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Mystery signal from a helicopter (windytan.com)
1101 points by schappim on Feb 1, 2014 | hide | past | web | favorite | 78 comments

We operate news helicopters at TV stations around the US and they all use a similar system.

Basically the right channel of audio is typically the microphone from the pilot or reporter and the left channel is the audio feed transmitting what is basically modem data back with the GPS coordinates of the helicopter and some other data.

This data is used at the antenna site to track the helicopter and point the antenna in the correct direction to keep a line-of-site since the transmission system from the helicopter to the receiver is line of site (microwave) based.

It's really cool to sit and watch the receive antenna track and move responding to the movements of the helicopter miles and miles away.

The antenna is on the belly of the chopper here - http://www.imt-broadcast.com/super-pod?cat=Products

The receiver is generally centrally located on a tower - http://www.imt-broadcast.com/silhouette?cat=Products - it's a parabolic dish inside a radome like this.

I'm not a native speaker, but still have a pet peeve for that one. It's "line-of-sight" (http://en.wikipedia.org/wiki/Line-of-sight).

Light travels in a straight line over (human-scale) long distances, so you need a straight line free from obstruction to see between two points. I'm sure you know this and just made a typo, of course.

Thanks for the additional information about this interesting solution.

Yup, just a typo!

How do you ensure that the receiver on the tower always has LOS to the mobile unit? I can't imagine its on a track to have full 360 degrees of motion around the tower.

Say you have the receiver on the top of a downtown building or a tower and you are transmitting from 1500', there's not too much that can get in the LOS. Receive systems can either be a single antenna, a series of antennas, or (like in the link above) a dish inside a radome - the dish generally can rotate the full 360 since the cable slips through the middle.

I love Oona Räisänen's blog. She's my hero. She combines a love of figuring stuff out with lots of fun hacks. And when I read her blog it makes me happy to see someone with that joy of uncovering mysteries.

You've probably already seen it, but in case you haven't: her talk at 30C3 was excellent and is worth a watch. https://www.youtube.com/watch?v=R-2k6TMPMRo

The talk was first given at 44CON in September in 2013 (but only available on the DVD, not posted on the youtube channel).

Funnily enough, this isn't the first time Oona's RDS adventures have been on HN[1]. Indeed, there's a comment from a while back on here[2] from the time I was trying to convince her to submit to 44CON. The thing that really surprises a lot of people is that Oona does this sort of thing in her spare time.

[1] - https://news.ycombinator.com/item?id=5656677

[2] - https://news.ycombinator.com/item?id=5657930

Been there, was cool indeed! If you aren't planning to already, you should come to the 31c3 :)

It is a certain commitment to Germany over Christmas.

It started the day after Christmas, though I did travel on second Christmas day. Totally worth it in my opinion. This was my first year and I can say that an extra day of that experience is always worth it.

Also WiFi totally rocked on day -1, but if you want that I guess you could also go wired any other day (symmetric gigabit and the best abuse policy in the world yay!).

But of course you can also travel on the day it starts and miss a few talks, it's up to you and your family how much of an issue it is to miss 2nd Christmas day. I was home in plenty of time for new year though, you still have that ;)

That and Hamburg is a beautiful town.

Having become hooked on watching the feeds over Christmas I actually have been thinking about going to the next one.

This was a fascinating talk. Thanks for sharing it.

I just wanted to reply here with "You are my hero" . But someone beat me to it. I must subscribe now.

This also fires up my drive to get more into Radio's and radio astronomy. It so fun to be able to grab all that stuff from the air that most people are unaware of. I got my first handheld a while ago, and people were like "WOW amaze" when There was a really clear night in Budapest, Hungary with the ISS 70deg. overhead, and I was picking up Russian com while pointing to the illuminated station. ( -- yes unlicensed (but I don't TX) ). And now planning to get a license because more bands for the win, and licensed radio operators actually get much love from government and rescue services.

There's also lots of stuff that you can do with a cheap DVB-T tuner and simple (DIY) aerial.

Reading it now for the first time, super fun.

e.g., about those restaurant light-up/buzz pagers: http://www.windytan.com/2013/09/the-burger-pager.html

She is cool&nice!

Wow. People on HN do a lot of cool things but that really blew me away. It all sounded so casual too.

This is real Hacker News.

If I'd seen this recreated in an episode of CSI, I would have thought "Bullshit."

Yup, I guess there is little overlap between the CS community and electronics community.

Also, surprisingly, after being exposed to things like this over and over again, people in CS community still underestimate the capabilities of technology.


I love this:

Several people in the restaurant were waiting for orders with their similar devices, which suggested to me this could be a pager system of some sort. Turning the receiver over, we see stickers with interesting information, including a UHF carrier frequency. For this kind of situations I often carry my RTL2832U-based television receiver dongle with me (the so-called rtl-sdr).

I knew I was forgetting something.


For this kind of situations I often carry my RTL2832U-based television receiver dongle with me

But of course.

As you would, really.


I think this is what she was talking about? Note to self: buy one! :D


It's so cheap it's practically free (I mean come on, 3.2MHz BW for that price?). The included antenna is horrible but it suffices for small things like finding out something about the restaurant buzzers.

Oona is doing an extremely important service for Ham radio communities etc. She is bringing the knowledge of all of this to a wider audience on the internet.

I've seen similar devices in a pizza joint. Does anyone know why they are so big and ugly?

So people don't steal them; so people have to hold them or put them on a table and don't miss when they buzz.

This is the kind of article that makes HN worth coming back for.

Nice bit of reverse-engineering, and an impressive hack from whoever built the system.

Quite funny - I know someone who uses a similar setup to send GPS Coordinates for a Quadcopter and a Plane via the audio channel of the video link (he flies the Quadcopter/Plane with video goggles) to the ground station. There, an Arduino controls some stepper motors to point directional antennas at the copter/plane.

Project is at https://github.com/KipK/Ghettostation - video at https://www.youtube.com/watch?v=tiO4YCX7hJo - you can hear the modulation in https://www.youtube.com/watch?v=nm0okv8mTsg

Cool! I hadn't seen that project before, although I'd seen a few other telemetry-over-audio solutions.

They usually use simple FSK just like the "real life" helicopter in the article.

Here's an example of a common simple protocol:


Not really a hack. Subcarriers are how they added color to TV, stereo to FM radio, and so on. WWV/WWVH also sends machine-readable time codes at 100Hz like this.

Indeed--it was more of a hack sixty years ago perhaps. Since then, similar tricks have been used for closed captions, Dolby surround (the original kind), Radio Data System (which puts song names on your car dashboard via FM radio), TV Second Audio Program (listen to shows dubbed in a second language, just press the SAP button on your ancient high-end TV!)....

Wow. I always thought SAP stood for Spanish Audio Program. TIL.

>Subcarriers are how they added [...] stereo to FM radio...

Set your time machine for 1958, or follow this link [0] to see how RCA did both FM stereo (throughout the article they call it "multiplex") and 67 KHz "SCA" subcarriers. Admire the awesome 5KW FM transmitter cabinetry and transformer shielding.

[0] http://fmamradios.com/BTE-10B.html

One thing I find particularly interesting about articles from this time period is that Hz is not yet the name for 1/s, and so everything is in "cycles per second".

According to Wikipedia, the transition started in 1960, so we're two years away when this article was published.

Using spare audio capacity like this is actually very common - prior to digital recording, engineers would dedicate one tape channel to recording a sync signal, and in mission-critical applications would even 'stripe' the tape with the sync signal beforehand (many pro recorders can have recording and read heads active simultaneously).

Oona's work is always amazing. Loads of interesting hardware stuff in her blog. I wish I would be better at some mechanical engineering/electrical engineering stuff, my work is mostly web-related or big data stuff, never really get my hands dirty :(

You can basically reverse those terms for me. I'm a mech engineer who wished he knew more/was better at the web-dev / big data stuff.

Sounds like you guys should be friends.

Sounds like if they ever met, big explosion !

I'd had a copy of her dialup decoded poster on my wall for a while; forgot where I found it but it was immediately something I wanted to hang up... glad to see its being monetized via poster sales, its quite a lovely work of art.

Of course, I also find chip tape outs beautiful, much to my wife's dismay.


While I love the blog and the very clear explanations, none of this is new nor amazing to the experienced RF engineer, enthusiast, or practitioner. What's awesome is that the hardware and software are currently cheap enough that more people can discover and enjoy the larger spectrum around them. Welcome!

Now go build some antennas! :)

It'd be interesting to scrape videos for this kind of information in order to map them en-masse. Great post!

I wonder how much audio compression the signal can go through before getting lost, and if joint-stereo affected it. Might have to check out the data myself :) Props to Oona.

Given that most audio compression techniques are perceptual, and the signal here falls into the "most important" 300hz to 3.4khz range that gets special attention in human hearing, I'd guess it is pretty resilient to run of the mill compressed audio codecs.

Another cool thing is that the author of Redis made a decoder for ADS-B to decode aircraft transponder signals: https://github.com/antirez/dump1090

The thing I love most about her work is it's witty, but to the point and I can always understand it.

I'm not a big fan of the tl;dr movement, but I also don't like dauntingly long blog posts. Nails it every time. Hats off!

What is the tl;dr movement?

He means people complaining that a post is too long and so they won't read it.

Amazing!!! I would have just dismissed it as static interference, and not even think twice.

Hmmm, makes me think twice about the beeping sounds I hear when my phone is right next to my radio. I know it's data being transmitted/received, but I wonder if it can be deciphered in this manner.

It's the phone checking in with the base station. As someone who records sound for a living, this is one of the banes of my professional life - I tell people on film sets to turn their phones off, and they think I mean 'don't use your phone' whereas in fact I mean 'shut it down completely.'

I didn't get how she can get so much sense out of what is basically line noise - until the comment that asked what tools she uses, and she says Perl. Ah, just a day in the life :)

Can someone explain how she plotted the car's position? Did she manually reconstruct it based on the video and the information she derived from the helicopter's noise?

I think so. It isn't a big deal to find the tennis courts in the beginning of the video:


I did that using her plots, but working backwards from the city hall to the freeway would only be tedious, not impossible.

The Kansas City Police helicopters are also equipped with 3 of our Augmented Reality mapping Systems:


Suddenly, my telecom courses in CS now seem a lot more fun than they really were. It's like magic.

Anyone else see the cop hit him @ https://www.youtube.com/watch?v=TCKRe4jJ0Qk#t=1140 (19min)?

Exception profile and work. I never seen anything similar and never really thought that this was possible. Really stunned :-)

This is also commonly used on Model Planes flying "First Person View". See http://www.youtube.com/watch?v=A_AnqOFAs3Q for an example.

Can anyone recommend some cheap hardware to hookup my TG-UV2 [1] to my PC?


And here I always consider myself paranoid when wondering whether odd patterns in anything mean anything. Guess I'm just too noob and inpersistent to find anything.

can this become/or is/ part of forensic science? if its possible to do the same on a mobile phone, and merely speaking into the phone, you give away your travel trajectory?

They're way ahead of you :- They can take any audio recording (from any source) in the UK and get an exact time-stamp of its recording and detect whether it has been edited in any way : http://www.bbc.co.uk/news/science-environment-20629671

No, in this case it was intentionally encoded in the audio.

I would love to try a "signals hacking" capture-the-flag.

I need to learn HW.

Could someone detail more information about the tools she used?

cool thing is, we can start watching them back ;/

and critique whether they're joy riding or actually working etc


wow.. this girl is superhuman...

hackers rule!

this is rad.



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