A few weeks ago we posted about some experimental work going on with Time Difference of Arrival (TDoA) direction finding techniques on KiwiSDR units. The idea is that public KiwiSDRs distributed around the world can be used to pinpoint the physical locations of any 0 - 30 MHz transmitter using the TDoA technique. This feature has recently been activated and can be accessed for free via any KiwiSDR.

The KiwiSDR is a US$299 HF SDR that can monitor the entire 0 - 30 MHz band at once. It is designed to be web-based and shared, meaning that the KiwiSDR owner, or anyone that they've given access, can tune and listen to it via a web browser over the internet. Many public KiwiSDRs can be found and browsed from the list at sdr.hu or by signal strength and location on this website.

One thing that KiwiSDRs have is a GPS input which allows the KiwiSDR to run from an accurate clock, as well as providing positional data. Time Difference of Arrival (TDoA) is a direction finding technique that relies on measuring the difference in time that a signal is received at over multiple receivers spread out over some distance. In order to do this an accurate clock that is synchronized with each receiver is required. GPS provides this and is able to accurately sync KiwiSDR clocks worldwide. 

Just recently all KiwiSDRs were pushed with a beta update (changelog) that enables easy TDoA direction finding to be performed with them. Since many KiwiSDRs are public, this means that right now anyone can browse to a KiwiSDR web interface and start a direction finding computation. You don't even need to own a KiwiSDR to do this so this is the first freely accessible RF direction finding system available to the public. This could be useful for locating signals like numbers stations, military transmissions, pirate stations, jammers and unknown sources of noise.

Usage

Running a TDoA job is as simple as using the KiwiSDR OpenWebRX GUI interface to select a signal and choose two or more receivers to use in the calculation.

If you want to try this out then it's easiest to start with VLF/LF or MW stations (less than 1.6 MHz) as these signals tend to propagate to receivers only via direct ground wave. HF sky wave signals are a bit more difficult to locate as they tend to travel longer distances by skipping, bouncing and refracting around the ionosphere, so it is difficult to determine exactly where they are coming from since the bounces result in a difficult to predict time delay. But if you know the rough location of the transmitter, you can try and select nearby KiwiSDR receivers, which will hopefully ensure that the signals are received directly via ground wave, and not via sky wave. More advanced users could try using receivers spaced further away, but at similar distances from the expected transmitter location. This will hopefully ensure that all the receivers have identical skip distances, and thus identical delays.

To get started follow these steps (and we also recommend reading the Help text, which is available by clicking the 'Help' button on the TDoA extension):

1. Open a KiwiSDR that can receive some signals that you are interested in locating. You can browse KiwiSDRs by map and signal strength quality on this website.
     
2. With the 'extension' drop down menu in the bottom right controls window choose TDoA and double check that the receiver modulation mode is set to 'IQ'.
    
3. You should now see a map on the top half of the screen. This map displays all KiwiSDRs in the world that have GPS enabled and thus can be used for TDoA.
   
   The map also displays several known transmitters in white with green markers that can be used as TDoA practice. Clicking on a known transmitter will automatically tune the KiwiSDR to that station.
    
4. Tune to the signal that you are interesting in locating. Make sure that the receiver bandwidth covers the signal.
    
5. Now you need to find two or more KiwiSDRs on the map that can receive the signal that you're interested in locating. (Two will give you a line of possible locations, whilst three may allow you to pinpoint the signal. But we recommend starting with only two or three first as more receivers can cause the calculation to fail).
    
   To test and see if a KiwiSDRs from the map can receive the signal, double click on its marker. This will open the selected KiwiSDR in a new browser window, with it tuned to the station of interest. If you have a rough idea on where the transmitter is located, try to select KiwiSDRs such that they surround the transmitter.
    
6. Once you've found a KiwiSDR that receives your signal of interest, close the second KiwiSDR receiver window that you just opened, and go back to the original KiwiSDR window. Now instead of double clicking just click once on the KiwiSDR pin on the map that you confirmed reception with. This will add that KiwiSDR to the window in the bottom left. This window displays the KiwiSDRs that will be used in the TDoA calculation.
   
   Make sure that it shows "XX GPS fixes/min" beside a selected KiwiSDR. If you get an error, remove that KiwiSDR and choose another.
    
7. When you've found two or more KiwiSDRs that receive the signal of interest, position the map to where you'd like the TDoA result heat map to be displayed. The positioning of the KiwiSDR map will determine where the TDoA heat map plot is displayed.
   
   
8. Click the 'submit' button to begin the TDoA calculations. The KiwiSDR server will gather 30 seconds of samples from each of your selected KiwiSDRs, and then run the TDoA algorithm on the KiwiSDR server. The whole process should take about 1-3 minutes to complete.
    
9. Once completed you can view the results by using the drop down menu next to the submit button to choose the 'TDoA Map'

The KiwiSDR TDoA feature is still in testing and can be a little buggy. If you get "Octave Error", try refreshing the KiwiSDR page and trying again with different receivers. Sometimes you'll also get an error saying that the GPS of a KiwiSDR hasn't updated in a while. In this case just remove that receiver and choose another one. We also find that if you're zoomed too far out on the map, the TDoA algorithm will sometimes return 'Octave error'. Try zooming in a bit closer to the approximately expected location. KiwiSDRs can also only support four simultaneous users at a time, so during peak periods it's possible that some may become busy.

Over on the KiwiSDR forums Martin G8JNJ has also provided a list of helpful tips that he's discovered. For example he recommends choosing KiwiSDRs that are spaced evenly around the estimated transmitter location (if known). Ideally they should also be chosen an opposing pairs (e.g. one pair north and south of the transmitter, and one east and west of it).

Results

We tested the new TDoA feature a few times. Below are some examples of the results we achieved.

USA: NLK @ 24.6 kHz.

This is a Naval transmitter located in Seattle, Washington. With three receivers surrounding the transmitter, we were able to get a pretty close location marker, that is confirmed with the known location.

Europe: DCF77 Time Beacon @ 77.5 kHz.

This is a German long wave time beacon transmitter. Again with three receivers we were able to pinpoint the signal fairly accurately.

Australia: Local MW Radio Station @ 549 kHz.

Here we tried to locate an Australian MW station. Unfortunately in Australia there is a lack of KiwiSDRs, and of the ones that are there, only three had GPS enabled and could receive the MW station, and two of those were right next to each other. With only effectively two stations we could only obtain a line of possible locations. Comparing with the known location plotted on Google Maps we confirmed that the transmitter is indeed located on the line.

We also tested a few signals at higher frequencies. As mentioned previously, anything above VLF/LF/MW (ie the HF bands) is a lot more difficult to locate since the signal can bounce around the atmosphere and can case extra delays to occur in the signal arrival time. The extra delays can cause problems with the time of arrival measurements. Thus for these signals it's important to find receivers close to the transmitter, or receivers spaced further away at the same distance so they each have identical skip distances, and thus identical delays.

When locating an HF signal that is in a completely unknown location we recommend starting with only two or three receivers, checking the heat map, and slowly adding more receivers in the hot parts of the heat map and removing receivers that turn out to be in the cooler areas. This way you can slowly narrow down the receivers to ones that are closer to the signal source, and are thus more likely to receive the signal directly, rather than via ionosphere bounces.

The Buzzer (UVB-76)

Using the just previously mentioned technique we attempted to locate the source of the Buzzer (UVB-76), a Russian numbers station at 4.625 MHz. Eventually we came to the results shown below. According to the heat map the buzzer appears to be located somewhere in the vicinity of St. Petersburg. Back in 2014 the numbers station researchers at priyom.org received an anonymous tip from a member citing a transmitter location just north of St. Petersburg. The TDoA heat map results seem to confirm that the anonymous tipper is correct.

Final Words

Right now the biggest problem appears to be the lack of active KiwiSDRs around the world. The more active KiwiSDRs there are, the better the direction finding results can be. At the moment Northern Europe and the USA are fairly well represented, but the rest of the world is not. Asia, Africa, Russia and South America are especially lacking. Also not all KiwiSDRs are utilizing the GPS feature. If you are running a KiwiSDR please do consider activating the GPS option. Another issue is that many KiwiSDRs suffer from poor reception and bad antenna setups, so not all active receivers are actually useful.

In the future we expect this feature to only improve, with the people behind it, John Seamons and Christoph Mayer, working hard to improve results. For example one possible future improvement is utilizing ray-tracing techniques to try and take into account delays caused by sky-bounce propagation. Update (15 July 2018): You can now also plot results over Google Maps.

If you want to purchase a KiwiSDR and contribute to the worlds first freely accessible TDoA system, you can purchase it immediately on Amazon or Seeed Studios for $299, or wait for a sale to occur on massdrop.com, where it is often discounted by up to US$100.