In this challenge, the start of a spectrum-painted flag is apparent when the signal is viewed in inspectrum, but as time progresses the signal narrows in frequency, widens in time, and is engulfed by an ever-increasing amount of noise. For instance, obstacles may absorb or reflect radio signals. Beware that some things other than distance from the transmitter also affect received signal power. When you get very close and your RF gain is already at a minimum, it may be necessary to switch to a worse antenna or remove the antenna completely. Once you’re close enough to the signal that the receiver is saturated (and audio begins clipping), reduce the RF input gain and keep searching for a stronger signal. Walking closer to the transmitter will then make the signal louder, and walking away will make it quieter. I recommend putting on a pair of headphones and disabling both hardware and software AGC, thus making the audio amplitude directly proportional to the incoming RF signal power. Gqrx does a fine job of receiving the Morse code, and it’s even possible to read it by eye from the waterfall:Īlthough directional antennas can be helpful to find hidden transmitters, most players got by with omnidirectional antennas. Each transmitter is powered by a 2000 mAh lithium cell, which is enough to provide four days of power. That makes it easy to tap out a message in Morse code. These boards are normally used to transmit and receive FSK signals, but they also have an on-off keying mode where the transmitter can be switched on and off using a GPIO pin. I built them with Adafruit Feather M0 RFM69HCW Packet Radios, which are available in 433 MHz and 915 MHz versions. Here’s a peek inside the hidden transmitters: Players had to determine the exact frequency of each transmitter, copy the flag which was included in the Morse code message, and then physically locate the transmitter to read another flag which was printed on it. The other transmitted at approximately 904 MHz, and was carried around by various conference organizers in their backpacks, making it a moving target. One ran at approximately 433 MHz, and was hidden at a fixed location in the conference area. I built two hidden transmitters (“foxes”) which transmitted Morse code. This challenge was inspired by amateur radio direction finding (also known as radio fox hunting) where competitors run through the woods searching for hidden radio transmitters. If you solved the SSTV challenge, you can also receive the SSTV images that the International Space Station occasionally broadcasts on 145.800 MHz! Fox hunting To test the challenges, I used the following: Most of the signals can be received directly within Gqrx, but a few require additional software. For GRCon22, I added M17, a promising open-source alterative to proprietary digital voice protocols. I later published the source code, and in 2019 I adapted it for use as a CTF challenge at BSides Ottawa. This challenge has its origins in a tutorial session I created for the Ottawa Amateur Radio Club in 2014. Flags were in the mono audio (L+R) channel, stereo audio (L-R) channel, and Radio Data System (RDS) subcarrier. Automatic Packet Reporting System (APRS).The flag appeared in a QR code within the image. FM-modulated Slow-Scan Television (SSTV).I expected that players might use the Signal Identification Wiki to help identify the signals and find decoding software. This challenge track consisted of 13 flags embedded in 11 signals, all contained in a single SigMF recording. If you don’t want any spoilers, stop reading here! Signal identification In this post, I’ll describe how I created my challenges, and how I anticipated they might be solved. Even though the competition period has ended, most of the challenges are still up and playable at. There were 52 challenges, and 71 teams submitted 686 valid flags. Luckily, I didn’t have to do it alone: Muad’Dib, Aerospace Corporation, Daniel Estévez, and Yamakaja all stepped up to contribute challenges of their own. The CTF is my favourite part of GRCon, so when the conference organizers asked whether I’d be willing to organize this year’s competition, I jumped at the opportunity. Secret messages (“flags”) are hidden in radio signals, which players have to find and then submit to a scoring system to earn points. Since 2016, the GNU Radio Conference has held a Capture the Flag (CTF) competition in parallel with its technical track.
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