DARPA developing Very Low Frequency (VLF) systems to provide GPS like position and timing technologies in contested, underwater and underground Environments

The GPS system provides critical positioning capabilities to military, civil, and commercial users around the world. However, in many environments in which military operates (inside buildings, in urban canyons, under dense foliage, underwater, and underground) have limited or no GPS access. It can be significantly degraded or unavailable during solar storms.