The Navstar Global Positioning System

Global positioning system (GPS) was the first operational global satnav system, and is designed and operated to provide service to civilian and to military users. GPS uses advisory messages, called Notice Advisories to Navstar Users (NANUs), to inform users of a change in the GPS constellation. This chapter provides a brief history of GPS and summarizes current plans in the first section, followed by an overview of GPS. Then, it provides detailed characteristics of GPS signals. The development of GPS in the 1970s built upon previously developed and explored technical concepts such as radio navigation from terrestrial transmitters, developments of space-based navigation systems using fundamentally different approaches from GPS, maturation of space-qualified atomic clocks, and application of concepts from digital communications. GPS originally transmitted three signals on two carrier frequencies. Satellites launched since 2005 have gradually added signals and then another carrier frequency.