SCAP: a MATLAB-based software package for satellite clock characterization and prediction in GNSS

In Global Navigation Satellite Systems (GNSS), accurate and stable atomic clocks need to be equipped in onboard satellites to ensure high-accuracy and reliable positioning, navigation and timing services. It is essential to continuously monitor behaviour of satellite clock in space and predict satellite clock correction for real-time GNSS applications especially for real-time precise point positioning (PPP). Some commercial software is available for clock characterization, but special attention has to be paid when referring to satellite clocks. In fact, the analysis and prediction of satellite clock behaviour may be complicated by outliers, 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 therefor not often suitable for clock applications in space. In this paper, we present a software package developed in MATLAB at the National Time Service Center (NTSC), more suitable for GNSS satellite clock characterization and prediction. The software package includes many subroutines and functionalities of particular interest in characterizing and predicting clock behaviour in space, such as the dynamic frequency stability evaluation, periodic fluctuation analysis and multi-step prediction of clock signals. The software package allows to handle with satellite clock data directly from RINEX clock files widely used in GNSS, perform quick characterization analysis and prediction of satellite clocks, and generate a visually graphic output.