Another failure of decoding UAT
bistromath at gmail.com
Wed Jan 22 17:33:14 UTC 2014
I'm assuming the GDL 39 is airborne when receiving -- there's no guarantee
of ground-based coverage, and unless you're close to a transmitter or have
line-of-sight due to elevation you might not have any signal to work with.
I think the first thing you should do is verify some sort of signal is
present, using osmocom_fft or whatever, and then record some samples you
can work with offline to decode. Maybe go for a flight near the uplink,
verify there's a signal using both the GDL 39 and the RTL dongle, and
record some nice strong samples to hack on later.
I haven't looked into decoding UAT with any seriousness apart from reading
DO-282B, but 2 samples per symbol should be enough to work with. Maybe I'm
being clueless here, but I don't see justification for saying the RTL
dongles can't sample fast enough for UAT. The usual noncoherent method -- a
differential quadrature demodulator -- should be fine, and saves you from
having to phase- and frequency-synchronize with the baseband signal. I
haven't looked closely at your code, but your description sounds pretty
close to that. You'll see (up to) 312.5kHz deviation on a '1' bit, and (up
to) -312.5kHz deviation on '0'.
You can probably get away with open-loop clock recovery since the packets
are so short, but you'll still have to estimate the center of the bit so
you can sample and slice.
Personally, as a registered GNU Radio fanboy, I'd be using GR to at least
get started demodulating UAT. It gets you graphical sinks to work with and
a set of proven signal processing blocks, so you don't have to worry as
much about an ad-hoc approach being valid or not.
On Wed, Jan 22, 2014 at 8:54 AM, Pete Zaitcev <zaitcev at kotori.zaitcev.us>wrote:
> I know someone who's reveiving FIS-B with a GDL 39, although his
> home base is about 100 km north-east. I'm somewhat confident that
> I should have the coverage. One problem is, from what I heard,
> the ground stations will not broadcast anything until a UAT-equipped
> aircraft checks in.
> Regardless, I was hoping that Nick or other experts chime in on
> the code itself. Honestly I have no clue what I'm doing here.
> I imagine that if signal is present, then vector pointed by Q+jI
> rotates at ~300 revolutions per second. Therefore, I calculate
> the phase angle difference between two samples taken at twice
> the UAT bit rate and see if it's anywhere reasonable. It may
> be a bogus technique for any number of reasons.
> -- Pete
> On Wed, 22 Jan 2014 08:25:57 -0800
> David Jacobowitz <david.jacobowitz at gmail.com> wrote:
> > This may be dumb because I haven't looked into it, but is it reasonable
> > expect to receive a signal?
> > Are you in the US? There really aren't that many UAT equipped aircraft
> > The FAA is rebroadcasting ADSB ES responses on UAT as well as providing
> > weather and other data, but that is all coming from ground stations. If
> > need line of sight and you're on the ground yourself you may be out of
> > - Dave J
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