David J. Vendittis, Paul Arveson, Errors due to Doppler Shift in Measured Levels from a Moving Ship, Journal of the Washington Academy of Sciences, Vol. 107, No. 1 (Spring 2021), pp. 29-34 ...

That’s when the inverse Doppler effect kicks in. Reflecting off a receding boundary, an ordinary electromagnetic wave would Doppler-shift downward by a minute fraction of its frequency.