Thanks, Patrick!

Yes, I can look at sixth field of the GPGGA sentence, it's very clear, you're right, so I'm going to do some testing and look at that more closely right now.

I'm also going to look at the number of satellites in the fix. I followed the link to the reference you cited, which shows that. I'd like to know whether, if I'm slowly closing the lid, if the number of satellites goes down slowly as the reception gets worse and worse. That will prove if it's simply the lid blocking the antenna. If it goes suddenly from 8 satellites to zero, then that's probably an indication that it's some kind of an on/off trigger.