Category: Applications

Using an RTL-SDR in Dual-Comb Spectroscopy using Diode Lasers

Thank you to Antonio from the Polytechnic University of Madrid, Department of Photonic Technology and Bioengineering for writing in and sharing with us his teams latest research titled "Dual-Comb Spectrometer Based on Gain-Switched Semiconductor Lasers and a Low-Cost Software-Defined Radio". The research involves the use of an RTL-SDR Blog V3 dongle in place of an expensive digital oscilloscope for measuring the output of a dual-comb spectrometer. The abstract of the paper reads as follows:

Dual-comb spectroscopy has become a topic of growing interest in recent years due to the advantages it offers in terms of frequency resolution, accuracy, acquisition speed, and signal-to-noise ratio, with respect to other existing spectroscopic techniques. In addition, its characteristic of mapping the optical frequencies into radio-frequency ranges opens up the possibility of using non-demanding digitizers.

In this paper, we show that a low-cost software defined radio platform can be used as a receiver to obtain such signals accurately using a dual-comb spectrometer based on gain-switched semiconductor lasers.

We compare its performance with that of a real-time digital oscilloscope, finding similar results for both digitizers. We measure an absorption line of a H13C14N cell and obtain that for an integration time of 1 s, the deviation obtained between the experimental data and the Voigt profile fitted to these data is around 0.97% using the low-cost digitizer while it is around 0.84% when using the high-end digitizer.

The use of both technologies, semiconductor lasers and low-cost software defined radio platforms, can pave the way towards the development of cost-efficient dual-comb spectrometers.

The paper can be freely accessed on IEEE Access which is open access.

We note that in the past we've also seen an RTL-SDR used as part of a low cost Ozone spectrometer experiment, and and Airspy used in an optical FM spectroscopy experiment.

Dual-comb Optical Spectroscopy setup with an RTL-SDR Blog V3

Receiving pH Readings from a Wireless Medical Implant with RTL-SDR

Over on Hackaday we've learned about an interesting investigation by James Wu who was recently implanted with a stomach pH (acidity) monitoring device called the "Medtronic Bravo Reflux Capsule". Whilst inspecting the patient demo capsule James noted that the device transmitted data wirelessly via a very small low power transmitter, in particular noticing a telltale "433" written on a component on the device, indicating that it uses the 433 MHz ISM band.

Back at home he pulled up the FCC filing for the device, which unveiled that it is OOK-PWM modulated, and operates at 433.92 MHz. The rest of the filing also had information noting that the implant transmits a 59-bit data packet every 12 seconds, and contained a nice breakdown of the packet structure, making it easy for decoding.

With all the information about the device's wireless transmissions now known, James grabbed his RTL-SDR and fired up SDR# to confirm that the signal was indeed transmitting every 12 seconds at 433.92 MHz. Next he was able to decode the data from the device by inputting the protocol information learned from the FCC filing into an rtl_433 command line string.

After a bit of further work James discovered that the pH data was actually two readings in one data string. At this stage he finally had the pH reading, however it was represented as an 8-bit ADC reading with a value between 0 to 255. James plotted the relationship between the 8-bit raw ADC reading, and the pH value shown on the official Medtronic receiver. With this he was able to determine a linear relationship between the ADC reading and real pH reading, but notes that there may be a more accurate calibration curve required for actual medical use.

Decoding pH readings from a stomach implant with an RTL-SDR

If you're interested in wireless medical devices, in the past we've seen how SDRs could be used to not only receive data coming from Minimed Insulin pumps, but to maliciously control them with a HackRF too. We've also seen that data could possibly be received from implanted heart defibrillators as well.

uSDR: A Lightweight Multimode SDR Receiver Program for Windows

Thank you to Viol Tailor for submitting news about the release of his general purpose multimode software defined radio receiver program for Windows called "uSDR" or "microSDR". Viol writes that uSDR is designed as a lightweight binary with a simple and compact user interface and highly optimized DSP to minimize CPU, hence the "micro" part of the name.

The software is compatible with RTL-SDR, Airspy, BladeRF, HackRF and LimeSDR radios. It has features including demodulation, base band and pass band recording, playback, and spectrum and waterfall visualizations.

uSDR aka microSDR. A lightweight SDR receiver program from Windows.

AIS-Catcher: A Dual Band Multiplatform AIS Receiver for RTL-SDR and Airspy HF+ with Multiple Decoding Models

Thank you to Jasper for writing in and letting us know about the release of his new open source software called "AIS-Catcher". AIS-Catcher is a MIT licensed dual band AIS receiver for Linux, Windows and Raspberry Pi. It is compatible with RTL-SDR dongles and the Airspy HF+.

AIS stands for Automatic Identification System and is used by marine vessels to broadcast their GPS locations in order to help avoid collisions and aide with rescues. An RTL-SDR with the right software can be used to receive and decode these signals, and plot ship positions on a map.

Jasper notes that his software was intended to be a platform for him to experiment with different receiving model algorithms. On the GitHub readme he explains how he's experimented with a coherent demodulation model that estimates the phase offset, a non-coherent model which is similar to what most existing decoders use, a modified non-coherent model with aggressive PLL, and an FM discriminator model which assumes the input is the output of an FM discriminator.

The readme goes on to show some comparison results indicating that the coherent model is the best although it uses 20% more computation time. He also compares AIS-Catcher against some other AIS decoders like AISRec and rtl-ais, showing that AIS-Catcher appears to be comparable or better than AISRec, which is one of the most sensitive decoders available for SDR dongles.

A Windows binary is provided on the releases page and compilation instructions for Linux are provided on the Github Readme.

Some results from AIS-Catcher. Different algorithms and different software compared.

LibreCellular: Easy 4G Cellular Network with LimeSDR and Intel NUC

We recently came across the LibreCellular project which is aiming to make it easy to implement 4G cellular networks with open source software and low cost SDRs. The project appears to be in the early stages, and seems to be focusing on deploying and modifying existing open source 4G basestation software known as srsRAN which will be used with a particular combination of hardware in order to create a reliable and easy to set up 4G basestation solution.

The reference hardware that they are recommending consists of an Intel NUC single board computer ($699), LimeSDR ($315), LimeRFE front end filtered power amplifier ($699), and Leo Bodnar Mini Precision GPS Reference Clock ($140). All together you can create a 4G basestation for around $1850.

LibreCellular Components for a 4G Basestation: LimeRFE, Leo Bodnar GPS Clock, LimeSDR, Intel NUC.

SDR4Space: Software Tools for SDR Based Satellite Ground Stations

Over on Reddit we've seen that SDR4Space, a provider of a satellite ground station receiver hardware and software has released a free feature limited lite version of their embedded software over on their GitHub page. In the Reddit comments the software is explained as follows:

It's a command-line tool using scripts, for SDR users. You can write your own scripts to: record IQ samples, predict satellite passes, start a record for a specific satellite and correct doppler at the same time.

It's also useful to record narrow subband IQ streams ( example: 48kHz wide instead of 2.048 MHz on rtlsdr, a single signal on HF is only few kHz wide).

You can work on IQ files: cut, resample, merge, convert formats and so on.

Having said that you can recognize features from predict, rx_sdr/rtl_sdr ,rtl_power/rx_power.

Regarding installation, a Debian package is provided, installing application and some examples in /opt/vmbase directory. Most of dependencies are installed by the package. But you should install SoapySDR and Soapy drivers for your SDR device first by yourself if not yet done !

To understand how it works, the best is perhaps starting download TLE and print a passes list, using scripts in ./sat/ directory.

For the next SSTV event I'd try to run unattended reception of ISS (from ./sat/sat_receiver directory).

From the examples, it appears that you can script a fully automated doppler corrected satellite signal receiver with the SDR interface connecting through Soapy, and all the DSP handled by the SDR4lite library.

SDR4Space Logo

Frugal Radio: How To Decode L band Satellite ACARS and CPDLC messages with JAERO and your SDR

In the latest episode of his YouTube series on Aviation monitoring Rob explores how to decode L-band satellite ACARS (Aircraft Communication Addressing and Reporting System) and CPDLC (Controller Pilot Data Link Communications) messages using JAERO, an SDR like an RTL-SDR, and a appropriate L-band antenna such as our RTL-SDR Blog Active L-Band Patch (currently out of stock).

In the video Rob shows examples of what you might receive such as CPDLC ATC instructions, digital ATIS information, arrival information and suggested landing data configuration instructions. He goes on to show satellite coverage maps, what hardware is required to receive these signals, and finally how to setup the receiving and decoding software.

How To Decode L band Satellite ACARS and CPDLC messages with JAERO and your SDR

A KerberosSDR Based Radio Direction Finding RC Boat

If you weren't already aware KerberosSDR is our 4-channel phase coherent capable RTL-SDR unit that we previously crowdfunded back in 2018. With a 4-channel phase coherent RTL-SDR interesting applications like radio direction finding (RDF), passive radar and beam forming become possible. It can also be used as four separate RTL-SDRs for multichannel monitoring.

KerberosSDR is soon to be replaced with the upgraded KrakenSDR, which will begin crowd funding on Crowd Supply later this year. Please note that we have had some pandemic related delays finalizing the design, but progress is being made.

Recently we came across a brief demonstration video on YouTube where it appears that students have embedded a KerberosSDR into an RC boat. The boat carries four direction finding antennas connected to the KerberosSDR and autonomously navigates towards a signal source.

KerberosSDR Direction Finding RC Boat
Kerberos SDR project result

KerberosSDR direction finding #2