A method, comprising: characterizing an electromagnetic radiation path based on at least a scattered electromagnetic radiation pattern resulting from propagation of electromagnetic radiation through the path; receiving a plurality of scattered portions of a modulated electromagnetic radiation at a plurality of receiver antennas; and processing, at a receiver, the plurality of scattered portions, in conjunction with the characterized electromagnetic radiation path, to determine a change in the electromagnetic radiation path over time. An apparatus comprises a transmitter which transmits modulated electromagnetic radiation along an electromagnetic radiation path subject to electromagnetic radiation scattering; a plurality of receiver antennas which receive scattered portions of modulated electromagnetic radiation; and a processor which analyzes respective scattered portions, to characterize the electromagnetic radiation path, and to determine a change in the scattered electromagnetic radiation path over time.