* https://arstechnica.com/information-technology/2013/07/profe...
The USN has brought back celestial navigation (sextants) as a backup mechanism in recent years:
* https://www.npr.org/2016/02/22/467210492/u-s-navy-brings-bac...
US is examining backup systems for GPS/GNSS, including bringing back (e)LORAN:
* https://www.defenseone.com/ideas/2020/12/we-need-backup-gps-...
Now that the UK has been kicked out of Galileo, perhaps the government might think about reinstating eLORAN, it would be a good use for some the Longwave AM broadcast infrastructure that will be surplus in the next 10 years or so.
* https://www.trinityhouse.co.uk/notice-to-mariners/27-15-enha...
* https://www.maritime-executive.com/article/europe-gives-up-o...
South Korea is having a go at it:
* http://www.digitaljournal.com/tech-and-science/technology/sh...
A government or three (NATO?) have to decide on a system (eLORAN or other), publish standards, and buy a bunch of equipment (transmit and receivers) to kick start things.