Category: RTL-SDR

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.

Combining Android Tasker and an RTL-SDR for Mobile Automated Frequency Power Scans

Over on YouTube Ian Grody has uploaded two videos demonstrating an early alpha project that he is working on which combines Android Tasker with RTL-SDR frequency scanning. Tasker is an Android automation app which allows users to define a task based on a context. For example, you could set it to turn on WiFi and open an app (task) every time you arrive at a certain location (context).

Ian's idea is to create a Tasker application that performs an rtl_power scan with the RTL-SDR whenever a certain context is detected. The current version of his Tasker app can perform an rtl_power scan over a certain frequency range at the tap of a button, detect the strongest frequencies in that range, and plot a marker at the current location on a Google map which displays the strongest frequency detected at that location. He eventually hopes to turn the application into a wardriving application that will scan 27 MHz - 1.7 GHz for active signals while on the move.

His Tasker alpha application is available via the link on his Reddit post.

Tasker and a Software Defined Radio

Tasker and an RTL SDR - Part II

IGate2: An RTL-SDR Compatible APRS iGate for Android

Thank you to Agrosi Luciano for submitting news about his new RTL-SDR compatible Android App called "IGate2". IGate2 is a receive only APRS IGate written for Android devices. There is a free and paid version of the app. The free version is limited to 100 packets forwarded per session. The paid version costs US$3.49 and has unlimited packet forwards. The description of the app is pasted below:

The RTL-SDR dongle tuner (cost starting from 10 €) and its antenna, receives the information contained in APRS packets transmitted from HAM radio stations, and then a phone device, with IGate2, forwards them to the world wide web using its internet connection (WiFi or 3G).

IGate2 acts as a Software Defined Radio Demodulator, a TNC Modem and an Internet Gate.

It needs the installation of a driver (Martin Marinov’s driver) for the SDR dongle that you can find in: https://play.google.com/store/apps/developer?id=Martin+Marinov.

If you already own an unused cellular phone or tablet, IGate2 represents a very cheap, compact and easy-to-use solution for suppling an IGATE service to radio amateur community.

Raw data contained in radio packets are visible on the phone screen and may be routed (if you check this option) to the APRS-IS network. All data convoyed and shared in APRS-IS network can be seen in maps and bulletins on particular websites, for example: http://aprs.fi/ .

To be authorized to send data to APRS-IS you must have a HAM CallSign and a PassCode. See aprs-is.net. If you are not a radio amateur, you may only use your equipment in receive only mode.
The app has an audio monitor useful for tuning the parameters of the Sdr receiver (it may not work well in old devices with low memory). In the main page there is a frequency switch, a hub with the text of received packets, two indicator lights: one for the Sdr connection and one for the Aprs-Is connection, three counters reporting the number of: received, forwardable and forwarded packets. When you leave the main page while the IGate is running, the app service will continue working in the background, you can recall the main page by tapping the service icon in the android status bar.

Since the device and the Sdr dongle drains much power from the phone battery, it is recommended to use the phone charger or a power bank. You will need an OTG power cable. It is not easy to find a working cable, maybe you can do it yourself. The reception quality of the IGate depends, above all, on the antenna connected to the Sdr dongle. With very strong FM broadcasts in your area, it may be helpful to manually adjust the gain of the receiver or use a band-stop filter.

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 IGate 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 IGates may also broadcast to the local RF network messages from APRS transmitters on the other side of the world.

IGate2: Android App that turns your phone and RTL-SDR into an APRS IGate.
IGate2: Android App that turns your phone and RTL-SDR into an APRS IGate.

 

OneSDR New Posts: RF Filter Primer, Bias Tees, SDR Precautions

Over on onesdr.com a new SDR tutorial website, the authors have put up three new posts. The first post is part 2 of their "How Not to Break your Software-defined Radio (SDR) Hardware" series. This post covers mechanical strain considerations on connectors and reference clock input voltages. 

The second post titled "Software-defined Radios and Bias Tees" covers the use of bias tee's and the different voltage and current specs of bias tee's on different SDRs. They explain how these specs affect which LNA's you can use, and how some bias tee's are protected against over-current damage.

The third post is titled "A Primer on RF Filters for Software-defined Radio". In this post they cover topics like types of filters, insertion loss and preselectors on SDRs. 

OneSDR's Image used to explain Band Pass filters.
OneSDR's Image used to explain Band Pass filters.

GQRX Updates: GR3.8, New Color Maps, Bug and Performance Fixes

Since mid-January 2020 the popular Linux and Mac compatible SDR program GQRX has seen a number of new code commits over on it's git repository. Some of the updates include moving to GNU Radio 3.8, new color maps, as well as various bug and performance fixes.

At the moment these updates only appear to be available on the latest git code, so to get them you'll need to install GQRX from source via the instructions on the git readme.

Also thank you to @devnulling for providing us with the screenshot posted below which shows off the various new color maps available for the FFT waterfall.

GQRX Updated Color Schemes
GQRX Updated Color Maps

ADSBExchange now using tar1090: Historical Flight Tracks, Military Aircraft Filters and more

ADSBExchange is an aircraft tracking website service which aggregates ADS-B data from contributors running RTL-SDR's or similar receivers worldwide.

However, unlike other flight tracking sites such as flightaware and flightradar24, ADSBExchange sets themselves apart by proudly refusing to censor the tracking of military and private jets that have requested privacy. One area where this refusal to self-censor helps is with the "Dictator Alert" service. This is a service that automatically tracks the movements of private aircraft owned by authoritarian regimes via the ADS-B data collected and shared by ADSBExchange. 

Recently ADSBExchange upgraded their web interface moving from the old Virtual Radar Server system to tar1090 which is a more fully featured open source display for dump1090. This new interface has some great features, like the ability to view the complete flight track history of any aircraft on a particular day, the ability to display only military aircraft and the ability to filter by altitude and aircraft type. 

ADSBExchange.com new tar1090 interface
ADSBExchange.com new tar1090 interface
Viewing the tracks of 1-day of police helicopter activity in the new ADSBExchange tar1090 interface.
Viewing the historic tracks of 1-day of police helicopter activity in the new ADSBExchange tar1090 interface.

New 978 MHz UAT Specialty RTL-SDR and Antenna and Dual 1090 & VHF Antenna from RadarBox

Over on Amazon we've recently seen the release of a 978 MHz UAT specialty RTL-SDR and 978 MHz tuned antenna by the flight tracking service known as AirNav RadarBox. The RTL-SDR appears to be similar to their 1090 MHz RTL-SDR version, which contains a SAW filter and LNA onboard the RTL-SDR. Due to the built in filter, this dongle will only work at the 978 MHz frequency. Like the 1090 MHz version, the dongle itself is priced at only US$14.95, and the antenna at US$49.95.

Universal Access Transceiver (UAT) is an alternative to ADS-B that is available only in the USA. It is typically used by smaller aircraft, transmits at 978 MHz, and apart from it's tracking system it has some additional advantages for pilots over 1090 MHz ADS-B, like the ability to receive alerts, weather data and radar plots. With an RTL-SDR and appropriate software these data services can also be received.

In addition they have also released some other interesting products including a 1090 MHz ADS-B with 118-136 MHz VHF airband antenna stacked on top of the ADS-B element for US$49.95, and an external ADS-B 1090 MHz filter for only US$14.95.

As an alternative to an RTL-SDR UAT receiver, we note that the Stratux has a hardware radio based UAT receiver available which has significantly lower power consumption. Although the receiver itself appears to be currently out of stock.

We note that we also currently have our 1090 MHz AirNav Radarbox Antenna + ADS-B optimized RTL-SDR set on sale for only US$39.95 + shipping, which is cheaper than you can find it elsewhere. Visit our store for ordering information.

The AirNav RadarBox 978 MHz UAT Optimized RTL-SDR and Antenna.
The AirNav RadarBox 978 MHz UAT Optimized RTL-SDR and Antenna.
New RadarBox 1090 MHz Filter and 1090 MHz & VHF Antenna single antenna.
New RadarBox 1090 MHz Filter and 1090 MHz & VHF Antenna single antenna.

OpenWebRX Version 0.18.0 Released: New Decoders for Digital Voice, Digital Ham Modes and More

Back in early January we posted about how the popular web based SDR and RTL-SDR compatible receiver software known as OpenWebRX was officially discontinued by the original author. However, thanks to it's open source licence, code contributor Jakob Ketterl (DD5JFK) has been able to continue developing the code and is taking over as the lead developer on his own fork of the code.

Recently he released version 0.18.0 of OpenWebRX which includes a few major upgrades including the much needed shift to Python 3, and the inclusion of multiple new decoders for DMR, D-Star, YSF, NXDN, FT8, FT4, WSPR, JT65, JT9, APRS and Pocsag.

Hello fellow radio enthusiasts,

with great excitement I would like to announce the availability of OpenWebRX Version 0.18.0 as public release. This is the first release of the project in some time, and the first release since I started working on it, so I’m more than happy to bring this to you.

What’s new? Quite a lot, actually. For those that haven’t had the chance to follow the progress of the project in the past months, here’s a quick overview:

    • Most of the server code has been rewritten for better flexibility, stability and performance. The project is now fully based on Python 3.
    • Large parts of the frontend code have been updated or polished.
    • The new core now supports multiple SDR devices simultaneously, as well as switching between multiple profiles per SDR, allowing users to navigate between multiple bands or frequencies.
    • Added support for demodulation of digital voice modes (DMR, D-Star, YSF, NXDN).
Added support for digital modes of the WSJT-X suite (FT8, FT4, WSPR, JT65, JT9).
  • Added support for APRS.
  • Added support for Pocsag.
  • Bookmarks allow easy navigation between known stations.
  • Background decoding can transform your receiver into an automatic reporting station, including automatic band scheduling.
  • The integrated map shows digimode spots as well as APRS and YSF positions.
OpenWebRX 0.18.0 is available via the following channels: Please check out our updated Setup Guide along with the rest of the documentation on the Wiki!

Questions, ideas, problems? Get in touch with the community at [email protected]!

Best regards and vy 73s

Jakob DD5JFK

We're so glad to see that this excellent software isn't dead in the water and is in fact thriving. We will continue to follow the Jakob's and the OpenWebRX communities' future developments. If you are interested, you can follow OpenWebRX development on the OpenWebRX groups.io forum.

OpenWebRX Screenshot
OpenWebRX Screenshot