Don't believe that the GPS system is ground based. While at sea you could be 1000 miles from the nearest land. The GPS antennas are directional and we always point the antennas straight up when mounting them on the ship. When doing this we are able to receive the signals from 3 or 4 of the many GPS satellites that are in orbit above the earth. If the earth were flat, we could always see the same satellites on our whole trip, but this isn't the case. Satellites come up over the horizon, provide us with our position data, and then go down into the horizon while another satellite takes it's place. There is a whole satellite constellation out there for us. If it were not the case, we would then have to resort to the old technology of the sextant, which we still carry aboard ship. Even while using the sextant you have to believe in the spherical earth, or that system wouldn't work either. All this comes from my every day work experience, not something that I read in a book, or saw on the internet. If someone believes this is a hoax, please tell me. I still have many friends at sea trying to make a living for themselves and their families. They might need to know what the true facts are so to protect them from any potential danger. I'm just a retired Merchant Marine officer and I sit at home now collecting a pension.