Category: Applications

Creating an APRS I-Gate and Digipeater with Baofeng and RTL-SDR

Over on YouTube ModernHam has uploaded a video showing how to create an APRS I-Gate and Digipeater with Baofeng and RTL-SDR. He also makes use of a Raspberry Pi as the computing module and an audio cable to connect the Baofeng to the audio jack of the Pi. The tutorial then consists of a walk through of the various software setup steps, and finally how to connect the Baofeng and RTL-SDR to the Pi.

If you weren't already aware, Automatic Packet Reporting System (APRS) is a digital VHF mode used in amateur radio. It allows for packets of data to be sent to receiving nodes over a local area via RF. Typical uses for it are vehicle tracking, weather station telemetry, text messages, announcements and other wireless device telemetry like high altitude balloons. An I-Gate is an internet connected node which receives local APRS RF signals and uploads them to the internet, to be seen on sites like aprs.fi. TX capable I-Gates may also broadcast to the local RF network messages from APRS transmitters on the other side of the world.

APRS I-Gate and Digipeater with Baofeng and RTL-SDR

DragonOS: Debian Linux with Preinstalled Open Source SDR Software

Thank you to Aaron for submitting news about his latest project called "DragonOS" which he's been working on while in COVID-19 lock down. DragonOS is a Debian Linux based operating system which comes with many open source software defined radio programs pre-installed. It supports SDRs like the RTL-SDR, HackRF and LimeSDR.

Aaron's video below shows how to set up DragonOS in a VirtualBox, and he has two other videos on his channel showing how to set up ADS-B reception with Kismet, and how to run GR-RDS in GNURadio. He aims to continue with more tutorial videos that make use of the software installed on DragonOS in the near future.

DragonOS 10 Installer (download in description)

Screenshot of the GR-RDS Tutorial

RTL-SDR AIS Android App Now with Waterfall Display for Frequency Offset Tuning

Thank you to Christian, author of the RTL-SDR AIS Android App for letting us know that he's updated his app and it now includes a waterfall display for tuning the AIS frequency. Tuning the AIS frequency is not required on higher end RTL-SDR dongles that come with a TCXO (Temperature Compensated Oscillator), but cheaper RTL-SDRs will have significant frequency offsets that will require the offset to be determined after a few minutes of warm up time. The easiest way to do this is with a waterfall display as that allows you to tune the frequency manually.

AIS stands for Automatic Identification System and is used by ships 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.

RTL SDR AIS Driver App PPM

SATSAGEN: Software to use a PlutoSDR as a Tracking Spectrum Analyzer

Thank you to Frank, HB9FXQ for submitting news about a new Windows program called SATSAGEN which allows you to use a PlutoSDR as a wideband spectrum analyzer. SATSAGEN was created by Alberto IU1KVL and is entirely free to use. This makes it possible to get wideband scans of RF components like filters and attenuators. Together with a directional coupler it could also be used to measure the SWR of antennas as HB9FXQ demonstrates in his Twitter post.

The PlutoSDR is a low cost RX/TX capable SDR with up to 56 MHz of bandwidth and 70 MHz to 6 GHz frequency range. It is typically priced anywhere between US$99 - US$149 depending on sales.

In the video below Alberto demonstrates SATSAGEN performing some wideband scans, and he shows the various features of the software. He writes that the system has a scan range from 70 MHz to 6 GHz and can show results in dBm. The spectrum analyzer works with the TX part of the PlutoSDR to provide a tracking generator with resolution of up to 1024 points. The software can also use the PlutoSDR as a frequency generator with 1 kHz of resolution.

There is also a support group available at groups.io/g/satsagen.

SATSAGEN Screenshot
SATSAGEN Screenshot

OpenEar: An Easy to Use Windows TETRA Voice Decoder

A new TETRA voice decoder called "OpenEar" has just been released. The program is a standalone Windows app that directly connects to an RTL-SDR. Decoding a TETRA voice signal is as simple as opening the program, tuning to the TETRA frequency and clicking on the signal. With good signal strength voice comes through very clearly. CPU usage on our PC is also minimal. 

The program source is currently not available as the author notes that he only intends to release it as open source in the future once the project is completed, and right now this is only the first early release. Right now the program is just an .exe with a few .dlls. You'll need to first install the Microsoft Visual C++ Redistributable Package linked in the Git readme. Just in case, we virus scanned the exe and tested the program in Sandboxie. It appears to be clean, and it works as intended.

In the future the author hopes to support many more protocols such as DMR, MPT1327, ACARS, AIR, GSM and more. In order to support his work he is asking for Bitcoin donations, and the donations link can be found on the Git readme.

UPDATE 1: If you're getting missing dll errors and you already installed the Visual C++ Redistributable, try downloading the missing dll's from dll-files.com. There should only be about 5 missing.

UPDATE 2: As pointed out in the comments by Steve M. from Osmocom, this software may be in violation of several GPL licences as no source code has been released and it appears to rely on GPL code and libraries. Please take this into account.

UPDATE 3: As per update 2, the author has decided to temporarily disable the TETRA functionality pending a rewrite of the code that he will complete within one to two months). Instead he has added DMR decoding.

OpenEar TETRA Voice Decoder Screenshot
OpenEar TETRA Voice Decoder Screenshot

YouTube Tutorial on Receiving Weather Images from NOAA Satellites

Over on YouTube the "Ham Radio Crash Course" channel has uploaded a new video showing how to receive APT images from NOAA weather satellites. There are many tutorials (such as ours here) and videos on this topic already, but more cannot hurt, and this one makes specific reference to how to download the WXtoIMG software now that the official website has been abandoned.

In the tutorial he uses an SDRplay with SDRuno as the receiver software, VBCable as the audio piping software, and WXtoIMG as the decoding software.

How To Receive Images Directly From NOAA Satellites

A Seminar on Setting up and Understanding a SatNOGS Satellite Ground Station

At the 2019 TAPR Digital Communications Conference (DCC), Corey Shields (KB9JHU) and Dan White (AD0CQ) presented a comprehensive guide on setting up your own SatNOGS satellite ground receiver station. The video of the presentation has just recently been uploaded to YouTube by Ham Radio 2.0.

SatNOGS is an open source project that aims to make it easy for volunteers to build and run satellite ground stations (typically based on RTL-SDR and Raspberry Pi hardware) that automatically receive RF satellite data, and automatically upload that data to the internet for public access. This is very useful for low budget cubesats launched by schools and small organizations who don't have the resources to run a worldwide satellite ground station network. Without global ground stations the majority of data and telemetry collected by the satellite could be lost as it would only pass over the owners ground station once or twice a day with limited time and bandwidth to downlink data. SatNOGS volunteers with distributed ground stations placed all over the world provide a free solution for this problem. 

Setting up a SatNOGS station and understanding the data coming down can be a pretty involved project, so Corey and Dan's 3.5 hr presentation gently guides us through the steps required. The guide focuses most on the software side, and does not include information about building their open source Yagi antenna rotator which can be used to receive satellites with lower power weak signals. Instead they focus on using a simpler fixed QFH antenna which is still capable of receiving data from a majority of satellites.

Learn to build and operate your own SatNOGS ground station. The Sunday Seminar is somewhat like the "anchor" topic of the entire weekend of the TAPR Digital Communications Conference. In 2019 we had the privilege of hearing from Corey, KB9JHU and Dan AD0CQ from the SatNOGS Team and they are going to give us, in detail, instructions for setting up a home satellite station.

(2:38) Intro
(7:46) Section 1: Satellite Building 101
(1:14:50) Section 2: Using SatNOGS
(2:19:55) Section 3: API and Contributing
(2:51:55) Section 4: RF Stack and Decoders

SatNOGS Ground Station Building Guide from TAPR DCC 2019

Thesis on Locating Transmitters with TDoA and RTL-SDRs

Jan Hrach of the Faculty of Mathematics and Physics at Univerzita Karlova in the Czech Republic recently defended his Masters thesis titled "Passive emitter tracking". The main theme of the thesis was the use of RTL-SDRs for tracking transmitters via the Time Difference of Arrival (TDoA) technique. TDoA works by having multiple receivers spread out over a region. As long as the receivers are synchronized in time, we can calculate the difference in time that a signal took to arrive at each receiver, which allows us to pinpoint the location of a transmitter. The challenge is in the timing synchronization, and receiver placement. The thesis abstract reads:

We have implemented a TDOA multilateration of transmitters on an unmodified rtl-sdr receiver using transmitters with known location as a timing reference. We present a brief theoretical background and describe the measurement process which includes several approaches that correct the timing and frequency errors between the receivers. Additionally, we have implemented an angle of arrival direction finder using coherent rtl-sdr.

The thesis and associated code is available on the universities website at this link and it is written in English. Jan also does have a presentation available on YouTube, however it is presented in Czech and automated subtitles do not appear to be available. The video and results section of the thesis shows some good results that indicate that transmitters were able to be pinpointed with very good accuracy, however, localization only worked well on signals with good cross-correlation properties, like DVB-T. Only about half the tested broadcast FM stations could be located due to interference, FM being low bandwidth and FM being transmitted at lower frequencies which suffer from reflections and multipath all of which result in poorer correlation.

TDoA results achieved with RTL-SDRs distributed around Prague.
TDoA results achieved with RTL-SDRs distributed around Prague.