This is one of the first early handheld Global Positioning System (GPS) receivers intended for civilian use. GPS is a navigational system that provides accurate and instantaneous position information to those equipped with receivers such as this one. The system relies on a set of 24 satellites placed in orbit approximately 18,000 km above the earth. Each satellite carries atomic clocks on board, and broadcasts a signal that is accurate to within 3 billionths of a second. GPS units are tuned to receive signals from these satellites, and if the receiver can lock on to at least four of them, it can determine its position and altitude on Earth.This unit was donated to NASM by its manufacturer, the Magellan Corporation. It shows positional data in traditional latitude and longitude coordinates. More recent models incorporate that data into maps and other graphic informaiton that is more understandable to laypersons.