The GPS in my HAND (Garmin GPS 60C) and in my JEEP (Garmin Streetpilot 2620) and my backup GPS (Magellan Explorist 100) all would not hold a fix in the valleys.

Here's a scan of part of the area concerned:


Note the deep valleys, really more indentations in the land. You just can't see much sky, plus I've always found Pine Trees are pretty bad in terms of RF attenuation. I'd have 3D lock, then 2d lock, then I'd loose all but 2 satellites, then I'd get a lock again. It wasn't a big deal, we didn't need the GPS at all, there were so many people to just walk the area and we used cell phones and HT's to coordinate.

And as far as Fog - it most certainly DOES affect RF signals in my experience.