Tagged: afsk

Unknown Signal Reverse Engineering and Decoding AFSK Signals Tutorial

Over on his blog "ele y ciencia" has written up two very useful blog posts - one on how to decode AFSK signals from scratch and the other on how to reverse engineer any unknown digital signal. The blog is written entirely in Spanish, but Google translate does a decent enough job at getting the message across (in Chrome right click anywhere on the page and select Translate to English or use the Google translate webpage).

The first post is about decoding an AFSK protocol and explains that you need to record the signal with an RTL-SDR or other SDR, apply a low pass filter to obtain the signal envelope and then apply thresholding with the known baud rate to obtain the demodulated digital signal. The tutorial is high level and just explains the process, but doesn't show how to do it in any software. Later on in the post he goes on to show how he reverse engineered a train-land radiotelephone system and a TCM3105 modem chip which utilizes a FSK system.

In the second post he shows how to decode any unknown digital signal using just an RTL-SDR and Audacity. He starts off with finding and recording an unknown digital signal with an RTL-SDR and then reverse engineers it in a sort of manual fashion without using any tools like Universal Radio Hacker. The post goes through the full details and steps that he took, and in the end he gets data out of the signal discovering that it is data from a Fleet Management System used in his country for monitoring data such as speed and engine data from commercial vehicles like trucks and buses.

The two posts are very detailed and could be an excellent reference for those interested in reverse engineering some unknown digital signals in your area.

Decoding an Unknown "Fleet Management" signal from scratch.
Decoding an Unknown "Fleet Management" signal from scratch.

Receiving ISS Data Comms with the RTL-SDR

YouTube user mutezone has uploaded a video showing some data communication packets from the International Space Station (ISS) being received with the RTL-SDR. To receive the packets he used SDRSharp, and piped the audio using a virtual audio cable to the Qtmm AFSK1200 Decoder.

I tried to get the ISS (International Space Station) data comms on 145.825 MHz while the satellite was in orbit close to my location & it worked, even though it can go off frequency due to atmospherics & such. On this day, I caught it when it orbited twice around my location in the space of almost three hours. The data comms was decoded on the 2nd attempt. The antenna I used was an omni placed outdoors, & also using a TV + radio signal booster.
For anyone interested in getting the ISS, you have to wait until it orbits close to your location, & I fully recommend a decent aerial that should be placed externally. You can check the ISS tracker websites to see live updates of when & where it will orbit. Here is a link to one website…


List of frequencies link…


Receiving ISS Data comms on RTL-SDR in UK, 6th June 2013

Transmitting Data with a Raspberry Pi and RTL-SDR

Hackaday brings to attention a simple hack where hacker Marc uses an antenna connected to a general purpose I/O (GPIO) pin on his Raspberry Pi to wirelessly transmit a wav file via AFSK modulation to his RTL-SDR. He uses a program called minimodem to encode the wav on the Raspberry Pi and then on the PC to decode the data received by the RTL-SDR.

Using this method, it is claimed that a signal can be transmitted up to 50m away, even through walls.

Raspberry Pi Transmitter
Raspberry Pi Transmitter Received with RTL-SDR