Where on Earth Am I?: Global Navigation Satellite Systems

Abstract

Measuring your absolute global position to within a few centimetres or relative changes in position anywhere on Earth to within a millimetre or so, seemed impossible just a decade or so ago. The Global Positioning System (GPS) has made this not just possible but routine. In addition everyone is familiar with the fact that ‘Satnavs’ will guide you with uncanny accuracy to any selected destination. How does GPS achieve such precision? This short review will explain the basic geometrical principles, the practical implementation and some of the burgeoning applications of GPS. But first a word about nomenclature. Although the ‘GPS’ is often used to describe the satellite based global location system, there are in fact several other systems in development which provide or will provide equal or even enhanced precision within the next few years. The generic description for these is Global Navigation Satellite Systems (GNSS), but like ‘Hoover’ and ‘Biro’, ‘GPS’ has become the generic acronym often casually used to refer to the overall system. Throughout this article the information and examples refer to the GPS. More about the proliferating rivals to GPS later.