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 naïve 
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.