Ch 11. GNSS Orbit Determination and Time Synchronization

Overview:

All navigation satellite systems in use today build on the measurement of pseudodistances to compute a user position. These reflect the time difference between signal transmission at the satellite and signal reception at the user, but are based on two clocks that are imperfectly synchronized to each other or to a global system time scale. Accordingly, both the satellite position and the satellite clock offset must be accurately known to compute a user position in a global or regional navigation satellite system (GNSS/RNSS). This chapter discusses the generation of precise orbit and clock information using ground‐ or space‐based observations. Evidently, orbit determination and time synchronization (ODTS) is an integral part of the control system of any GNSS/RNSS provider, where the motion of the satellites and the variation of their clock must be continuously observed, predicted, and distributed to the user as part of the navigation message. Similarly, a variety of scientific institutions, such as the International GNSS Service (IGS; [1]), routinely determine precise GNSS satellite orbits and clock offsets. These GNSS products provide the basis for precise point positioning (PPP) applications in engineering, surveying, and geodesy. As a by‐product, the precise orbit determination (POD) process delivers information on Earth rotation and the terrestrial reference frame, as well as the state of Earth’s atmosphere. While precise GNSS products have typically shown latencies of days or weeks, an increasing interest in real‐time applications has resulted in various real‐time correction services that generate and distribute orbit and clock information to their users in a near‐instantaneous manner.