## 1 Introduction  
Navigation is the process of controlling the movement of objects [1]. Its purpose is to determine the position coordinates of an object as a function of time, and to influence (control) the direction and velocity of its movement. The introduction of the earliest satellite positioning systems such as Transit (USA) or TSIKADA (USSR) to use in the middle of the 20 th century initiated the era of satellite navigation that continues to this day. The Navigation Satellite System (NSS) is a general term encompassing Global Navigation Satellite Systems (GNSS) and Regional Navigation Satellite Systems (RNSS) which enable the positioning of receivers in space and the transmission of time signals. The systems that ensure global operating zone include the American NAVSTAR Global Positioning System (GPS), Russian GLObal NAvigation Satellite System (GLONASS), and the Chinese BeiDou Navigation Satellite System (BDS). The European satellite navigation system (Galileo) is currently under development and is likely to become fully operational soon. Navigation satellite systems with a regional operating zone include the Indian Regional Navigation Satellite System (IRNSS) which provides preliminary services and the Japanese Quasi-Zenith Satellite System (QZSS). The current status of the autonomous navigation system constellations is presented in Table 1[2]. It is reasonable to assume that, within the next few years, the number of navigation satellite systems all over the world will exceed 140 units, which will allow users to use them simultaneously, naturally within the available location. The summary provided in Table 1 does not take into account augmentation systems or the number of satellites still to be launched as part of supplementing them to obtain full constellations. It follows from the presented analysis that multi-GNSS receivers are nowadays able to use over 140 satellites of different navigation satellite systems. It should be noted, however, that a single satellite is capable of transmitting several signals on multiple frequencies at the same time, which considerably increases the number of pseudorange measurements that can be used by the receiver. This is why multi-GNSS receivers are now widely used in car navigation systems, other mobile applications, smartphones and sports [3; 4; 5; 6; 7; 8], as well as even including modern dual-frequency solutions [9; 10; 11; 12]. Their very rapid development, associated with increasing the positioning accuracy, particularly in urbanised areas, results primarily from the rapidly increasing number of satellites of systems under construction [13; 14; 15; 16; 17; 18]. The parameter informing about the spatial distribution of satellites in relation to the observer, on which the positioning accuracy using a NSS is dependent, is the Dilution Of Precision (DOP) which is the subject of this article [19; 20; 21]. Before discussing this parameter in detail, the factors affecting the positioning accuracy in GNSS systems will be presented. According to [22; 23; 24; 25], the positioning accuracy is determined by the value of the selected DOP and the User Equivalent Range Error (UERE), which comprises the User Range \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Type of System**} & \multicolumn{3}{c}{**Constellation Status**} & \multicolumn{3}{c}{**Type of Orbit**} & \multirow{2}{*}{**Attentions**} \\  & **Current Number of Operational Satellites** & & & & & & \\ \hline GPS & 34 & 30 & 34 & – & – & – & 3 SVs—in reserve \\  & & & & & & & 1 SV—testing \\ \hline GLONASS & 26 & 23 & 26 & – & – & – & 2 SVs—testing \\  & & & & & & 1 SV—in maintenance \\ \hline Galileo & 26 & 22 & 26 & – & – & – & 3 SVs—not usable \\  & & & & & & 1 SV—not available \\ \hline BDS & 48 & 42 & 28 & 8 & – & 12 & 6 SVs—testing \\ \hline IRNSS & 8 & 7 & – & 3 & – & 5 & – \\ \hline QZSS & 4 & 4 & – & 1 & 3 & – & – \\ \hline Total satellites & 146 & 128 & 114 & 12 & 3 & 17 & – \\ \hline \hline \end{tabular}
\end{table}