Navigation is the science of determining our position and our course of motion. Because it involves such fundamental quantities, navigation has played an essential role in human existence for millennia. This role has hardly diminished, even though we now have the capability to determine where we are with global radio navigation systems such as the Global Positioning System (GPS). If anything, the opposite is true. We are finding new ways of using navigation information to make our travels safer, faster, less costly, and generally more convenient.