The evocation of radar in this passage is to Rabkin's knowledge, the "one invention that ever appeared first in science fiction in adequate form and detail to count as a true prediction." Guglielmo Marconi didn't create a working device that could detect remote objects by signals until 1933.

Ralph 124C 41+ by Hugo Gernsback:

A pulsating polarized ether wave, if directed on a metal object can be reflected in the same manner as a light ray is reflected from a bright surface ... By manipulating the entire apparatus like a searchlight, waves would be sent over a large area. Sooner or later these waves would strike a space flyer. A small part of these waves would strike the metal body of the flyer, and these rays would be reflected back to the sending apparatus. Here they would fall on the Actinoscope, which records only the reflected waves, not direct ones.

... From the intensity and elapsed time of the reflected impulses, the distance between the earth and the flyer can then be accurately estimated.

* Bing: The history of radar

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