Decoding News Helicopter Signals on YouTube

news-heli2

A serendipitous YouTube video recommendation led [Oona] to a raw copy of a news helicopter car chase video. While watching the video she noticed an odd sound playing from her left speaker. That was all it took to put [Oona] on the hunt. Decoding mystery signals is a bit of an obsession for her. We last saw [Oona] decoding radio signals for bus stop displays. She isolated the left audio channel and sent it through baudline software, which helped her determine it was a binary frequency shift keyed (BFSK) signal. A bit more work with SoX, and she had a 1200 baud bit stream.

Opening up the decoded file in a hex editor revealed the data. Packets were 47 bytes each. Most of the data packets was static. However, thee groups of bytes continuously changed. [Oona] decoded these numbers as latitude and longitude, and plotted the resulting data on Google Earth. Plotting her data against the position of the car in the video revealed a match. [Oona] had a complete track of the news helicopter as it followed the car. The telemetry data is in 7-bit Bell 202 ASCII, and is most likely part of an Interruptible Foldback (IFB) system used by the helicopter news crew and the studio producers. Click past the break for the YouTube video that started this all.

[News Helicopter Image Source]

Comments

  1. cooperised says:

    That is some genuinely excellent sleuthing. I love it. :-)

  2. CorrosiveOne says:

    I really love the blog this came from.. I was going to send it in but, of course its so good that HAD already has it here.

  3. Chas says:

    This is pretty awesome!

  4. Dougmsbbs says:

    Sweet! Nice work!!

  5. Chad says:

    Great story, but the pic of the heli is not a Bell helicopter. It’s a Eurocopter. 3 blades, not 2 or 4 like the American copters.

    I know, I’m knit-picking

  6. Alex Calder says:

    The data packets are something called NavTrak. The GPS location of the helicopter from inside the aircraft is muxed into the audio signal on one of the audio channels (the other channel is used for reporter audio), and the nearby receiver on a mountain top watches the audio channel with the GPS data on it and is able to automatically point the receiver to the inverse heading (the data is the heading from the helicopter to the mountain, so the inverse is the direction the mountain needs to point to the helicopter), and automatic microwave tracking (on both ends, helicopter and mountain top receiver) is achieved. Very cool stuff. It works perfectly, as long as nothing gets in between the mountain top receiver and the helicopter (i.e. another mountain), at which point the GPS-embedded audio is not received and the mountain top receiver is left clueless as to which direction to point. Still, very cool technology. (It should be noted that the helicopter transmitter and the hilltop receiver use directional antennas. They need to be pointed directly at each other or the signal diminishes to the point of being unusuable. So this technology is vital is a signal is to be maintained from a moving aircraft.)

  7. crispyjones says:

    The system I am familiar with has been in some version of production for 20+ years by a company called NSI (or N-systems). Originally gps rs232 data from the helicopter’s nav system was used, a Trimble protocol called RNAV r1, sent to a bell 202 v.23 modem running at 1200 baud. This audio signal was sent to one of the audio inputs of the microwave transmitter on board the ship, the same one transmitting the live video. The audio was pulled off at the receive site, decoded locally by the tracking system that drove the receive antenna to automatically follow the live coordinates from the moving helicopter. As gps has become more common the units no longer depend on flight computers, but due to backwards compatibility issues they still use the strange protocol. The earlier system was waypoint based, the copter transmitter antenna was pointed at a known waypoint (the receiver) this bearing-to-waypoint info was simply reversed 180 by the receive site to point at the helicopter, but this system doubled any pointing errors. The newer systems simply track the lat/long coordinates. Minimodem http://www.whence.com/minimodem/ works great to decode the afsk.

    It is not an IFB channel. IFB is what reporters listen to for cues from the producer and is a voice channel, phone, 2-way etc, not data.

  8. No Hack says:

    Wow this is really a great hack. I love Oona’s work… Oona, marry me?

  9. Ryan says:

    Wow, I’m impressed at Oona’s data-sleuthing (seriously)! I as well came across another raw helicopter ENG video feed on YouTube that had the same data in one of the audio channels, and I totally recognized by ear that the data was 1200bps Bell-202 FSK, since that is the same low-level protocol used for “Packet” radio and APRS (Automatic Packet/Position Reporting System, which uses packet) on the amateur “ham” radio bands. I myself am an licensed amateur radio operator who has futzed around now and then with packet & APRS. I was gonna try to encode it, but got waylaid by other goings-on….

    Had I known she was doing this, I could’ve told her just to directly feed the data audio of this news feed into the input of a hardware TNC instead (like a Kantronics KPC-3) or a software TNC (like AGWPE), it would’ve decoded it to ASCII instantly, but that would’ve been cheating. :)

    I’m assuming the NavTrak system (which is also awesome, automatic microwave ENG/RPU tracking!) that this data is a part of took some cues from amateur packet (or has engineers on their staff who are hams). I’m suprised NavTrak doesn’t use APRS instead, but I’d imagine this approach of constant data xmission of GPS coords (APRS position reports are usually transmitted sporadically) is a bit more real-time and required for the constant auto-alignment of the microwave link that NavTrak provides.

    I admire that she took a more forensic (and more educational) approach using baudline, SoX, and a hex editor to decode the data. Her “sleuthing” skills of data transmission I highly admire (especially her feat of intercepting/decoding OTA bus stop sign data)! Bravo Oona, you’re my newest tech-hero! :) (Is she a ham, perhaps?)

  10. fred says:

    something similar is used on UAV antena tracking systems from model airplanes, drones, etc.

    The “drone” gps puts the coordinates via some video/audio overlay (sometimes even n the non used tv lines) and the tracker on the ground decodes this message and points the directional antenna towards the drone

  11. asdf the third says:

    Hot damn, that’s amazing work.

  12. wretch says:

    Wow! R.E.S.P.E.C.T.

  13. teanau says:

    http://sanfrancisco.cbslocal.com/live-video/

    sounds like the audio is baked into this live feed.
    our neighborhood is being shot up a little at the moment.
    id love to see a live feed of this data.

  14. HackerK says:

    Now, this is the reason I come to HaD! Good one.

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