Thriller sign from a helicopter

2023-01-24 16:43:45

Final evening, YouTube steered a video for me. It was a uncooked clip from a information helicopter filming a police chase in Kansas Metropolis, Missouri. I shortly observed a bizarre interference within the audio, particularly the left channel, and thought it have to be attributable to the chopper’s engine. I turned up the quantity and realized it is not interference in any respect, however a mysterious digital sign! And off we went once more.

The sign sits alone on the left audio channel, so I can utterly isolate it. Judging from the spectrogram, the modulation scheme appears to be BFSK, switching the provider between 1200 and 2200 Hz. I demodulated it by filtering it with a lowpass and highpass sinc in SoX and evaluating outputs. Now I had a bitstream at 1200 bps.

[Image: A nondescript oscillogram of the data signal, and below it, the signal after FM demodulation, showing a clear pattern characteristic of binary FSK switching at 1200 bps.]

The bitstream consists of packets of 47 bytes every, synchronized by begin and cease bits and separated by repetitions of the byte 0x80. Most bits keep fixed in the course of the video, however three distinct teams of bytes include various information, marked blue under:

[Image: A time-stamped hex dump of the byte stream, arranged in packets with only a few bytes changing over time.]

What might or not it’s? Location telemetry from the helicopter? Details about the digicam route? Video timestamps?

The primary guess appears to be right. It’s supported by the connection of two of the three byte teams. If the 4 first bits of every byte are ignored, the info kinds a easy gradient of three-digit numbers in base-10. When plotted parametrically, they type an intriguing winding curve. It is rather much like this plot of the automotive’s place (blue, yellow) together with viewing angles from the helicopter (inexperienced), derived from the video by manually following landmarks (solely the primary jiffy proven):

[Image: Screenshot from Google Earth, showing time-stamped placemarks tracing the roads of a suburb, accompanied by an X-Y plot of the changing FSK bytes that draws a very similar picture.]

When the acquired curve is overlaid with the automotive’s location hint, we see that 100 steps on the curve scale corresponds to precisely 1 minute of arc on the map!

Utilizing this relative info, and the truth that the helicopter circled across the police station ultimately, we will plot all of the acquired information factors in Google Earth to see the situation hint of the helicopter:

[Image: Coordinates from the whole data signal plotted on top of a Google Earth satellite photo several miles across, with a lot of circling around.]

Replace: Apparently the video downlink to floor was transmitted utilizing a transmitter much like Nucomm Skymaster TX that is ready to ship dwell GPS coordinates. And that is how they appear to do it.

Replace 2: Sure, it is 7-bit Bell 202 ASCII. I attempted decoding it as 7-bit information earlier, ignoring parity, however will need to have gotten the bit order fallacious! So I simply selected a roundabout approach and saved wanting on the hex. When absolutely decoded, the stream says:

#L N390386 W09434208YJ
#L N390386 W09434208YJ
#L N390384 W09434208YJ
#L N390384 W09434208YJ
#L N390381 W09434198YJ
#L N390381 W09434198YJ
#L N390379 W09434188YJ

These are the complete lat/lon pairs of coordinates (39° 3.86′ N, 94° 34.20′ W). Nucomm says the system allows viewing the helicopter “on a transferring map system”. Additionally, it might allow the receiving antenna to be locked onto the helicopter’s place, to permit uninterrupted video downlink.

Due to all of the readers for added hints!

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