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. Currently the signal is received and processed in short blocks with each block starting with a burst of calibration noise.