There’s an already-extant ground-based vessel navigation system, Loran-C, though I’m sure that it’s possible to improve on it and I have no idea how much of the receiver hardware is still out there.

https://en.wikipedia.org/wiki/Loran-C

The introduction of civilian satellite navigation in the 1990s led to a rapid drop-off in Loran-C use. Discussions about the future of Loran-C began in the 1990s; several turn-off dates were announced and then cancelled. In 2010, the US and Canadian systems were shut down, along with Loran-C/CHAYKA stations that were shared with Russia.[2][3] Several other chains remained active; some were upgraded for continued use. At the end of 2015, navigation chains in most of Europe were turned off.[4] In December 2015 in the United States, there was also renewed discussion of funding an eLoran system,[5] and NIST offered to fund development of a microchip-sized eLoran receiver for distribution of timing signals.[6]

The National Timing Resilience and Security Act of 2017, proposed resurrecting Loran as a backup for the United States in case of a GPS outage caused by space weather or attack.[7][8]