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A really interesting project combining cheap software defined radios with open-source signal processing software and some smart hardware techniques to create a very powerful platform for doing almost anything in the normally-used radio spectrum.

Specifically, these 4 radios can be easily made to be phase coherent, meaning that software like multi-rtl can be used to listen in on GSM, cancel out noise for better SNR, and enable a whole new regime of software defined radio on low-cost hobbyist platforms.

From a slightly more security-focused lens, four radios spaced not that far apart can be used for very effective passive radar and actually can form a phased array to do single-site multilateration (“fox hunting”), which is a huge issue for security.