Over on YouTube user Tatu Peltola has uploaded a video showing his RTL-SDR based phase correlative direction finder in action. This set up uses three RTL-SDR dongles and three antennas to measure phase differences and thus determine the direction towards a signal source. All three RTL-SDR’s must be coherent, meaning that all three of their 28.8 MHz clock signals must come from the same source.
In the video Tatu walks around the three antennas with a handheld radio. An arrow on a laptop screen points in the direction of the transmitter.
A known problem with RTL-SDR’s is that even with the clock sources synchronized there is still an unknown cause of additional phase shift. To solve this problem Tatu writes:
Each rtl-sdr is fed from the same reference clock to make their phase shift remain constant. They still have unknown phase shifts and sampling time differences relative to each other. This is calibrated by disconnecting them from antennas and connecting every receiver to the same noise source. Cross correlation of the noise gives their time and phase differences so that it can be corrected.