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

From: Cory Papenfuss
Subject: Re: [Discuss-gnuradio] Over the Air Signal Capture
Date: Mon, 17 Jan 2005 07:06:04 -0500 (EST)

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.

DME is up by the transponder frequencies...around 1GHz. The glideslope signal is around 300Mhz UHF.

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.

Some are localizer, not VOR within that frequency band. Those are 90 and 150Hz AM out a center line of the runway. The nav instrument reads the strength of each to determine if centered down the runway.... centered needle=equal strength=lined up. The glideslope signal is more or less the same (at different frequencies), but vertically oriented as opposed to horizontally.

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).

I don't know if the FAA/FCC is working too hard on another split in the US. From what I understand, some/(all?) countries in Europe are using an 8.33MHz split already. I know I'll be pissed if they obsolete the radios I just put in my plane.....

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.

... and some do not provide even altitude information. "Mode-A" provides only the 4 octal digit code assigned to them by ATC. "Mode-C" also provides absolute static air pressure (from which ATC can correct for altimeter setting and get indicated altitude).


* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *

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