ABSTRACT
Global navigation satellite systems (GNSS) provide positioning services across the globe (Hofmann-Wellenhof et al. 2008). Utilization of these systems gained huge popularity because of many advantages that they provide such as constant availability (although rare cases of downtimes are known), free of charge service, and good positioning accuracy (Gleason and Gebre-Egziabher 2009, 2010, Je§rey 2010). In general, GNSS might be internally split into three segments: space, control, and users (Prasad and Ruggieri 2005, Groves 2008). Space segment is comprised of satellites, which are constantly orbiting around the globe. Orbits of individual satellites, within particular systems (constellation), are arranged in a way, which allows for observation of at least four satellites, from almost any point of Earth surface, at the same time (Petrovski and Tsujii 2012). Satellites send ceaselessly information with time stamp, precise orbit coordinates (ephemeris), description of satellites constellation (almanac), error corrections as well as health state indicating their operational state (Ward et al. 2005a). GNSS receiver obtains navigation signals and utilizes its internal clock in order to determine delay between time of navigation message transmission and time of message acquisition by the receiver antenna. As a result, pseudorange between transmitting satellite and receiver might
be determined (calculated value is called pseudorange, because various errors can in©uence obtained results) (Kaplan et al. 2005, Grewal et al. 2001, Xu 2007). If signals from four satellites are available, then based on obtained pseudoranges, and ephemeris of satellites, receiver is able to calculate coordinates in three-dimensional space (Gleason and Gebre-Egziabher 2009). It is worthy to mention that contemporary receivers are able to utilize many visible navigation satellites at the same time, and combined data obtained from various GNSS in order to increase the precision of obtained coordinates. System requires high time precision and synchronization of time between satellites, even small errors in time synchronization lead to signi¦cant errors at the level of the Earth’s coordinates calculations. In order to achieve these tasks, satellites are equipped with several clocks (i.e., a battery of cesium and rubidium atomic clocks) that are regularly synchronized by control segment.
