Out of interest how well can you tell your location using the time, the position of an available known celestial object (sun, constellation, or moon) and a compass? Would that combination plus good instrumentation and computer provide a decent backup. I guess nighttime and cloudy poses an issue.

Is that a real question? As in you don't actually know?

That's how ships navigated for centuries. The instruments are simple by modern standards. You don't need a computer, although it's nice to have. The fixes aren't (normally) as precise as you get from GPS, but they're much better than you'll ever get from gravimetry. It can take time to get one, though.

Celestial navigation was a huge driver for the development of timekeeping and supporting technologies. They still teach it in "ship school". It was big news when the US Naval Academy stopped requiring it, and I think they've now backed off and brought it back because of the GPS jamming. It can be and has been automated, although the resulting instrument is a lot bigger, touchier, and more expensive than a GPS receiver.

If you need a lot of accuracy, you normally use more than one object and don't trust the compass. Individual stars are fair game and have a lot of advantages over extended objects.

But, yeah, clouds. I assume clouds are the reason people are building these big complicated expensive devices. It also gets harder to do it, at least manually, if the ship is rolling around a lot.

"I must go down to the seas again, to the lonely sea and the sky And all I ask is a tall ship and a star to steer her by."

Nighttime makes it easier, I suppose (if no clouds).