General method for extracting source distance information from any kind of received radiation, including electromagnetic and acoustic, without involving round-trip time or phase in any form, and thus more truly passive than existing passive radars. The method exploits the facts that radiation from a real source must comprise wavepackets of nonzero bandwidth, that the individual frequency components of a wavepacket must have consistent phase at the source, and that their instantaneous phases must increase linearly along the path in proportion to the respective frequencies, so that the phase gradient across the components must be proportional to the distance travelled. The invention simplifies over naive phase gradient measurement by scanning the phase gradient at a controlled rate, thereby converting the gradient into normalized frequency shifts proportional to the scanning rate and the source distance. It mimics the cosmological redshift and acceleration, but at measurable levels over any desired range and even with sound.Potential applications include stealth and "unjammable" radars for the military, ranging capability for emergency services, commodity low-power vehicular and personal radars, simplification and improvements in radar and diagnostic imaging, improved ranging in general all the way from ground to inter-galactic space, "interference-free" communication systems including radio and television receivers, source-distance (or range-division) multiplexing improved cellphone power control and battery life, and continuous, transparent diagnostics for optical fibres, integrated circuits and transmission lines.
