Extremely Low and Very Low Frequency Radio
Aside from underwater telephones, the only other viable method of communicating with submerged submarines is via Extremely Low Frequency (ELF) radio or Very Low Frequency radio (VLF) There are variable definitions, but the most common defines ELF as radio in the range of 30 Hz to 300 Hz and VLF as between 3 kHz to 30kHz.
For the most part, electromagnetic waves do not travel through water easily. But VLF and ELF radio are not absorbed by seawater in the same way that higher frequencies are. So while light cannot get to the lower depths of the ocean becaus VLF and ELF frequencies can. This means that if someone could reliably transmit ELF information, deeply submerged submarines would still be able to receive the signal being transmitted.
Some submarines also float a radio antenna up toward the surface so that higher frequencies can be detected. Higher frequencies mean transmitting and receiving at a higher data rate. A higher data rate means more pertinent information can be sent in less time.
For the most part, electromagnetic waves do not travel through water easily. But VLF and ELF radio are not absorbed by seawater in the same way that higher frequencies are. So while light cannot get to the lower depths of the ocean becaus VLF and ELF frequencies can. This means that if someone could reliably transmit ELF information, deeply submerged submarines would still be able to receive the signal being transmitted.
Some submarines also float a radio antenna up toward the surface so that higher frequencies can be detected. Higher frequencies mean transmitting and receiving at a higher data rate. A higher data rate means more pertinent information can be sent in less time.