That sounds a lot like the ELF radio system they used, which basically was used to order submarines to surface (or near enough) to receive new orders as the ban

That sounds a lot like the ELF radio system they used, which basically was used to order submarines to surface (or near enough) to receive new orders as the bandwidth was <1 baud. https://en.m.wikipedia.org/wiki/Extremely_low_frequency https://en.m.wikipedia.org/wiki/Extremely_low_frequency