Science Explained: How does sat-nav work?
18 August 2010 Last updated at 11:48 BST
Satellite-navigation systems have become so common that we take for granted the technology - in space and here on Earth - that makes them possible.
Satellites, atomic clocks and good old-fashioned geometry fit into the equation.
But how do sat-nav systems actually work?
Professor Cathryn Mitchell from the University of Bath explains as she finds herself navigating around London.
Please note: The graphics used in this video are not to scale