SOMETIMES with a GPS, the satellites are to low in the hemisphere or there are to many in one direction to lock on with a really good system. I have a $50,000 GPS I survey with and there are times it won't get a lock, even with 9 satellites.

I would assume that with it being a good GPS system that it updates the ephemeris every time it is turned on.

The ephemeris is a set of parameters that the GPS uses to accurately determine your position and is updated about every 4 hours or so as more data is collected by the satallites