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Nick Foster bistromath at gmail.comI'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. --n 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 > to > > expect to receive a signal? > > > > Are you in the US? There really aren't that many UAT equipped aircraft > yet. > > 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 > you > > need line of sight and you're on the ground yourself you may be out of > luck. > > > > - Dave J > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.osmocom.org/pipermail/osmocom-sdr/attachments/20140122/cc304fe3/attachment.htm>