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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: Mon, 14 May 2007 14:57:14 -0400
User-agent: Thunderbird (X11/20060911)

Boris Maryshev wrote:
> Great news indeed, but still no code in svn?


I am planning post-Dayton for svn.  Things are not complete.

I got some panel mount N connectors and made up a simple 1/4 wave
groundplane for 1090 MHz.  I tie-wrapped a 40 foot or so extending tent
pole to my back deck and was able to go 10 feet above the Discone.  The
1/4 wave on the pole added about 20 miles of range over the Discone.  I
also tried my AOR 5000 with the out-board IF Output amplifier into the
LFRX daughterboard and was able to get about 100 mile range on the
discone and 120 mile range on the 1/4 wave.  I also got some surface
position reports from Logan.  With Google Earth most of the surface
positions were located on Runways.  One was in a East Boston
neighborhood but it is likely due to INS errors.  Most planes takeoff
down the centerline of the runway but some are way off.

Instead of getting ready for the Dayton trip, I spent yesterday evening
using NEC to make a curly coil colinear with 1/4 wave and 5/8  wave
elements.  I have not built it yet.

I implemented the new preamble detection and it works better.  I need to
do the bit error detect and correction part.  I thought I was getting a
lot of bit errors on some messages.  I reread my background information
and found the interrogator id is XOR with the CRC.  On other messages
the airframe id is XOR with the CRC.  Reminds me of RDS used on the FM
broadcast band.  The ADS-B and ACAS messages have an interrogator id of
zero so they are easier to check.  I did a quick check of the file
generated using the AOR 5000 with the 1/4 wave antenna and found all 15
possible interrogator ids in use.  I looked at the all-call replies of a
few aircraft and it was not unusual for the transponder to respond to
multiple radars.

Because I have interest in doing other aircraft receivers, I am thinking
of calling the project directory gr_air.  I was also thinking about
gr_aero but people may mistake it for the Windows Vista video sink.
Mode S has a different meaning for the AMSAT crowd so it is not a good

73 Eric

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