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.
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.
Funnily enough, this isn't the first time Oona's RDS adventures have been on HN. Indeed, there's a comment from a while back on here 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.
 - https://news.ycombinator.com/item?id=5656677
 - https://news.ycombinator.com/item?id=5657930
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 ;)
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.
e.g., about those restaurant light-up/buzz pagers: http://www.windytan.com/2013/09/the-burger-pager.html
This is real Hacker News.
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.
But of course.
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.
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
They usually use simple FSK just like the "real life" helicopter in the article.
Here's an example of a common simple protocol:
Set your time machine for 1958, or follow this link  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.
According to Wikipedia, the transition started in 1960, so we're two years away when this article was published.
Of course, I also find chip tape outs beautiful, much to my wife's dismay.
Now go build some antennas! :)
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!
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.
I did that using her plots, but working backwards from the city hall to the freeway would only be tedious, not impossible.
Could someone detail more information about the tools she used?
and critique whether they're joy riding or actually working etc