I rather doubt that you have a solid aluminium roof.
Those of you who fly into CPT will have seen the 'informal settlements' under the approach path. They have corrugated iron roofs. How do you know I am not resident in one of those?
Seriously, I thought that the GPS had to have a more or less clear 'line of sight' to the sky. I was surprised to find it works indoors and also in the car even if it not against a window. (I can now use it to navigate in my drunken stupors from front door to bedroom without crashing into obstacles in the hallway and waking up HAH !)