Pseudo-doppler radio direction finding (RDF) has traditionally been done in hardware where a timing circuit is used to control the switching of the antenna array, and filter out the pseudo-doppler frequency from the connected audio output of a separate FM radio.
This presentation shows how the same can be achieved in an integrated fashion and with a minimum of hardware (USRP 1 and RF switch IC). The rest of the hardware has been moved into the software domain. The USRP's FPGA code is modified to output a timing signal, synchronised to the captured samples, which controls the antenna switch. GNU Radio is used to perform DSP, which results in a single value: the direction of the transmitter.
This SDRDF concept has been realised as a 'mobile' setup in a vehicle (with array of four antennas on the roof). An additional GUI runs on a second laptop, which receives real-time data via UDP from GNU Radio, to enable track mapping with GPS.
RDF generally has some associated black magic (RF reflections, etc), which also presented itself here since testing was done in an urban area.
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