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

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] Re: Mode S and ADS-B
Date: Sun, 29 Apr 2007 12:08:42 +0200
User-agent: Debian Thunderbird 1.0.2 (X11/20070113)

Eric A. Cottrell wrote:
> Hello,
> I got Mode S and ADS-B receiving working.  I dump the messages to
> standard output for redirect to a simple log file. Simple programs take
> the log and decode it into English.  I have not decoded all the messages
> yet like Airborne Velocity.
This is great news.
Have you already got an svn accoupt so you can commit your code into a 
development branche.
Or is your code available in an other way?

> Tick:  Sat Apr 28 08:43:52 2007
>  Extended Squitter  A/F: a6b55a Extend Cap: Unassigned 5
>    Airborne Position O ln: -70.949388 la: 42.521745 Alt: 6100
>  All-Call Reply     A/F: a6b55a Extend Cap: Unassigned 5
>  Short Altitude     A/F: a6b55a Airborne No Downlink Request Alt: 6100
> Tick:  Sat Apr 28 08:43:54 2007
>  Extended Squitter  A/F: a6b55a Extend Cap: Unassigned 5
>    Airborne Velocity
> I was having problems with GNURadio stopping while trying to receive
> from the USRP.  It seemed like the USB stream would stop.  I thought it
> was the laptop so I setup a Core 2 Duo system.  The problem still
> occurred.  I "fixed" the problem by removing the TVRX board from RXB and
> moving the DVB board from RXA to RXB.  Now I can log all day.
> One neat little program I use makes a kml file out of the positions and
> I use Google Earth to see how good the range is.  I can only receive
> stuff up to 20 or so miles out.  There are a number of Aircraft with
> ADS-B that use Boston or fly over on the way to Europe.
> As I am reading technical articles and other information I see the
> techniques progressed as 1090 MHz got more crowded and more powerful DSP
> chips became available.  Mode S started out using simple receivers with
> pulse edge timing or centered sampling of pulse amplitudes.  A radar
> site sends interrogations at their Pulse Repetition Frequency so the
> maximum reply rate was under 450 per second.  FRUIT became a problem as
> more sites came on-line and aircraft started using 1090 MHz for TCAS.
> FRUIT as in False Replies Uncorrelated In Time.  Basically overlapping
> of different Mode S signals or Mode S with Mode A/C pulses.  One
> interesting article outlines using 10 or 8 MSPS for the sampling rate
> and using factors such as amplitudes of pulses being within 3 db of each
> other and the effects of beat frequencies of interfering Mode S signals
> to determine if a bit is a one or a zero.  So this is turning into an
> interesting DSP and data decoding experiment.
> I suspect that I am in a high FRUIT environment and my limited range is
> due to the basic receiver techniques I am using and the Discone Antenna.
>  I am currently implementing a better receiver in code and may put up a
> 1090 MHz antenna with a preamp.
> Another interesting project is to also receive the 1030 MHz
> interrogations and figure out the range and bearing of Mode S aircraft
> by the timing between the interrogations and replies.
> 73 Eric
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