"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