How accurate is GPS? Is the GPS signal available everywhere?

GPS signals available to civilian GPS receivers can be used to compute position to within 10 meters (33 feet) most of the time. However, GPS signals are sometimes diluted in certain sensitive areas.
GPS signals are available all over the world. However, please note that GPS receivers need signals from three or more satellites to compute position and, in general, this requires that an antenna connected to the GPS receiver be placed with a clear view of the sky. GPS reception may, therefore, be inconsistent under certain extreme weather conditions, or:

• Indoors, in parking garages.
• under heavy foliage
• Near tall buildings, etc.