"The GPS signal strength measured at the surface of the Earth is 
about -160dBw (1 x 10^-16 watts), which is roughly equivalent to 
viewing a 25-watt light bulb from a distance of 10,000 miles."

Since it is a pretty weak signal, it is hampered by just about 
anything that is denser than air!  I have definitely seen the 
precision degrade under a dense canopy of trees and in canyons (both 
natural and manmade).  A concentration of overhead power lines will 
also give you fits (anybody have a link to a good explanation of the 
physics of that phenomenon?)

Tim
Team AZFastFeet

>Being new to GPS receivers I am curious about sensitivity. My 
>receiver is a cheaper magellan model. It doesn't work inside 
>buildings or airplanes. Are there any receivers that work well 
>anywhere?
>
>Mark