What is GPS?
GPS stands for Global Positioning System. GPS units provide satellite-based navigation systems allowing you to pinpoint your longitude, latitude and altitude. GPS units utilize 24 satellites. Three satellite connections are required to determine your latitude and longitude. If four or more satellite connections are obtained your GPS unit will provide your latitude, longitude and altitude.
Where will I receive a signal?
Civilian GPS units (GPS satellites are maintained by the United States Department of Defense) are transmitted in a straight line. The signal is capable of traveling through clouds and mildly forested area. The signals are not capable of traveling through mountains or other solid objects. Therefore if you are directly next to a mountain or in a canyon it may be difficult to obtain the required three satellite connections.
The twenty-four satellites are designed so that your position can be determined from any location in the world (that is not obstructed by mountains etc).Full Article