The year 1978 saw the launch of the first Global Positioning System (GPS) satellite. Today, GPS, as the first and currently only operational global navigation satellite system (GNSS), is widely used and is a vital technology for many of society’s economic, scientific and social activities. Applications can be found everywhere, such as spacecraft navigation, geodesy, surveying and mapping, precision navigation, machine guidance, vehicle fleet management and “intelligent transport systems” (ITS), emergency services and “location based services” (LBS). Clearly the development of GPS has revolutionised what are now termed “positioning, navigation and timing” (PNT) activities.