I'm wondering if there have been any studies which attempt to analyze or characterize GPS error distribution over different situations/conditions (e.g. surrounded by tall buildings/mountains, precipitation/clouds/fog, rural areas, etc.) to understand how these errors may change as compared to 'perfect' conditions.
The general term for this is "Multi-path Error", which can be described as what happens when a GPS signal bounces on it's way to the receiver from the satellite instead of a direct line of sight.
Lighter reading is available in the top results from the following search: http://www.google.com/search?btnG=1&pws=0&q=GPS+multipath+error