[hpsdr] Fw: [Discuss-gnuradio] Software Defined Radio DirectionFinding (SDRDF)

[Phil Harman]

Hi Joe,

Great to hear of your continuing DF work. Regarding the random phase 
differences at power on. I guess there are a number of potential sources for 
this but one suggestion is to use a common CORDIC for each receiver.

There is a spare LVDS  I/O on each Mercury board so it may be possible to 
use one Mercury as the master CORDIC and reticulate to the others via LVDS.

I've not looked at this in detail, and its early morning here, but it may be 
worth considering.

73 Phil....VK6APH

-----Original Message----- 
From: Joe Martin K5SO
Sent: Monday, March 05, 2012 12:51 AM
To: HPSDR list
Subject: Re: [hpsdr] Fw: [Discuss-gnuradio] Software Defined Radio 
DirectionFinding (SDRDF)

***** High Performance Software Defined Radio Discussion List *****


Hi Jeff!

That's very slick!   Thanks for sharing it.

As you likely know, currently I am working hard trying to develop a suitable 
direction finding (DF) capability using our multiple-receiver HPSDR rig at 
HF frequencies.  The YouTube video referenced in your post briefly mentions 
DF at HF frequencies but, of course, the actual unit shown and demonstrated 
is working at much higher frequencies than HF.  The magnitude of Doppler 
shifts are directly proportional to the operating frequency, therefore 
Doppler-based DF is usually not done at HF;  instead, it is much more common 
to use Doppler shift DF at VHF and higher frequencies simply because the 
measured shifts are much larger and therefore the S/N is better.

At HF, I've been developing a different approach that has been around for 
some time that is very different from Doppler DF methods, namely, using 
measured phase-shift differences among three antennas/Mercury receivers to 
calculate the angle of arrival of the rf wave.  This approach is showing 
quite good promise except for a "little" problem that, as it turns out, is a 
fundamental characteristic of our HPSDR radio design.

The problem centers around the fact that when we start up multiple Mercury 
boards the relative phase relationships between the receivers is random. 
Try and try as I might I have not yet been able to devise a suitable method 
to force all receivers to be in phase at startup.  I've tried numerous 
methods to do this, most centered around holding the receivers in "reset" 
until everything else stabilizes but, while that sort of works it isn't good 
enough for accurate DF work at HF.  I'm still working hard at this issue and 
believe I will eventually find a solution that works for our HPSDR radio.  I 
have a suitable GUI coded already to display everything needed, with a 
modified v2.2.3 PowerSDR program, so that's ready to roll as soon as I 
figure out a suitable method to cure the start up phase problem.

Thanks again for the info!  Good stuff.

73,  Joe K5SO

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