Ch 47. Atomic Clocks for GNSS

Overview

Space‐based navigation systems rely on stable atomic clocks on satellites in well‐defined orbits that transmit microwave signals to Earth and serve as a space‐time reference frame providing Time (capital “T” epoch) and range information. A minimum of four satellite signals is required to determine position and time unambiguously. If the receiver has its own accurate clock that provides time in the same reference frame as the satellites, a minimum of three satellite signals is sufficient to determine position. Obviously, signals from additional satellites provide increased precision and can identify inconsistent data and other problems.

In this chapter, we focus primarily on the atomic frequency references (AFRs) that are used in GNSS systems, and particularly the atomic clocks used on the satellites (rather than those used in the ground‐based control systems or receivers). Several excellent books discuss atomic clocks more generally and in greater detail, including the classic two‐volume reference by Vanier and Audoin and its updated version by Vanier and Tomescu, and another fairly recent text by Riehle. There are also several excellent and modern review articles. This chapter provides some introductory context, the basic concept of an atomic clock and related terminology, and then provides more specific descriptions of the three types of atomic clocks (Rb, Cs, and H) that are used in GNSS space systems. In this chapter, to distinguish between the variable time, t, and time interval measurements, ∆t, from the specification of a Time epoch, T, we will use uppercase T to indicate an epoch for Time‐keeping and lowercase t for the time variable and time interval. A Time epoch must have a specified reference frame that includes a location (e.g. center of Earth) and starting time (e.g. UTC).