Since extremely-low-frequency radio waves are used by submarines for some simple, low-transmission-rate communications, why can't those same wavelengths be used for submarine radar? It may not be ideal, or highly precise, but couldn't it still provide some useful information?

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    $\begingroup$ According to the Wiki page, en.wikipedia.org/wiki/Communication_with_submarines , ELF is not used by submarines because they can't take a feasible antenna with them. Also you couldn't use ELF to pinpoint an object on the scale of tens of meters because the ELF wavelength is a million meters. $\endgroup$ – HiddenBabel May 20 '19 at 21:54

There are likely many reasons why ELF radar is impractical. Here are two. The wavelength at ELF frequencies exceeds 10,000 km. A radar using that large a wavelength will not be able to locate anything very precisely. In addition, radiating and receiving radio signals efficiently requires an antenna size of a significant fraction of a wavelength; probably too big to fit on a submrarine.

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