How can a drone ship hold its designated position while it's blocking gps?

By stopping jamming for a few seconds every few hours , and navigating using dead reckoning in between? With just a compass, a log, and a watch I can sail for days and end up within two kilometers of my destination.

Shouldn't it be possible for you to reconstruct the original GPS signal if you can control the jamming pattern?

Eh, I doubt it, GPS signal is so weak, once the EM field in the air has so much power, I think the literal hardware ADC of any receiver would be overwhelmed/clipped with a high power jammer nearby. I mean it's possible that the front end will be literally fried, not just clipping the analog front end. GPS is usually -130dBM, it's absurdly weak.

But in theory you're not wrong.

Beyond what everyone else has said it could also have sonar and keep track of its position relative to the local ocean floor like an optical mouse.

Anchor? ;)

Electro-mechanical gyroscope

Probably just turn off the jammer for 2 seconds

Astral navigation is probably accurate enough

Dead reckoning with a minute or two to get a gps fix with the jammer off once a day.

What's to stop someone spoofing a GPS signal during that fix?

Nothing, and if it were subtle enough it could be effective at making it drift around. Lots of ways to get a rough fix though, and you don’t need very good absolute accuracy offshore, using a compass and log to DR your way around. Even better if you have a wind vane and anemometer. The majority of your drift will be wind driven.

A camera that looks at the stars / sun and a very accurate clock...

Or one of the many methods described in TFA.

accelerometers can do that job. Also it can stop the jamming every once in a while to check.

INS