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Not RDF, as there is no "direction finding"! It's "geolocation". Not that modern, either! Remember LORAN? Or even the WW2 Gee system? Both TDOA geolocation systems in reverse, i.e. "hyperbolic navigation" systems. And, guess what---GPS is a TDOA navigation system, too!
Time difference of arrival (TDOA) and it's lesser known cousin, frequency difference of arrival (FDOA), have been used in military geolocation systems for a long time. They have not been popular because of the difficulty in distributing precise time and frequency, and also the difficulty of networking the measurements back to a geolocation computer. However nowadays distributing precise time and frequency is trivial with GPS, and networking is trivial using wireless networks. Complex digital signal processing is also becoming relatively trivial as exemplified in our SDRs. One very interesting modern application of TDOA geolocation is in the rapid replacement of secondary surveillance radars (SSRs) in the US with distributed "multilateration" arrays. These arrays more rapidly, precisely and, most important, less expensively locate the transponders on aircraft flying over the US. Note that there are a lot of hams claiming they have built TDOA direction finding systems. However the use of TDOA and DF is a contradiction in terms. What these hams have accomplished is a corruption of the terminology. These so-called "TDOA DF systems" are really simple, two-element beamformers. You could say that they are a special case of phase interferometer, however the two element arrays require them to be mechanically rotated. A true multi-element phase interferometer requires no rotation of the array. Or maybe I'm just sensitive because I've been involved with a lot of these systems over the years 
Interestingly, given a good GPSDO with 1PPS output, there is sufficient horsepower in the hardware of the Flex and Apache Labs SDRs to implement honest to goodness, highly accurate TDOA nodes given the right firmware. |