... A GPS unit establishes contact with at least three of these satellites and uses the information received to determine altitude, longitude, and latitude. ... In order to calculate altitude, the unit must have access to at least four satellites. ... GPS receivers are extremely accurate due to their parallel multi-channel operation. ... Most portable GPS receivers are accurate to within 10 feet. ... These receivers can lock onto more than one satellite at the same time. ... This is important if you plan to use the unit while hiking or camping. ... This is an important consideration if you want to carry the GPS device in your pocket. ... Salt water can ruin an ordinary GPS receiver, so look for a marine rated unit if it will be operated in a boat. ... You might want to purchase a spare set of batteries and use the power adapter in your RV or boat whenever possible. ... Roads and highways change all the time and the installed map will eventually become obsolete. ... It can be surprisingly expensive in some models. ... Batteries can run down, or the unit might lose contact with the satellites ...