###### Abstract
Several studies found that Multipath effects can account for about 50% of the horizontal position error observed [45; 46; 42]. The combination of multi-frequency reception and the relatively new possibility of reading out raw GNSS observation data from smartphones makes it possible to use different positioning variants. Thus, code- and carrier-phase observations can be analysed and thereby Multipath can be minimised. The positioning accuracy of a GNSS module on a smartphone was typically between 3 and 5 m under good Multipath conditions and over 10 m under harsh Multipath environments [47]. However, measurements of smartphone positioning detected rapid Multipath variations over time [48; 49]. Even when positioning was conducted using geodetic class receivers, considerable variability in positioning accuracies occurred due to the Multipath effect [50]. Additionally, the satellite constellations also do not behave in the same way. For example, Galileo measurements have a smaller Multipath error compared to GPS measurements [44]. Several studies have been carried out to investigate the positioning accuracy based on smartphone observations. The accuracies that can be achieved depend crucially on the positioning algorithm used, and results usually reach in the range of metres to decime-tres [51; 52; 53]. A study by [PERSON] et al. [54] in 2014 showed that cm solutions are possible in principle, using smartphone antennas, and that the high Multipath sensitivity of smartphones can be overcome. Studies in recent years have shown that centimetre accuracies using smartphones are achievable under good conditions (open sky) [55; 56; 57], even with the same (Xiaomi Mi 8) or comparable (Huawei P30) smartphones as used in this study [58; 59]. A decisive factor in reception quality and position accuracy under forest conditions is the number of available, tracked, and usable satellites. The assumption is that the more satellites in view/track, the better the position accuracy. However, the achievable accuracy also depends on many other factors, such as the distribution of the satellites, the evaluability, and the correction of the delay error variables [21; 22; 60; 61]. In the early research stages, only the American NAVSTAR-GPS (navigation signal timing and ranging--global positioning system) and/or the Russian system GLONASS (globalnaja nawigazionnaja sputnikowaja sistema) could be used. Most modern smartphones now support a multi-constellation of satellite systems and use a combination to determine their position; therefore, they can also use the European GNSS system GALILEO, the Chinese GNSS system BeiDou, the Indian RNSS (Regional Navigation Satellite System) system NavIC (Navigation with Indian Constellation), and the Japanese RNSS system QZSS (Quasi-Zenith Satellite System). Additionally, the number of satellites that can be used for positioning and the number of frequencies available from the satellites has increased significantly over the last few years, contributing to higher satellite availability, especially in the forest. Figure 1 shows the available operational GNSS/RNSS satellites as a function of years and systems for the years 1978 to 2020. The number of active satellites available for positioning has tripled since 2002 and has doubled since 2010, with 130 active by the end of 2020. This increase is mainly due to the implementation of the GALILEO and BeiDou systems. A total of 130 active potential satellites was available for positioning the receivers at the beginning of 2021. The current quantitative installation of satellite systems is thus largely complete [62; 63; 64; 65; 66; 67; 68; 69]. This high number is particularly noticeable under difficult conditions (e.g., under a forest canopy), as, in most cases, sufficient satellites can now be used in a favourable constellation and thus the DOP (dilution of precision) values are also very good. GNSS satellite signals may be reflected by the ground and thus influence position determination.