Way back in the 1980s, when I was a young naval officer, the Global Positioning System was still in its experimental stage. If you were in the middle of the ocean on a cloudy night, there was pretty much only one reliable way to know where you were: Loran-C, the hyperbolic low-frequency radio navigation system. Using a global network of terrestrial radio beacons, Loran-C gave navigators aboard ships and aircraft the ability to get a fix on their location within a few hundred feet by using the difference in the timing of two or more beacon signals.
An evolution of World War II technology (LORAN was an acronym for long-range navigation), Loran-C was considered obsolete by many once GPS was widely available. In 2010, after the US Coast Guard declared that it was no longer required, the US and Canada shut down their Loran-C beacons. Between 2010 and 2015, nearly everyone else shut down their radio beacons, too. The trial of an enhanced Loran service called eLoran that was accurate within 20 meters (65 feet) also wrapped up during this time.
But now there's increasing concern about over-reliance in the navigational realm on GPS. Since GPS signals from satellites are relatively weak, they are prone to interference, accidental or deliberate. And GPS can be jammed or spoofed—portable equipment can easily drown them out or broadcast fake signals that can make GPS receivers give incorrect position data. The same is true of the Russian-built GLONASS system.