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

[Eric Cottrell]

Hello,

Articles I read say that a 10MSPS rate gives better performance that a 8MSPS 
rate.  I used the 8MSPS rate and still got good performance.

The oversampling is used in determining if the pulse is valid or part of 
interference.  Some receivers use a huge ROM lookup table to convert all the 
sample values of the PPM slot into a one or zero.

73 Eric
----- Start Original Message -----
Sent: Sun, 24 Oct 2010 20:40:18 +0200
From: Andrea Montefusco <address@hidden>
To: address@hidden
Subject: Re: [Discuss-gnuradio] Gnuradio Mode S project released

> 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*
> 
> ---------------------------------------------------------
> Andrea Montefusco iw0hdv        http://www.montefusco.com
> tel: +393356992791                     fax: +390623318709
> ---------------------------------------------------------
> 
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----- End Original Message -----