SCAP: A MATLAB-based software package for satellite clock characterization and prediction in the Global Navigation Satellite System

In Global Navigation Satellite Systems, accurate and stable atomic clocks need to be equipped on satellites to ensure reliable, high-accuracy positioning, navigation, and timing services. It is essential to continuously monitor the behavior of satellite clocks in space and predict satellite clock correction for real-time Global Navigation Satellite Systems applications especially for precise point positioning. Some commercial software is available for clock characterization, but special attention has to be paid when referring to satellite clocks, the analysis and prediction of which may be complicated by outliers, data gaps, and periodic fluctuations in onboard clock data, not often encountered by clock data from a timekeeping laboratory. The typical approaches for clock characterization and prediction currently employed in a timekeeping laboratory are therefore unsuitable for clock applications in space. We present a software package developed in MATLAB at the National Time Service Center, intended for satellite clock characterization and prediction. The software package includes many subroutines and functionalities of particular interest in characterizing and predicting clock behavior in space, such as dynamic frequency stability evaluation, periodic fluctuation analysis, and multi-step prediction of clock signals. The software package allows handling of satellite clock data directly from Receiver Independent Exchange Format clock files widely used in Global Navigation Satellite Systems, facilitating quick characterization analysis and prediction of satellite clocks, with graphically visualized output.