Category: Applications

Logic Trunked Radio Analyzer and the RTL-SDR

Over on YouTube user k2nccvids has posted a short video showing the Logic Trunked Radio (LTR) Analyzer software working with SDR# and the RTL-SDR. Logic Trunked Radio is a type of radio trunking system that uses distributed control channels modulated into the analogue voice channel instead of using just one signal control channel.

LTR Analyzer Sample

RTL-SDR Tutorial: Following Trunked Radio with Unitrunker

The popular trunking decoding software Unitrunker now supports the RTL2832U R820T RTL-SDR directly in its new version. This means that extra SDR receiver software like SDR# is no longer required to use Unitrunker.

You can download the latest version of Unitrunker here.

In a normal radio system, one company (or talkgroup) might use a single frequency for radio communications. However, this is very inefficient as the frequency may not be in use for the majority of the time. In a trunked radio system, a small set number of frequencies are shared between a large number of talkgroups. Each radio receives a special computer controlled control channel. The control channel determines a vacant frequency that a particular talkgroup should use. This helps to make radio frequency allocations more efficient.

Because a talkgroup might switch between various frequencies often, it can make listening to a conversation difficult for radio scanners. Unitrunker can be used to decode the control channel and follow a voice conversation as it hops across various frequencies. With two RTL-SDR dongles you can set up a trunking receiver station with just Unitrunker. What follows below is a tutorial on how to set this up.

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Decoding a Weather Temperature Sensor with the RTL-SDR and GNU Radio

Over on YouTube user MrCircuitMatt has uploaded two videos on how he was able to decode a temperature weather sensor using an RTL-SDR and GNU Radio. His videos go through the GNU Radio technical steps as well as the signal encoding theory he used to decode the temperature.

Using RTL-SDR to read temperature from outdoor sensor, part 1
Using RTL-SDR to read temperature from outdoor sensor, part 2

Reverse Engineering a RF Controlled Ceiling Fan with the RTL-SDR

Using an RTL-SDR Clayton Smith was able to reverse engineer his remote controlled ceiling fan. To do this he first used his BladeRF to determine that the remote control was transmitting a signal at 303.747 MHz. He then used a simple GNU Radio flow graph with the RTL-SDR to plot the amplitude of the signal over time which suggested that the signal was using on-off keying. From the plot he was then able to visually determine the bit pattern sent from each button on the ceiling fan remote.

Next he used his bladeRF and another GNU Radio flowgraph to replicate and transmit the the bit pattern which was able to control the ceiling fan from the PC.

Clayton notes that all this reverse engineering was done in half an hour, demonstrating the power of software defined radio.

Ceiling Fan Bit Pattern Recovered with an RTL-SDR and GNU Radio
Ceiling Fan Bit Pattern Recovered with an RTL-SDR and GNU Radio

Direct Sampling Mode with No Hardware Modifcations

Over on the Reddit RTL-SDR forums user Jengal has posted a modified RTLSDR.dll dll file for SDR# which allows the direct sampling mode to be used on the R820T without the need for any hardware modifications. The modified dll is compiled from keenerds experimental branch of the RTL-SDR driver, which uses code for the no hardware mod written by tejeez. The no hardware direct sampling code was inspired by Anonofish’s discovery where he found that the E4000 based RTL-SDR could tune to AM radio without the need for the direct sampling mod (though this appears to be now patched in the newer rtlsdr drivers).

The direct sampling mod is a hardware modification to the RTL-SDR which allows it to receive HF frequencies between 0-14.4 MHz without the need for an upconverter. It works by connecting an antenna directly to the RTL2832U chip, thus bypassing the tuner. Teejez’s modification tells the RTL-SDR to bypass the tuner in software, allowing antennas to be connected to the normal antenna port. HF reception with the experimental driver is very poor in comparison to the direct sampling hardware mod or an upconverter, but even so Jengal was able to receive AM Radio, an SSB ham radio signal and an HF weather report with a simple longwire antenna.

To use the modified dll, simply download it from this link, rename it to rtlsdr.dll, and replace the original rtlsdr.dll in the SDR# folder. Then connect an HF antenna to the normal antenna port and in SDR# tune to a frequency between 0-14.4 MHz. Next turn ON the RTL AGC option in the configure menu. Jengal replaced the function of the RTL AGC option with the direct sampling mod. He found that best reception occurred when he set the gain to 48 dB.

No Hardware Mod Direct Sampling in SDR# Receiving AM Radio
No Hardware Mod Direct Sampling in SDR# Receiving AM Radio

Decoding 2x AIS Channels with SDR Console, AISMon and OpenCPN

YouTube user k2nccvids has posted a video showing him decoding and plotting both AIS channels simultaneously on a Windows PC with free software. To do this he uses SDR Console and two AISMon instances. SDR Console is a general purpose SDR GUI that supports the RTL-SDR. One of it’s major advantages over other SDR software is that it can tune to multiple signals in the same swath of tuned bandwidth simultaneously and output their audio to different virtual audio cables.

k2nccvids used two AISMon instances, each one connected to a separate virtual audio cable outputting AIS audio from SDR Console. He set AISMon to output decoded UDP packets on two different ports. Then he created two UDP listeners in OpenCPN for plotting, one for each port.

AIS RTL-SDR w/MultiPSK, AISmon (x2) OpenCPN,

RTL-SDR DSD Call Log Recorder

Programmer Tyler Watt has been working on software that automatically logs each call from an RTL-SDR running DSD (Digital Speech Decoder) and then stores it in a time stamped database as an mp3 file. There is also a web front end for the database which allows public users to search and play recorded calls.

Recently Tyler updated his code by rewriting it in PHP and making it multiplatform.

The old version of his web front end in action for his local P25 calls can be viewed here, and the newer one can be found here.

Reverse Engineering Radio Controlled Power Outlets with Help from the RTL-SDR

Radio controlled electricity power outlets are outlets that can be turned on or off using a wireless radio controlled remote. Over on the blog the author has written an article showing how he was able to reverse engineer the wireless power outlets radio protocol.

The author used an RTL-SDR and SDR# to listen to the outlets wireless AM transmissions at 434 MHz. He then recorded the signal audio and then used audacity to view the waveform. By analyzing the audio output he discovered that the signal was a Non-Return-To-Zero (NRZ), pulse width modulated (PWM), Amplitude Shift Keying / On Off Keying (ASK/OOK) signal.

Later he was also able to use the RFCat USB dongle to transmit an on off signal from his computer. RFCat is an USB dongle that is capable of transmitting on 433 MHz.

RTL-SDR Software Radio used to Reverse Engineer the Wireless Power Outlet
RTL-SDR RTL2832U Software Radio Audio output Analyzed in Audacity for Reverse Engineering a Wireless Power Outlet
Remote Control Outlet Replay With RFCat

Source Hackaday

Decoding APRS with SDR#, APRSISCE32 and an RTL-SDR

Over on YouTube user k2nccvids has posted a video showing how he was able to decode APRS signals and plot them on a map using APRSISCE32. APRSISCE32 is an advanced Automatic Packet Reporting System (APRS) decoder which has mapping capabilities and can also connect to the internet as an iGate.

APRS is used by amateur radio hobbyists to send data like messages, announcements and also GPS coordinates.

Decoding APRS using SDR#, APRSIS32, RTL-SDR

Real Time GPS Positioning with the BladeRF

Over on YouTube user taroz1461 shows real time GPS positioning done in software using a BladeRF. The BladeRF is a ~$400 software defined radio which similar specs to the HackRF and compared to the RTL-SDR is capable of receiving much larger bandwidths and transmitting.

To do this decoding he used RTKLIB and his own GNSS-SDRLIB software which is a Windows GUI program. We aren’t sure if this software will work with the RTL-SDR, but we note that other people have had success with GPS positioning and the RTL-SDR.

Real-time GPS positioning with bladeRF