Re: [Discuss-gnuradio] Mode S and ADS-B

[Eric A. Cottrell]

Martin Dvh wrote:
> Hi Eric,
> I tried out your code.
> Does it run on your machine in realtime?
Yes.  It works quite well on my Core 2 Duo.
> I have a dual-core athlon X2 3800 and both processor go to 100 % when I run 
> it.
> (I didn't know gnuradio could use both processors , I though we still had the 
> single-threaded-scheduler)
> 
> This gives me a lot of USRP overruns (UoUoUo) so it doesn't work very well 
> this way.
> Does mode-s really need a samplerate of 8000000
The methods I used to implement the code do require 8000000 and that is
the slower rate with 10000000 the preferred rate.  I may be wasting some
CPU cycles.

One thing to try is to use the scope program to set the threshold.  The
default is low for alot of front ends including the DBSRX.  The DBSRX
tends to be deaf.  Also I find a gain setting of 32 to 36 seems to work
best for the DBSRX.  If the threshold is set too low I find that so much
data is being processed that even my Core 2 Duo will start UoUoUo a
little.  I set the threshold so that most of the noise is below the
threshold value.  Try a gain of 32 and a threshold of 75 to start.  If
you set the scope to sync on channel 2 going negative you should see
Mode Select frames of various amplitudes.

Getting the antenna up in the clear is important.  A discone will work
if you have a good receiver but I found building a simple 1/4 wave
ground plane on an N connector beats out the discone.

I had good results using my AOR 5000 as a front end.  I used the
wideband IF output through a 20 dB amplifier to the LFRX board.  I have
tried other receivers with tapped IF outputs and did not get good
results.  I think it is the combination of the low noise front end of
the AOR 5000 and being able to turn off AGC.

I am experimenting with antennas and want to find a GAsFet preamp that
covers the band here in the US.
> 
> Another question.
> I know 1090 Mhz is the frequency they use in the US.
> I googled for info on frequencies for mode-s and ads-b and it seems multiple 
> frequencies could be used.
> Does anyone know of an overview of the actual frequencies in use, at which 
> places.
> (I live in the netherlands)
1090 MHz is the international frequency.  You may be thinking about UAT
for smaller private aircraft.

73 Eric