We seek to understand and document all radio transmissions, legal and otherwise, as part of the radio listening hobby. We do not encourage any radio operations contrary to regulations. Always consult with the appropriate authorities if you have questions concerning what is permissible in your locale.

Author Topic: Kiwi TDoA Direction Finding Increases SW Pirate Institutional Paranoia Levels  (Read 8293 times)

Offline Azimuth Coordinator

  • Full Member
  • ***
  • Posts: 247
  • Listen for Us on 4185 Khz or 5185 Khz
    • View Profile
    • Email
My take is.  I'm not comfortable about the technology.   
QTH: A Clandestine location on the East Coast
Watkins Johnson WJ-8716, WJ-8718A, WJ-8618B
Radio.Illuminati6150@Gmail.com

Offline ION Radio

  • Sr. Member
  • ****
  • Posts: 331
  • LLFR!
    • View Profile
    • Email
My take is.  I'm not comfortable about the technology.

I'm not comfy with this at all myself AC. It just seems too easy.     :o
« Last Edit: July 17, 2018, 2330 UTC by ION Radio »
QTH- East of everywhere.
ICOM and 40m Dipole
ionradioshortwave@gmail.com

Offline ChrisSmolinski

  • Administrator
  • Marconi Class DXer
  • *****
  • Posts: 31106
  • Westminster, MD USA
    • View Profile
    • Black Cat Systems
The bad news is listeners can sometimes find out where a station is located within 50 miles or so... if everything works.

The good news is listeners can sometimes get a completely wrong result (for example, I tried some SWBC stations and it was off by a few hundred miles some times), and not know the result is wrong.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Token

  • Global Moderator
  • DX Legend
  • *****
  • Posts: 2118
    • View Profile
Yeah, nvis done right will result in a df fix straight up.

The direction and angle of arrival might well be up at a high angle, however TDOA does not use angle cuts to plot position.  So TDOA can arrive at accurate position fixes despite high angles.

The good news is that while TDOA CAN arrive at very good positional fixes, it can also plot positions way far from reality that look great.  And the user, especially the average, uninformed user, may never know the difference.

T!
T!
Mojave Desert, California USA

Offline Monophonia

  • Jr. Member
  • **
  • Posts: 32
    • View Profile

The good news is that while TDOA CAN arrive at very good positional fixes, it can also plot positions way far from reality that look great.  And the user, especially the average, uninformed user, may never know the difference.


Yep, Like the first time I attempted a DF test right when the feature launched. It "found" WRMI very firmly in North Carolina.

Offline ChrisSmolinski

  • Administrator
  • Marconi Class DXer
  • *****
  • Posts: 31106
  • Westminster, MD USA
    • View Profile
    • Black Cat Systems

The good news is that while TDOA CAN arrive at very good positional fixes, it can also plot positions way far from reality that look great.  And the user, especially the average, uninformed user, may never know the difference.


Yep, Like the first time I attempted a DF test right when the feature launched. It "found" WRMI very firmly in North Carolina.

Kiwi TDoA works extremely well if you already know where the transmitter is located, so you can filter out bogus results  ;D
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline KaySeeks

  • DX Legend
  • ******
  • Posts: 1246
  • Quebec. Vive la différence.
    • View Profile
So, good signal strength (good SNR) and all receive locations evenly distributed around the target and roughly the same distance from the target can yield very good results, skywave or not. 

Yes, in fact back around the time that you wrote this, I realized that I had gravitated toward an understanding that one RX being much closer or farther away than the others would skew the results. I had more-or-less come to a belief of exactly what you say here - equidistant RXs are optimal - through trial and error.

Of course, this is more complicated if you don't know where the TX is (which is the point of the whole exercise). You use an iterative process to narrow it down. However, even with this knowledge, things can and will go wrong.

Because of my belief in "robust" solutions to things, I like to attempt to achieve basically the same location result with a mostly or completely different set of RXs. If it's a correct solution, I believe that you should achieve a corroborating result. If the RXs are roughly equidistant, I have been able to achieve a similar result this way, which is reassuring.

Sometimes I try to test it to be sure that it is leading me to the right solution by picking RXs that are not centered on the previous result, perhaps not surrounded by the RXs at all. The hope is that it at least tells me that the TX location is somewhere off in the direction of the previous result, i.e., corroborating. Unfortunately, this works maybe 50% of the time. The other 50% of the time, I get a completely different outcome that is also equidistant from those RXs in use for that second run, nowhere near the previous result and not a result that says “No, no. You are headed in the wrong direction.”

If you think about the implications of this, that is what makes the process of using TDoA on an unknown TX very difficult. It is very easy to be lead astray.
Just somebody with a radio, a computer and a pair of headphones...