Positioning on the Earth Current status and future perspectives of GNSS positioning: from global constellations to integrated PNT

Positioning on the Earth is a broad topic, undergoing an unprecedented transformation and involving a variety of technologies and methods. This review contribution focuses on outdoor positioning based on true satellite signals on direct line-of-sight. With this limitation, the status and the modernization perspectives of positioning based both on Global Navigation Satellite Systems (GNSS) (US GPS, Russian GLONASS, Chinese Beidou and EU Galileo), on Regional Navigation Satellite Systems (RNSS) (Indian NavIC and Japanese QZSS) and on new constellations of Low Earth Orbit (LEO) satellites are addressed, together with the evolution of the International Terrestrial Reference Frame (ITRF) to which positions are referred. Principles of a variety of positioning methods are recalled, together with their present performances in terms of accuracy and real-time/off-line availability. In this frame, positioning is intended in a broader sense, considering not only positions, but also other kinematic parameters (velocities, accelerations). The overall goal is to outline the status and trajectory toward a resilient and outdoor seamless Positioning, Navigation, and Timing (PNT) ecosystem.