Oona Räisänen determined the GPS coordinates of the helicopter from YouTube’s sound
Finnish engineer and hacker Oona Räisänen, who is keen on sound processing, accidentally discovered YouTube video from a helicopter while chasing a car. Una was not fascinated by the adventures of the police, but she was very interested in the unusual sound she heard from the speakers, especially from the left channel.
At first, Uona thought that this was interference from the engine, but when she isolated the left channel and amplified the sound ( selostus2.mp3 ), it immediately became clear that this was some kind of mysterious digital signal!
Curiosity made the girl take on the puzzle . Judging by the spectrogram of the left channel, the device used binary frequency modulation (BFSK), when values of 0 and 1 are assigned to sounds of a certain frequency. In this case, the frequency changed between 1200 and 2200 Hz. UNA demodulated the signal using a low and high pass filter in an open-source program for audio processing SoX (Sound eXchange). The output is a digital stream at 1200 baud.
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A stream consists of packets of 47 bytes, synchronized by the initial and final bits and separated by a 0x80 byte. Most of the bits remain the same throughout the entire video, but three separate groups are definitely changing. In the screenshot they are marked in blue.
Did she start to wonder what it could be? Maybe telemetry data? Or information about the direction of shooting from a video camera? Any time stamp on frames?
After analyzing the three groups of data, Räisänen came to the conclusion that this is precisely telemetry. If we discard the first four bits in each byte, then the remaining bytes can be represented as a pair of three-digit numbers encoded in base-10. If you plot them on the x and y coordinate axes, you’ll get a graph that is very similar to the actual movement pattern of a helicopter. The yellow map shows its approximate coordinates, judging by the picture from the video, and at the top left, a graph with values from the mysterious data stream (the first few minutes of the video are displayed).
If you look more closely, you can see that the 100 coordinate pairs exactly correspond to one minute of video.
Knowing the final coordinates of the helicopter (at the end it circled over the police station), you can tie the resulting trajectory to the Google Earth map.
Thanks to the advice of her knowledgeable comrades from the Internet, Oona Räisänen realized that the 7-bit encoding used is Bell 202 ASCII. After decoding, the values look like this:
These are full pairs of longitude and latitude with an accuracy of four digits (39.0386 ° N, 94.3420 ° W).
PS By the way, a year ago, Una Raisanen made a clear flowchart with a description of all the sounds that a dialup modem makes when establishing a connection.
PPS Oona told about the decoding of a digital signal from open radio programs in a report at the hacker conference Chaos Communication Congress in Hamburg on December 30, 2013 ( video ).
At first, Uona thought that this was interference from the engine, but when she isolated the left channel and amplified the sound ( selostus2.mp3 ), it immediately became clear that this was some kind of mysterious digital signal!
Curiosity made the girl take on the puzzle . Judging by the spectrogram of the left channel, the device used binary frequency modulation (BFSK), when values of 0 and 1 are assigned to sounds of a certain frequency. In this case, the frequency changed between 1200 and 2200 Hz. UNA demodulated the signal using a low and high pass filter in an open-source program for audio processing SoX (Sound eXchange). The output is a digital stream at 1200 baud.
')
A stream consists of packets of 47 bytes, synchronized by the initial and final bits and separated by a 0x80 byte. Most of the bits remain the same throughout the entire video, but three separate groups are definitely changing. In the screenshot they are marked in blue.
Did she start to wonder what it could be? Maybe telemetry data? Or information about the direction of shooting from a video camera? Any time stamp on frames?
After analyzing the three groups of data, Räisänen came to the conclusion that this is precisely telemetry. If we discard the first four bits in each byte, then the remaining bytes can be represented as a pair of three-digit numbers encoded in base-10. If you plot them on the x and y coordinate axes, you’ll get a graph that is very similar to the actual movement pattern of a helicopter. The yellow map shows its approximate coordinates, judging by the picture from the video, and at the top left, a graph with values from the mysterious data stream (the first few minutes of the video are displayed).
If you look more closely, you can see that the 100 coordinate pairs exactly correspond to one minute of video.
Knowing the final coordinates of the helicopter (at the end it circled over the police station), you can tie the resulting trajectory to the Google Earth map.
Thanks to the advice of her knowledgeable comrades from the Internet, Oona Räisänen realized that the 7-bit encoding used is Bell 202 ASCII. After decoding, the values look like this:
#L N390386 W09434208YJ
#L N390386 W09434208YJ
#L N390384 W09434208YJ
#L N390384 W09434208YJ
#L N390381 W09434198YJ
#L N390381 W09434198YJ
#L N390379 W09434188YJ
These are full pairs of longitude and latitude with an accuracy of four digits (39.0386 ° N, 94.3420 ° W).
PS By the way, a year ago, Una Raisanen made a clear flowchart with a description of all the sounds that a dialup modem makes when establishing a connection.
PPS Oona told about the decoding of a digital signal from open radio programs in a report at the hacker conference Chaos Communication Congress in Hamburg on December 30, 2013 ( video ).
Source: https://habr.com/ru/post/211128/
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