## 1 Introduction
### Energy Production and Accuracy Features of Dual Tracking PV Systems  
With the help of a solar tracking system, the orbit of the Sun can be monitored with sensors. An ideal solar tracker allows PV panels or complete systems to precisely follow the sun within an accuracy of less than 0.1 degree. This makes it possible to maximize the current energy production since sun tracking systems keep the PV modules perpendicular to the incoming solar radiation. [19; 22; 23]. In order to obtain maximum efficiency, three main factors are crucial:  
* an optimal configuration of the tracking axes [24; 25],
* an optimal configuration of the control systems [26],
* the optimization of the moving fixtures [27]. Currently, there are two different classes of solar tracking systems, whose characteristics have been examined by many manuscripts (Figure 2):  
Figure 1: Estimated global solar photovoltaic capacity, by country and region in 2018, based on [18]. * solar tracking systems with one axis (rotating around one axis) [28; 29; 30; 31; 32],
* solar tracking systems with two axes (rotating around two axes) [22; 33; 34; 35]. The efficiency of a solar tracking solution will be higher than that of a fixed PV system. Single- and dual-axis solar tracking PV systems can easily increase their energy produced by 12-42%, depending on the installation site and mounting type [22; 37; 38; 39]. Focusing on Europe, [PERSON] et al. (2015) collected many (\(>\)100 pieces) real, detailed energy production measurement data from single- and dual-axis PV plants all over Greece. They investigated the real (measured) data comparison between dual-axis and fix-mounted PV systems in 8 towns in Greece (Kilkis, Ilia, Serres, Trikala, Evros, Karditsa, Magnisisa, and Rodope). It can be stated that the percentage of the average difference between the average performances of dual-axis and fix-mounted PV systems was 34.8%. Results delivered by other similar researches in other geographic locations (Canada, Turkey, Spain) were close to the average of the figures obtained in Greece (Table 1) [19]. Several sources suggest that dual-axis tracking solutions are more efficient and can also be cost-effective if bigger systems are implemented. Nevertheless, the implementation of solar tracking devices is only justified if the amount of electrical energy produced compensates for the costs of the equipment, the maintenance and the energy consumed by the moving parts [23; 38; 40; 41; 42]. Regarding solar tracking technologies, the implementation of good tracking and control algorithms is also crucial. These systems are generally classified into open- and closed loop systems depending on the type of control strategy involved and the mode of signal operation. [23; 43; 44]. An important aspect is the performance and the error of solar tracking systems. [PERSON] et al. (2009) present the performance and error relationship of many precise solar tracking systems and algorithms. It is apparent from the research that there is typically a 0.0003\({}^{\circ}\)-1\({}^{\circ}\) tracking error tolerance, but nowadays the investment costs of such systems are high [45]. Nevertheless, there are solar tracking systems and sensors which are easily and cheaply available to the public but their tracking error tolerance is at least 1.5\({}^{\circ}\)[46]. According to [PERSON] et al. (2018) solar tracking system devices are exceptionally attractive and always yield more than 98.5% of their full tracking efficiency even when the aim is off by 10\({}^{\circ}\). Notwithstanding, it is necessary to minimize small tracking errors to ensure the maximum energy gain on a long term basis [23; 47]. An alternative option to control a two-axis PV tracking system without sensors is based on a mathematical sun tracking model for the specified geolocation (Zenith angle calculation). This model was presented in the study by [PERSON] et al., 2013, which could give the perfect position but did not consider the diffuse radiation [48]. Figure 2: Types of sun tracker solutions based on [36].