signals, to map the Hydrogen line and to detect pulsars.
Building a Giant 2m Corner Reflector Antenna For Less than $200 (For Goes-16, Pulsars and More!)
Hydrogen tends to emit radio signals in the 21cm (1.4 GHz) region of the frequency spectrum. An emission from a single Hydrogen atom is very rare, but since there is so much Hydrogen in space a bump at 1.4 GHz can be observed on the frequency spectrum if a sensitive radio is used with a directional antenna pointing up at the sky. This is a moderate difficulty experiment that can be performed by amateur radio astronomers today with cheap RTL-SDRs or other SDRs together with some LNAs.
The astronomers in this experiment focus on a distortion in the 21cm line signal that is expected to have been created when the first stars formed. The their paper they write:
After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz.
The results show a successful detection of the expected phenomena at 78 MHz, confirming the age at when the first stars have been predicted to have begun forming. The phenomena is detected at 78 MHz instead of 1.4 GHz because the wavelength of a Hydrogren line signal gets stretched the further the source is from us, due to the redshift doppler effect from the expansion of the Universe. This detection is from some of the furthest (and thus oldest) stars in the Universe, so a big stretch is expected.
The experiment consisted of a broadband blade dipole which was set up in the Australian outback. Since the cosmic signal is expected to be detected right in the middle of the broadcast FM band, a dedicated radio-quiet location is required to stand any chance of detection. The receiving SDR hardware consists of an LNA, line amp, filtering and a 14-bit ADC that is connected to a PC.
It seems possible that this experiment could be repeated by amateur radio astronomers with commercial SDR hardware, but the biggest challenge would probably be finding a very radio-quiet location without broadcast FM radio signals.
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.
DK8OK also explains in his post how you can use SDR-Console V3 to create a level diagram which shows power vs time, allowing you to count reflections and visualize the response of the reflection.
Any SDR that can tune to VHF frequencies such an an RTL-SDR can be used for monitoring reflections like this. If you aren’t in Europe you might consider looking for distant strong transmitters such as for TV/FM which you could also monitor for reflections.
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 since space and the galaxy is filled with many hydrogen atoms 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.
In his tutorial Adam discusses important technical points such as noise figure and filtering. Essentially, when trying to receive the hydrogen line you need a system with a low noise figure and good filtering. The RTL-SDR has a fairly poor noise figure of about 6dB at 1420MHz. But it turns out that the first amplifier element in the receive chain is the one that dominates the noise figure value. So by placing an LNA with a low noise figure right by the antenna, the system noise figure can be brought down to about 1dB, and losses in coax and filters become negligible as well. At the end of the tutorial he also discusses some supplementary points such as ESD protection, bias tees and IP3.
One note from us is that Adam writes that the RTL-SDR V3 bias tee can only provide 50mA, but it can actually provide up to 200mA continuously assuming the host can provide it (keep the dongle in a cool shaded area though). Most modern USB 2.0 and USB3.0 ports on PCs should have no problem providing up to 1A or more. We’ve also tested the LP5907 based Airspy bias tee at up to 150mA without trouble, so the 50mA rating is probably quite conservative. So these bias tee options should be okay for powering 2xLNA4ALL.
Finally Adam writes that in the future he will write a paper discussing homebrew hydrogen line antennas which should complete the tutorial allowing anyone to build a cheap hydrogen line radio telescope.
Over on his blog Dave Venne has been documenting his attempts at using National Weather Service (NWS) broadcasts for forward scatter meteor detection with an RTL-SDR. Forward scatter meteor detection is a passive method for detecting meteors as they enter the atmosphere. When a meteor enters the atmosphere it leaves behind a trail of highly RF reflective ionized air. This ionized air can reflect far away signals from strong transmitters directly into your receiving antenna, thus detecting a meteor.
Typically signals from analog TV and broadcast FM stations are preferred as they are near the optimal frequency for reflection of the ionized trails. However, Dave lives in an area where the broadcast FM spectrum is completely saturated with signals, leaving no empty frequencies to detect meteors. Instead Dave decided to try and use NWS signals at 160 MHz. In the USA there are seven frequencies for NWS and they are physically spaced out so that normally only one transmitter can be heard. Thus tuning to a far away station should produce nothing but static unless a meteor is reflecting its signal. Dave however does note that the 160 MHz frequency is less than optimal for detection and you can expect about 14 dB less reflected signal from meteors.
So far Dave has been able to detect several ‘blips’ with his cross-dipole antenna, RTL-SDR and SDR#. He also uses the Chronolapse freeware software to perform timelapse screenshots of the SDR# waterfall, so that the waterfall can be reviewed later. Unfortunately, most of the blips appear to have been aircraft as they seem to coincide with local air activity, and exhibit a Doppler shift characteristic that is typical of aircraft. He notes that the idea may still work for others who do not live near an airport.
We note that if you are interested in detecting aircraft via passive forward scatter and their Doppler patterns, then this previous post on just that may interest you.
Over on our forums Andy (M0CYP) has posted about his new meteor scatter detection program which works with HDSDR and any supported SDR like an RTL-SDR. It works in an interesting way, as instead of analyzing sound files for blips of meteor scatter activity it analyzes screenshots of the HDSDR waterfall. The software automatically grabs the screenshots and determines if a signal is present on any given frequency. You can set a preconfigured detection frequency for a far away transmitter, and if the waterfall shows a reflection it will record that as a meteor.
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 (although that site does not use Andy’s software).
Andy writes that his meteor scatter detection software is still in beta so there might be some bugs. You can write feedback on the forum post, in the comments here, or contact Andy directly via the link on his website.