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Re: [Discuss-gnuradio] Over the Air Signal Capture

From: Rick Parrish
Subject: Re: [Discuss-gnuradio] Over the Air Signal Capture
Date: Sun, 16 Jan 2005 17:05:31 -0600
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Martin Dvh wrote:

Also nice would be air-traffic control samples.
These are narrow AM-modulated in the 120-130 Mhz band.
I think a bandwidth of 10 or 20 kHz should do (didn't check for exact specs yet)

I'll jump in on the air-band example:

108-118 Mhz is aero navigation; 50 khz spacing. The potential to construct a receiver with a 10 + mhz wide front-end that could simultaneously track up to "N" number of VOR or VORTAC ground stations would be very useful for classic "old school" aero navigation (old school meaning non-GPS). Traditional cockpit mounted NAV receivers only follow one signal at a time. A single VOR only gives you bearing information ... with two or more VOR signals you can triangulate. Some VOR stations also offer DME - a sort of distance ranging "ping" protocol. I think DME is up in the 300 mhz range so it would likely require a second receiver to capture this additional band. The VOR signal carries two components - a 30 hz reference and a 30 hz bearing signal. Comparing the phase of the two gives bearing (eg. 0 to 360 degrees) to the VOR site.

118-136 Mhz is aero voice communication. Listening to a half-dozen simultaneous conversations isn't very useful for a pilot but being able to simultaneously scan dozens of air-band channels is a dream come true for many scanner enthusiasts. Fifteen years ago, channel spacing was 50 khz, then split to 25 khz. The FAA and FCC may be working to split the band yet again (I'm way out of touch on some of this).

1030 - 1090 Mhz is the range for primary radar interrogation and aircraft transponders' replies. Some transponders provide only altitude information; others provide GPS XYZ coordinates. By capturing all of the transponder replies in a given area, you could build your own collision avoidance system.


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