U.S. Navy developing Acoustic Undersea Navigation and Positioning system to provide GPS-like accuracy anywhere in the ocean

The unmanned underwater vehicles (UUVs) depend on stealth as they conduct surveillance and reconnaissance and other missions in the deep oceans. With GPS signals unable to penetrate the ocean’s surface, these UUVs can rely on inertial sensors to provide acceptable positioning information during short missions. On longer missions, however, inertial sensors accumulate error, forcing the vehicles to risk exposing themselves to enemies as they periodically surface to obtain a GPS fix.

GPS rely on Radio waves  that do not propagate well underwater due to the high attenuation. In fact, radio waves propagate at long distances through conductive salty water only at extra low frequencies (30− 300Hz), which require large antennae and high transmission power. However, the acoustic waves are low frequency waves which offer small bandwidth but have long wavelengths. Thus, acoustic waves can travel long distances and are used for relaying information over kilometers