## 1 Introduction  
To navigate safely or carry out positioning-related surveys (hydrographic or land), it is advisable to be equipped with a positioning system that complies with all operational characteristics assigned to a specific navigation task at the same time. These include: accuracy, operating zone, availability, reliability, continuity, and integrity. These requirements are defined in the radionavigation plans [1; 2; 3; 4], other recommendations or standards related to a specific activity that require positioning, e.g., hydrography [5], railway engineering [6], aviation [7], and land [8] or marine navigation [9]. An analysis of these documents indicated that the number of navigation system applications is increasing each year; however, in terms of the numerical value, the minimum requirements for positioning accuracy and other characteristics assigned to specific applications have remained unchanged over the years. Contrary to the positioning accuracy requirements that have generally been constant over the years, radionavigation positioning systems (particularly GNSS (Global Navigation Satellite System)) continue to increase their positioning accuracy with the result that they can be used in a particular application for which they were not suitable only a few years ago due to the insufficient positional precision. For example, over the last several years, the GPS (Global Positioning System) has continually enhanced the positioning accuracy and other operational characteristics. In 1993, the accuracy of this system on the horizontal plane was 100 m (\(p\) = 0.95) [10]. In 2001, after selective availability (SA) was turned off, it increased to 13 m (\(p\) = 0.95) [11]. However, in 2008 it reached 9 m (\(p\) = 0.95) [12]. The increase in positioning accuracy has resulted in an increase in the number of applications for the system. For example, in 1993, the system still failed to meet the positioning requirements for car navigation due to the accuracy being too low (100 m, \(p\) = 0.95), while in 2001 it could be successfully used in this application, which required an accuracy of a few meters. A similar increase in positioning accuracy has been observed for all GPS augmentation systems [13; 14; 15; 16; 17]. However, the greatest increase in positioning accuracy has been noted over the last few years for a solution based on several GNSS systems. Multi-system, or multi-GNSS receivers, are currently widely used in smartphones, car navigation systems, sport, recreation, and other mobile applications [18; 19; 20; 21; 22; 23], including dual-frequency solutions [24; 25; 26]. Their very rapid development is associated with increasing the positioning accuracy, particularly in urban areas, which results from the rapidly increasing number of satellite systems under construction, i.e., BDS (BeiDou Navigation Satellite System) and Galileo. Hydrographic surveys are among the navigation applications where GNSS systems are commonly applied. In accordance with the IHO (International Hydrographic Organization) S-44 standard [5], they can be carried out in four orders: special, 1a, 1b, and 2. Each of them was assigned a number of requirements, of which two navigation parameters for positioning systems were primarily defined: maximum allowable positioning accuracy and its confidence level, i.e., the availability of a specific position error value, which is identical for all orders and amounts to 95%. The positioning accuracy and availability requirements imposed on hydrographic operations are among numerous regulations governing the use of positioning systems [27; 28; 29; 30]. Figure 1 shows a synthesis of the requirements imposed on (air, maritime, and land) transport applications as a function of positioning accuracy (the X-axis) and availability (the Y-axis). The diagram was prepared based on an analysis of the data included in global publications defining the requirements for the navigation process, which most frequently were radionavigation plans [1; 2; 3; 4].