INRIM tool for satellite clock characterization in GNSS

In Global Navigation Satellite Systems (GNSS) the role of atomic clocks is essential for the determination of the user position and due to their excellent stability they are fundamental to ensure the adequate performances required in the positioning service. Since a degradation of the clock stability would immediately impact the navigation performances, the on board clock behaviour has to be continuously monitored. In this work, we illustrate a software tool developed at INRIM, optimized for GNSS clock characterization and monitoring. Although some commercial software are available for clock characterization, special attention has to be paid when dealing with space clocks. In fact, the space clock behaviour analysis may be complicated by different aspects: data from satellite clocks often present gaps and outliers as well as periodic fluctuations. On the contrary, data from timing laboratories are equally spaced and usually not affected by many outliers. Hence, often, the typical methods for clock characterization currently used in timing laboratories are not sufficient for space applications: new approaches, more suitable for GNSS applications, are required. To overcome these limitations, we developed the Clock Analysis Tool: a software, developed in Matlab, for clock characterization to be used either in metrological laboratories or for space applications. To fulfill GNSS needs, the software has been enhanced and extended, including new routines and functionalities of particular interest when characterizing the space clock behaviour, such as the dynamic stability analysis, as well as the addition of the uncertainty evaluation in the clock parameter estimation, or the inclusion of the system noise estimate in the frequency stability analysis. Moreover, the algorithms commonly used for clock characterization, such as the Allan Deviation computation and the frequency drift estimation, have been adapted to deal with space clock features, namely data gaps and outliers, treating missing data with the best possible procedure to allow the estimate of the stability over long observation intervals. In addition, the software has been provided with a graphical user interface, allowing an easy handling of satellite clock data and permitting a quick estimate and graphic representation of the clock key parameters.