The Kiwi SDR network or receivers allows you to direction find HF transmitters, by using the inbuilt Time Difference of Arrival (TDoA) functionality. TDoA is a method of measuring RF propagation from two or more receivers in an effort to establish the location of the transmitter.
You can watch a demonstration of how TDoA works here.
The Kiwi SDR network allows you to direction find HF signals from the comfort of your own home.
This feature would have previously been reserved for government departments or security services.
Other Kiwi receivers http://kiwisdr.com/public/
You can see the calculated location in this instance was only 3 km from the actual location of the 909 kHz transmitter.