SDR direction finding

[Antennae]

Say I have an hour's worth of SDR recording from an SDR receiver with dual channel input (2 receiving antennas). 
Can I find the direction of all the transmissions that are within the spectrum of that hour that I recorded? 

[Token]

Below is my understanding of the situation.  I have never done it myself but have given it a little thought and research.  When the dual channel NetSDR (RFSpace) was announced I thought this might be a cool direction to go.  While it is a heck of a nice thought, at a really simple level the answer to your question is no.

First is the two antenna / sample channel issue.  With only two inputs / antennas and if everything else is right you end up with two possible directions for any signal, it does not narrow down to one direction.  Grating lobes are an issue, so even those 2 directions can be in question and the array probably has general directions of optimal and suboptimal performance.  To get one direction you need at least 3 inputs / antennas, and 4 is more or less the best staring point.  2 antennas would be great for steering a lobe or a null though.  Spacing of the antennas are going to realistically limit bands of operation.  Feedline variables must be known but should be able to be calibrated out.

Then comes questions about the inputs of the SDR.  Is it separate ADC’s?  Or is it one device?  If it is multiple ADCs are they disciplined to a single source so that the phase relationship (time of arrival) of the inputs are known?  If each ADC is free running then the phase relationship of the signals in each is in question, and direction cannot be derived.

Then is the aspect of the software at the hobby level, at the pro level there are lots of off the shelf options.  At the hobby level there are several people playing around with this, and GNU Radio has some interesting stuff, but there is no simple interface you can use right now to display a direction, even if you have everything else right.  Everyone who is doing this appears to be writing their own stuff, possibly building off others works as a starting point.

T!

[Antennae]

It sounds like its in its infancy. Mostly its my paranoid side that is curious.
 
I was thinking about 3 receivers placed across America for triangulation. And if you use something like Afedri SDR-Net x2 receivers, their 2 channels use the same clock so you could accurately steer the null. 

Then my paranoid side wonders about a govt. agency making their own system and automating the triangulation through some fancy software and *viola* they have a database of transmissions and their rough transmitter locals!

[skeezix]

The FCC doesn't have something so rudimentary like that.

They have their own HFDF system. The following link is a request for private lines for their sites.  
http://transition.fcc.gov/omd/contracts/pre-award/RFQ11000024.pdf

Here are the sites:
https://maps.google.com/maps/ms?msa=0&msid=205652917633915171076.0004d938f34fa585f2329&dg=feature

Photos of the Livermore, CA site:
https://www.flickr.com/photos/coldwararchaeology/sets/72157626210688595/


Have heard that they can triangulate to an area of a small city in a very short time.

They still have the limitations of the laws of physics. There is no magic.

[Token]

It sounds like its in its infancy. Mostly its my paranoid side that is curious.
 
I was thinking about 3 receivers placed across America for triangulation. And if you use something like Afedri SDR-Net x2 receivers, their 2 channels use the same clock so you could accurately steer the null. 

Then my paranoid side wonders about a govt. agency making their own system and automating the triangulation through some fancy software and *viola* they have a database of transmissions and their rough transmitter locals!

Note I was pretty careful to talk about the hobby side of things.  At a professional level and at DoD / gov agency levels this has been going on for many years.  But nothing as narrow or limited as the AFEDRI, think more along the lines of the entire HF spectrum at once.  A digitizer with 500 MHz of instantaneous bandwidth today is no big deal. 

Direction finding based on TDOA and interferometry has been in operational use for a couple decades now.  For example almost every major RWR and RHWR sold today uses such techniques.  Triangulation and geolocation based on those measurements are almost as mature.  But even before SDR technology was in deep pocket agency use there were similar techniques used with analogue receive technologies.

A nice little paper on the basics here, it is describing a VHF/UHF application, but frequency is only a matter of scale.  http://www.dtic.mil/dtic/tr/fulltext/u2/a567399.pdf

T!

[Antennae]

Whew, a wealth of information! A map of the locals, what more could you ask for?
There aren't many direction finders on the West side of America.
Token, I wasn't able to view the .pdf so I didn't read it.  500mhz is big, to view that I would like a circular room with a screen all the way around and an office chair with good wheels on it. 

So my big question is, are these DF stations recording transmissions 24/7 so they can go back and triangulate them at a later time if need be?  Similar to what the NSA is doing with the internet traffic: recording it for later scrutiny.

[IQ_imbalance]

There's always this:
https://goo.gl/maps/rQu7O

[Token]

That FLR-9 (Elmandorf) was the last of the breed I think, but I am pretty sure I heard it shut down in 2013.  Misawa shut down a year or two before.  I don't think any of the FLR-9's or FRD-10's are still active, although there are some other CDAA's in use around the world.

T!