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

From: Eric A. Cottrell
Subject: Re: [Discuss-gnuradio] Re: Mode S and ADS-B
Date: Sun, 29 Apr 2007 01:17:46 -0400
User-agent: Thunderbird (X11/20060911)


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.

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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