RogerBW's Blog

Automatic Dependent Surveillance – Broadcast 04 August 2015

I've been playing with software-defined radio and broadcast aviation data.

A couple of years ago I got hold of a cheap "DVB-T + DAB + FM" USB stick, which is based on the RealTek RTL2832 chipset. (You can tell it's a classy manufacturer - the USB-registered serial number is "1".) This is meant to be a digital TV/radio tuner for Windows, but I don't care about that: much more interestingly, it's a radio that can be tuned and controlled by software.

The easiest way to capture data off it is not to mess about with the full gnuradio toolchain but rather to use the RTL-SDR suite from Osmocom (in Debian), which comes with a number of pre-set utilities for common applications. Once I'd verified it worked for plain old FM radio (very disconcerting to hear The Archers after years of avoiding it), I chose to work with ADSB, an aircraft traffic information system whereby aircraft broadcast their position and other data. Sure enough, even with a basic stubby antenna borrowed from an airband receiver (the ideal simple antenna would be a 137.5mm dipole), up it came:

*8d400d8f3a5155e8d9d0fcc5f692;
*8d4ca6e390ab022dc1c3c7643ef8;
*90801530c0782030aa032010d79e;
*a0000a15d02a3f23a2c41ddee6f6;
*a0000a15a9080030680000d0ca48;
*a0001530b0e9e12ae01402828003;
*8545ac5099147c2060f882920d29;

Well, all right, that's recognisable but not entirely helpful. Fortunately there are various decoders about, so I didn't have to write my own; I chose dump1090 on the basis that it was open-source and not thoroughly tied to other systems.

That can give you a live view overlaid on Google Maps, but more interestingly to me it produces reasonably readable data in the informally-defined SBS-1 format:

MSG,1,,,484B02,,,,,,KLM1069 ,,,,,,,,0,0,0,0
MSG,3,,,4CA9D2,,,,,,,13200,,,51.37125,-0.95039,,,0,0,0,0
MSG,6,,,4CA9D2,,,,,,,,,,,,,4465,0,0,0,0
MSG,3,,,4CA9D2,,,,,,,13125,,,51.36648,-0.94620,,,0,0,0,0

which I was able to capture, mangle a bit, and feed into a map plotter. (Weirdly, none of this timestamps the captured data, so I did that myself.) Here's what I picked up for the hour between 20:05 and 21:05 last night:

Blobs represent the later ends of the tracks. You can see plenty of aircraft coming westbound out of Heathrow, then turning north and north-east on their standard routes. (Wind was out of the south and west, and I didn't pick up any obvious landing aircraft.) Other aircraft are in transit at higher altitude.

Reception would probably be improved if I didn't have the bulk of the house between the antenna and points north. Even so: nifty.


  1. Posted by chris at 10:57am on 04 August 2015

    You forgot to mention that what you first got from Radio 4 was one of the five-second silences between scenes in The Archers, which made it even more upsetting when someone spoke.But I have to ask: how did you know that it was The Archers? Was it the silence that was the giveaway? So many of the voices have got new actors - fifteen of the regular characters have changed voice in the past two years, which is more than in the previous twenty - that it's often quite hard to know which of them is speaking and to whom even if you are a regular listener.

  2. Posted by RogerBW at 11:00am on 04 August 2015

    Yes, five seconds of dead air seems to me a bit excessive. I wouldn't put up with it in my own podcast, never mind on something that's meant to be produced by people who know what they're doing.

    I knew it was The Archers because someone was talking in a fake-sounding accent about Rob and Helen, and living in the same house as you I can't help knowing what those names signify.

  3. Posted by Owen Smith at 12:32am on 05 August 2015

    What are the min and max frequencies this device can recieve? Any limits on modulation schemes it can handle?

  4. Posted by RogerBW at 09:22am on 05 August 2015

    The Osmocom page I linked above says "24-1766MHz" for the R820T tuner that this unit appears to have. A quick series of tests suggests I can extend the low end down to around 10MHz on this particular device.

  5. Posted by RogerBW at 09:24am on 05 August 2015

    The dead-easy-to-use decoder covers "fm, wbfm, raw, am, usb, lsb" (wbfm being wide-band commercial FM), but the thing can be plumbed into gnuradio or another block-based system to decode other things too.

  6. Posted by Owen Smith at 03:34pm on 05 August 2015

    That's a very wide frequeny range it can handle.

    While we're on the subject, my Arcam DT91 FM/DAB tuner is a software defined radio for both FM and DAB using a Radioscape module. It is by far the best FM tuner I have ever used, it's much better at full stereo quieting (no annoying hiss) with the barely adequate FM signal I have (Cambridge is a bad location for FM reception, I receive from Peterborough). I'm told (by RF engineers at Sepura) software defined FM is particularly good at stereo quieting, whereas when it comes to ultimate sensitiviy and selectivity traditional radio designs are better.

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