Normally I'm expecting to rely on the chart plotter to tell me where I am. As long as it is working I don't plan to "navigate" as such, although pilotage is of course still necessary when required.

What if the chart plotter goes wrong? In fact, nowadays it almost seems like every device in the boat (except me) knows where it is thanks to GPS. Thus, if the chart plotter breaks down the AIS beacon gives me a lat and long position which is displayed on the VHF radio. If either of them are also not working, for example, the 12V supply has failed, then the handheld VHF, my waterproof camera, and my iPhone (in waterproof case) are all battery powered devices capable of telling me my position*. So provided the GPS system is working I should know my position, and I do carry paper charts on which to plot it!

What if the GPS system stops working? There seems little chance of it and the Russian and European counterparts being "turned off" - too many systems rely on it. However it is certainly possible that a solar storm could disable it. The symptom of something being wrong would be that GPS derived velocities would become highly eratic, or the devices would complain that they couldn't get a fix. The chart plotter displays the track, and it should be possible to work out where the positions started going wrong and I also take photos and I could use the position information recorded in these to determine my last reliably known position. To determine new position estimates I have charts and a sighting compass (which is in addition to the boat's fixed compass and the iPhone compass app). I also carry OS maps covering the coast since I would be more confident in identifying a landmark on these in order to take a bearing, rather than doing the same from a chart. So as long as I can see land I think I should be able to determine my position OK!