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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