Re: [Discuss-gnuradio] Gnuradio Mode S project released

[Andrea Montefusco]

On 10/24/2010 07:22 PM, Andrea Montefusco wrote:

> is it thinkable to operate the decoder at a lower sample rate ?
> (e.g. at 2 MS/s or even less)

I found this reference

http://www.tech-software.net/adsb_decode1.php

in a Eric A. Cottrell message that explains why we need an high sample rate:

[...]
I thought that the USRP and DVB tuner board could also receive ADS-B and
Mode S.  I did not know if my discone and long coax run would work.  So
I did some experiments.

I used the fft and oscilloscope programs to check for activity.  I did
see activity on 1090 MHz.  I then modified the oscilloscope program to
do AM detection with the gr.complex_to_mag() function.  I saw various
pulse trains that look correct.  There were some short ones that look
like old transponders and longer ones that look like Mode S.  I found
the range of good gain settings was narrow (31 to 35) with 33 to be the
best.  I found the decimation of 16 gave good pulse shapes.  The pulse
timings are in 500 nS units so you need 250 nS sample times at least.
[...]

So, I believe there is no hope to decode this stuff with HAM SDR hardware.

        *am*

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Andrea Montefusco iw0hdv        http://www.montefusco.com
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