Deja vu
An exercise like this was done by Trinity House some time back (I forget when exactly but around 20 years ago). They sailed a trawler up and down past Grimsby and jammed its GPS signal (awfully easy to do). There were two interestig findings: [1] at one point it broadcast that it was doing around 200 knots over land, and [2] the "fallback" radar navigation system didn't work because it used the GPS clock for synchronisation.