As long as the external antenna has a clear view of the sky, even to the horizon for low satellites, it will give highly accurate positions. Some folks have mounted it under the boat deck, thugh that isn't recommended. Any metal, metallic paint, wood, etc will block signals. There's no way you could use low-horizon satellites, your best ones up there, far better that those overhead.
The internal antenna is much less forgiving. The head unit can't be flush mounted, in fact must be positioned with top straight up. If you had to tilt it back to view it, chances are you couldn't get a position. They won't do on a bow deck since that requires the screen aiming up at you.
The 520C replaces the 332C.
Tom already indicated the best way to match antenna and transducer. If when setting a waypoint you take action with the pop up window you see after pressing WPT, you can get accuracy to within 5' for whatever sonar view you have (like a brush pile) assuming antenna is over the transducer. Choose point averaging and let the unit collect 30 second's worth before creating the waypoint. I let it go longer if the boat is still, taking time to assign a peculiar name that describes the spot. if you end up with a long list of default names like Wpt 001, Wpt 002, etc, they lose value later when you have no recall of what was there or why you made it. "Brush Pile" would be a better name. "Laydown". "Ridge". "Ledge". There are many ways to organize your data. I'm going to write an article on it.
Jim
