No GPS fix

[nwn]

We have just experienced a long period of "no GPS Fix". I want to ask if anybody has had similar experience.
Last week, Aug 11 we sailed from Calkis towards Eretria on Euboia in Greece. It is a large sound with an Island on the East side and the main land on the West side. My Foruno plotter gave alarm for no GPS fix many times. At first I thought there was something wrong with the GPS receiver. But then I discovered that my computer which gets its signal from the AIS transmitter also had problems now and then. Furthermore, the iPhone Navionics could not establish the direction we were sailing. Next day was in the beginning the same when we continued south along Euboia. Gradually, the problem disappeared. The following day, we sailed down to Cape Sounion without any incidence and we had GPS fixes all the time.
Thus, it seemed like the signals were distorted in the area south of Calkis. But how and why? What can distort the GPS signal like that?
Has anybody had similar experiences?

[bensolomon]

I've sailed that area long ago, it is gorgeous! Especially cape sounion. My home is Chesapeake Bay, United States. I have had a similar experience. The explanation was that there were military assets in the area with GPS jamming technology. I later found out there was a nuclear submarine that was coming up through the bay that jammed GPS.