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

A Low Cost 2.4 GHz Downconverter from off the Shelf Dev Boards

Over on GitHub Ian Wraith has released his design and microcontroller code for a low cost 2.4 GHz downconverter circuit. A downconverter is a hardware device that shifts the signals that it receives into a lower frequency band. This is useful in the case of RTL-SDRs and Airspy SDRs, as their maximum frequency range is only 1.7 GHz. Ian's 2.4 GHz downconverter reduces those 2.4 GHz signals down to 1 GHz, which can then be received with his Airspy.

Rather than designing a circuit from scratch, Ian's design makes use of several very cheap Chinese evaluation/development boards that he found on eBay. It costs of a mixer board, oscillator board, and an STM32 development board for controlling the oscillator board via SPI. The whole set of hardware cost him less than £30 (~37 USD).

After spending some time working through the difficulties in programming the SPI interface on the STM32 board, he was able to get the downconverter circuit fully working. He notes that he's been able to receive WiFi, Zigbee, Bluetooth and ISM band signals at 2.4 GHz, as well as 3G and 4G cellular signals at 2.6 GHz.

Ian Wraith's Downconverter consisting of three off the shelf cheap Chinese eBay boards.
Ian Wraith's Downconverter consisting of three off the shelf cheap Chinese eBay boards.

SignalsEverywhere: Improving HF Reception with Impedance Matching

This week on the SignalsEverywhere YouTube channel host Corrosive explores why impedance matching matters when trying to obtain the best reception possible. To do this he reviews the NooElec 1:9 Balun, which is designed to convert the (roughly) 450 Ohm impedance of a long wire antenna or ladder line dipole back down to 50 Ohms, which is the standard impedance that an SDR expects. Small amounts of impedance mismatch are negligible for RX, but larger mismatches can result in poor reception.

SDR Nooelec 1:9 Balun | Why Impedance Matters in Radio

Investigating the Galileo Satellite Navigation System Outage with a LimeSDR

Galileo is a European Union owned satellite navigation system. Galileo was created so that the EU does not need to rely on the US GPS or the Russian GLONASS satellites, as there is no guarantee that these systems won't be purposely turned off or degraded by their governments at any time.

Unfortunately since July 11 the Galileo system has been out of service. Not much information about the outage has been provided, but it appears to be related to problems with the Italian ground based Precise Timing Facility which consists of two ultra high precision atomic clocks that keep the Galileo systems' reference time. (We note that recently within the last few hours of this post, most satellites seem to have come back into operational status, but the EGSA website still reports an outage.)

Over on his blog, Daniel Estevez has been using his LimeSDR and a small patch antenna to gather some more information about the outage directly from the Galileo satellites. His investigations found that the modulation and signal itself are still working correctly. However, by using the GNSS-SDR software to investigate the signal data he was able to obtain the ephemeris, and see that the ephemeris is stuck in the past. The ephemeris data is used to calculate compensations for orbital drift and without frequent ephermis updates, orbital errors add up within hours resulting in poor positioning accuracy. In order to generate the ephermis, the Precise Timing Facility must be operational.

Daniel's post goes into further technical details about the information he's collected, and it's definitely an interesting read. One interesting bit of information that you can read from his post explains why the service has gone from initially just heavily degraded accuracy from July 11, to completely nonsense results from July 15 onwards.

Upcoming KerberosSDR Software Updates: Automatically Estimate TX Location and Navigate There

KerberosSDR is our 4x Coherent RTL-SDR that we've developed together with Othernet. It can be used for tasks such as direction finding and passive radar. KerberosSDR was successfully crowdfunded over on Indiegogo, and the first batch has already been shipped. Currently we are taking discounted pre-orders for a second production batch on Indiegogo. Please note that the discounted pricing will expire when we ship, which according to the manufacturing schedule should be next month, so please get in quick if you're interested!

If you'd like to back the KerberosSDR project and purchase a unit, please see our Indiegogo page.

Below are some recent updates to the project:

Android App Software Improvements

The Android App allows a KerberosSDR user to drive around in a car, collecting angle of arrival data for a signal. Driving around and collecting multiple data points solves the multipath issue. In a single location it is possible for a signal's direction of arrival to be skewed or incorrect as it can bounce off multiple surfaces and appear to be arriving from a wrong direction. If we collect data from many locations, we can average out the multipath.

We've recently been working on improvements to the direction finding capabilities of the KerberosSDR, and in particular to our free Android App which records and plots data from the KerberosSDR server. We are still testing and finalizing these new features, but hope to release the updated app before the end of this month.

Recently added features to the app include:

  • Added the ability to determine the estimated location of a transmitter, providing there has been sufficient data collected.
  • Added a heatmap grid of the collected data which can be used to determine where most angle lines cross. Can take into account RF power data too.
  • Added the ability for the software to automatically navigate you to the estimated TX location via MapBox GPS turn by turn navigation.

Bellow are screenshots showing some of the new features. In this experiment we located an 858 MHz TETRA transmit tower. Initially the app will navigate you to the edge of the grid, in the direction that most DoA lines are pointing to. When there is sufficient data to be able to confidently pinpoint the TX location, it will begin navigating you to the estimated location. In the screenshots the placemarker represents the known location of the transmitter, and the circles indicate the location estimated from direction finding.

Below is screenshots from a 415 MHz DMR tower that we located with KerberosSDR. The antenna array was purposely kept small, with a diameter of only 12cm. Even with the small antenna array we were able to pinpoint the transmitter down to about 100 - 200 meters.

The app should also now be able to handle intermittent signals, via a squelch filtering function, although this has not been fully tested yet.

In order to navigate you must have a 3G/4G data plan on your phone, and your phone must have the ability to create a WiFi hotspot. The KerberosSDR server running on a Pi 3 or similar will then automatically connect to a WiFi hotspot named "KerberosSDR" running on your phone and provide data to the app via WiFi.

Batch 2 Manufacturing Updates

Batch 2 production is in full swing, and at the moment we're expecting completion by mid August. This batch will ship directly from China, so we should be able to ship them off fairly quickly rather than needing to first wait for them to arrive in the USA.

Magnetic Whip Antennas

We have been disappointed that it has been difficult to find low cost but good quality magnetic whip antennas to use with KerberosSDR and vehicles. The quality of antennas used in direction finding equipment can matter, as any signals leaking into the coax, or radiation pattern skew can affect results. We are working on sourcing some high quality magnetic whip antennas that have good ground coupling. These will be sold at a reasonable price on our store.

Future Updates

We are still working on improving the server software further too and future updates will include things like the ability to notch out unwanted signals during phase calibration, a simplified DoA set up wizard, an improved buffering scheme so that additional data and processing gain can be applied, and more.

The Raspberry Pi 4 looks to be an excellent candidate to be used with the KerberosSDR. We will begin releasing ready to use images for the Pi 4 in the future.


Every sale of a KerberosSDR helps fund further developments to the software and possible future iterations of the hardware. So we'd like to thank all backers once again!

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)

Artemis 3 Released: Offline Signal Identification Database

The Signal ID Wiki (sigidwiki) is our sister site that we started a few years ago as a way to collect and catalog various types of signals that an SDR user might see and hear on the airwaves. The idea is that a user could search the database to learn about and identify unknown signals. Over time it has grown significantly, with now almost 400 known signals with both waterfall images and sound samples available in the database. Special thanks to lead admin Carl Colena for maintaining and playing a huge role in the databases' growth.

Artemis is an open source Windows/Linux/MacOS compatible application initially programmed by Marco Dalla Tiezza. It brings the sigidwiki website into an offline searchable database with an easy to use UI. Today version 3.0 was released to the public. The new version has been completely rewritten from scratch in Python, as the previous versions were written in BASIC (a now abandoned programming language). The new version has an improved UI, and paves the path for future improvements. 

Marco notes that in the future they hope to add an Autocorrelation function, which might help users automatically identify certain types of repetitive signals simply by playing the raw audio into Artemis.

Note that in order to download the software you will need to sign up to their forum, which is free.

Artemis 3.0 Screenshot
Artemis 3.0 Screenshot

rtlSpectrum: A New GUI for rtl_power

Thank you to Andrey for submitting news about the release of his new open source program called "rtlSpectrum". rtlSpectrum is a GUI for rtl_power, which allows you to scan a wide swath of bandwidth with an RTL-SDR dongle. The scan can then be plotted with rtlSpectrum, and peaks of activity can then be determined. Some of the features include:

  • load from .csv file produced by rtl_power
  • run rtl_power directly. it should be available in the $PATH
  • add multiple graphs for analysis
  • subtract one graph from another
  • save/export graph in the rtl_power based format

In particular, the ability to subtract one graph from another is useful for analyzing filters. Andrey has posted about doing this with rtlSpectrum, a noise source and an RTL-SDR V3 over on his blog (note that the blog is in Russian, so please use Google Translate if necessary). He notes how the dynamic range of the RTL-SDR is limited, so that the true blocking power of a filter cannot be determined, but it is enough to see the shape and frequency response.

rtlSpectrum Screenshots
rtlSpectrum Screenshots