Any frequency selective device with an ongoing drift will cause observed spectra to be variously and simultaneously
scaled in proportion to their source distances. The reason is that detectors after the drifting selection will integrate
instantaneous electric or magnetic field values from successive sinusoids, and these sinusoids would differ in both
frequency and phase. Phase differences between frequencies are ordinarily irrelevant, and recalibration procedures
at most correct for frequency differences. With drifting selection, however, each integrated field value comes from
the sinusoid of the instantaneously selected frequency at its instantaneous received phase, hence the waveform
constructed by the integration will follow the drifting selection with a phase acceleration given by the drift rate
times the slope of the received phase spectrum. A phase acceleration is literally a frequency shift, and the phase
spectrum slope of a received waveform is an asymptotic measure of the source distance, as the path delay presents
phase offsets proportional to frequency times the distance, and eventually exceeding all initial phase differences.
Tunable optics may soon be fast enough for realizing such shifts by Fourier switching, and could lead to pocket
X-ray devices; sources continuously variable from RF to gamma rays; capacity multiplication with jamming and
noise immunity in both fibre and radio channels, passive ranging from ground to deep space; etc.