# Evaluating Optical Clock Performance for GNSS Positioning
###### Abstract  
Atomic clocks are highly precise timing devices used in numerous Positioning, Navigation, and Timing (PNT) applications on the ground and in outer space. In recent years, however, more precise timing solutions based on optical technology have been introduced as current technology capabilities advance. State-of-the-art optical clocks--predicted to be the next level of their predecessor atomic clocks--have achieved ultimate uncertainty of \(1\times 10^{-18}\) and beyond, which exceeds the best atomic clock's performance by two orders of magnitude. Hence, the successful development of optical clocks has drawn significant attention in academia and industry to exploit many more opportunities. This paper first provides an overview of the emerging optical clock technology, its current development, and characteristics, followed by a clock stability analysis of some of the successfully developed optical clocks against current Global Navigation Satellite System (GNSS) satellite clocks to discuss the optical clock potentiality in GNSS positioning. The overlapping Allan Deviation (ADEV) method is applied to estimate the satellite clock stability from International GNSS Service (IGS) clock products, whereas the optical clock details are sourced from the existing literature. The findings are (a) the optical clocks are more stable than that of atomic clocks onboard GNSS satellites, though they may require further technological maturity to meet spacecraft payload requirements, and (b) in GNSS positioning, optical clocks could potentially offer less than a 1 mm range error (clock-related) in 30 s and at least 10 times better timing performance after 900 s in contrast to the Galileo satellite atomic clocks--which is determined in this study as the most stable GNSS atomic clock type used in satellite positioning. optical clocks; satellite atomic clocks; clock stability analysis; Allan deviation; GNSS; Positioning, Navigation, and Timing (PNT) 2023232023202320232023202320232023202320232023202023202320232020232023202023202320202320202320232020223202022320202202202202202220220220220220220220220220222022022022022022  
frequency stability is considered, which describes how consistent the clock frequencies are over different averaging times. The more stable the clock frequencies, the better the timing performance. ADEV is a statistical measure of frequency stability widely used to assess or compare high-precision clocks. Further details regarding ADEV are provided in the following section. In PNT, likewise, optical clocks are drawing great attention as GNSS positioning is reliant on precise timing. Typically, Positioning and Navigation are to accurately determine the geographic location of any point and to calculate a suitable route between two locations, respectively. Timing, in contrast, is to provide global standard time scales by synchronising time and frequency standards between different clocks situated around the world [5; 6]. GNSS is a widely used PNT technology that consists of four main constellations such as Global Positioning System (GPS), Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), Galileo, and BeiDou [7]. A typical GNSS system consists of three fundamental segments, namely (a) space segment, (b) control segment, and (c) user segment. The GNSS space segment, usually a constellation of satellites, is designed to provide PNT services with the use of artificial satellites orbiting Earth. Each satellite carries several (normally three or four) atomic clocks onboard as the primary source of time and frequency standards [7; 8; 9]. In Positioning and Navigation, which is the focus of this study, precise timing is crucial because a signal travel time between satellite and receiver multiplied by the speed of light, 299,792,458 m/s, determines the range between them. And using a minimum of four ranges, i.e., receiving signals from a minimum of four different satellites simultaneously, can determine the receiver position and the receiver clock time.