22 Jan
2014
22 Jan
'14
5:54 a.m.
Guys,
As you know, our sample rate is not good enough to decode UAT's FSK in
a conventional way, so I thought to try something else: use 2 samples
per bit to detect the frequency shift. This is obviously unreliable,
but I hoped to see something at least. But it didn't work: seems like
decoding noise.
The code is here:
https://github.com/zaitcev/ruat
Anyone is willing to critique? Where did I go wrong?
Thanks,
-- Pete
22 Jan
22 Jan
11:35 a.m.
Have you checked your hardware? Mostly antenna and grounds...
BR,
Nikos
On Wed, Jan 22, 2014 at 6:54 AM, Pete Zaitcev <zaitcev(a)kotori.zaitcev.us>wrote;wrote:
Guys,
As you know, our sample rate is not good enough to decode UAT's FSK in
a conventional way, so I thought to try something else: use 2 samples
per bit to detect the frequency shift. This is obviously unreliable,
but I hoped to see something at least. But it didn't work: seems like
decoding noise.
The code is here:
https://github.com/zaitcev/ruat
Anyone is willing to critique? Where did I go wrong?
Thanks,
-- Pete
4:36 p.m.
The hardware receives the Extended Squitter at 1090 MHz, so we may
assume that it works. I'm using a little monopole antenna sized for
978 MHz for this, sitting right on the connector.
-- Pete
On Wed, 22 Jan 2014 12:35:35 +0200
Nikos Balkanas <nikos.balkanas(a)eyeonix.com> wrote:
Have you checked your hardware? Mostly antenna and
grounds...
BR,
Nikos
On Wed, Jan 22, 2014 at 6:54 AM, Pete Zaitcev <zaitcev(a)kotori.zaitcev.us>wrote;wrote:
> Guys,
>
> As you know, our sample rate is not good enough to decode UAT's FSK in
> a conventional way, so I thought to try something else: use 2 samples
> per bit to detect the frequency shift. This is obviously unreliable,
> but I hoped to see something at least. But it didn't work: seems like
> decoding noise.
>
> The code is here:
> https://github.com/zaitcev/ruat
>
> Anyone is willing to critique? Where did I go wrong?
>
> Thanks,
> -- Pete
5:25 p.m.
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
On Jan 22, 2014 7:36 AM, "Pete Zaitcev" <zaitcev(a)kotori.zaitcev.us>
wrote:
The hardware receives the Extended Squitter at 1090
MHz, so we may
assume that it works. I'm using a little monopole antenna sized for
978 MHz for this, sitting right on the connector.
-- Pete
On Wed, 22 Jan 2014 12:35:35 +0200
Nikos Balkanas <nikos.balkanas(a)eyeonix.com> wrote:
Have you checked your hardware? Mostly antenna
and grounds...
BR,
Nikos
On Wed, Jan 22, 2014 at 6:54 AM, Pete Zaitcev <zaitcev(a)kotori.zaitcev.us
wrote:
> Guys,
>
> As you know, our sample rate is not good enough to decode UAT's FSK in
> a conventional way, so I thought to try something else: use 2 samples
> per bit to detect the frequency shift. This is obviously unreliable,
> but I hoped to see something at least. But it didn't work: seems like
> decoding noise.
>
> The code is here:
> https://github.com/zaitcev/ruat
>
> Anyone is willing to critique? Where did I go wrong?
>
> Thanks,
> -- Pete
5:54 p.m.
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(a)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
6:33 p.m.
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.
--n
On Wed, Jan 22, 2014 at 8:54 AM, Pete Zaitcev <zaitcev(a)kotori.zaitcev.us>wrote;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(a)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
3754
days inactive
3754
days old
5 comments
4 participants
participants (4)
-
David Jacobowitz -
Nick Foster -
Nikos Balkanas -
Pete Zaitcev