And glass is a solid substance, the same as any other roof, so it has an attenuating quality. The light has no idea that it is transparent to human eye.
And yes, it needs atleast three satellites to triangulate your position somewhat accurately, so there's more to it than just a line of sight upwards.
"Software is like sex, it's better when it's free." ~Torvalds
GT-I9100 (cm10.1 nightly, modem: XXLS8) – GT-P6800 (cm10.1 nightly, modem: XXLPW)