GPS - (Global Positioning System) - A worldwide radio-navigation system originally developed by the US. Department of Defense that allows users to determine their exact location, anywhere in the world, using the time difference for signals from different satellites to reach the receiver.
Portable - Portable GPS systems tends to be battery powered and not require any permanent installation. Hence they can be easilly transported from vehicle to vehicle. They are not usually the sort of thing you would want to slip in your pocket!