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

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] ADS-B and Mode-S Transponders
Date: Sun, 25 Mar 2007 07:49:38 -0700
User-agent: Mutt/1.5.9i

On Sat, Mar 24, 2007 at 01:17:28PM -0400, Eric A. Cottrell wrote:
> Hello,
> Yet another project I am interested in.  Number 157 on the list. 8)
> ADS-B is an interesting system that allows aircraft to send position and
> other information.  It uses the 1090 MHz Transponder frequency.
> http://www.garmin.com/aviation/adsb.html
> ADS-B is more progressed over in Europe.  It is in a testing phase on
> the East Coast of the US.  It is out of range from my QTH.
> http://www.garmin.com/aviation/Safe_Flight_21_v0.69a.pdf
> The UAT is used for general aviation on a frequency of 978 MHz.
> I was interested in the SBS-1 which decodes Mode S and ADS-B.
> http://www.kinetic-avionics.co.uk/sbs-1.php
> The design uses SDR techniques.  It appears the decoding is done in a
> Xilinx Spartan.  http://www.tech-software.net/inside_sbs1.php
> There are other interesting things on the website.
> http://www.tech-software.net
> Some interesting info about the data being encrypted and delayed
> 5 minutes over TCP/IP.  So no direct Linux support.
> http://www.kinetic-avionics.co.uk/forums/viewtopic.php?=&p=22004
> I talked to someone with the SBS-1 and they said there is some ADS-B
> activity as some US aircraft fleets have it installed.
> 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.
> This is the data format. http://www.tech-software.net/adsb_decode1.php
> The next step is to write a decoder block that takes in the samples and
> prints out type and raw data to the standard output.

This is great!  Looking forward to what you come up with!


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