A dream has become true: direction finding (DF) in the range of 0 to 30 MHz for all! The method is “time difference of arrival”. This is a speaking term, and it works exactly like it says: measuring the different times of arrival of a signal at at least three different receivers and calculating the position of the transmitter.
Base is the net of KiwiSDRs. If connected to GPS, each of this SDRs is able to deliver an I/Q stream with time stamps of a very high resolution. If you compare the recordings of different receivers tuned to the same station at the same time, you will find as slight time difference from receiver to receiver. This is due to the “time of flight” from transmitter to receiver. HF is traveling 300 km per 1 millisecond – so time is distance.
However, you don’t know the absolute transmitting time on transmitters other than time signal stations. So “Time Difference on Arrival”, or TDoA, measures the time differences of arrival. The first step is to compare the I/Q streams to find at which time difference their content does correlate. From these time differences, there are calculated curves onto a map. They cross in that region, where most likely the transmitter is located – see the screenshot at the top which I did to locate the STANAG 4285 transmitter on 12.666,5 kHz.
Due to concept, most reliable results (and, hence: sharp regions where the curves meet) are achieved only with the same propagation mode prevailing at all receiving stations. This is most strictly the case on VLF and long wave, where we mostly have one and stable waveguide-like propagation. But you can achieve also stunning results on HF if you carefully choose the SDRs – see next picture.
As soon as we approach skywave propagation, you must care for more or less the same propagation mode, of which a one-hop propagation (e.g. 1 x F2) should be preferred. Often even a bit bigger heat map of correlation gives a significant clue from where the transmitter is operating see next picture.
The software has been developed by some smart people around Christoph Mayer who also provides detailed information on the concept of this approach on his website. It quickly has become an extension of the KiwiSDR net. This makes it very comfortable to use – if you take care of what is said above.