Watson et al. (http://plan.geomatics.ucalgary.ca/papers/watson%20et%20al_ion%20navigation_winter06.pdf) analyzed what matters most when receiving GPS signals in-door. The GPS system was not designed for this but with good antennas, modern receivers and long integration times it is possible. Their graph below shows signal strengths from different satellites plotted on a hemisphere circle. The result indicates that reflections within the building are not picked up – only the attenuated “line-of-sight” direction reaches the antenna. Another interesting conclusion from their study is that besides an antenna that picks up signals from the horizon well, it is only the quality of the clock signal that matters (assuming the receive chain is a typically good one of course).
The Sony Ericsson Equinox phone (http://www.sonyericsson.com/cws/support/phones/topic/locationservices/equinox?cc=us&lc=en) is a good example that integrated antenna and clocking well. It obtained the best ever GPS availability when operators tested it in field, and was implemented according to new A-GPS integration guidelines (authored by Joakim Pettersson, founder of Quflow) that in detail explains how to stabilize antenna and clocking also in very small mechanics.