###### Abstract
When using a ground plane to avoid reflections, higher accuracies are possible under overshadowing [70]. Moreover, an accurate determination of horizontally arranged smartphones can possibly reach other values [53; 71]. However, both use cases do not correspond with the practical application of smartphones in the forest, where a smartphone is usually held vertically by the user. Due to the many position-influencing variables under the forest canopy, the signals' correction plays a decisive role (differential global navigation satellite system: DGNSS), achieving an accurate position. However, the various correction signals are often not  
Figure 1: Available operational GNSS/RNSS satellites as a function of years and systems from 1978 to mid-2020 (data sources: [62; 63; 64; 65; 66; 67; 68; 69]). available over a wide area in many areas. The satellite-based augmentation systems (SBASs) are often blocked in forest conditions due to their primarily geostationary satellite altitude and are only available to a limited extent. The same applies to radio- or internet-based DGNSS systems (e.g., SAPOS and BEACON), whose geographical coverage varies greatly and is not comprehensive [16]. In 2018, a chipset from Broadcom BCM47755 that supports two frequencies each for three satellite systems (GPS L1 C/A, L5; QZSS L1, L5; Galileo E1, E5a) was launched; it can also be used on mobile phones [72]. The Xiaomi MI 8 smartphone was the first to use this new multi-frequency chipset, and it was launched in June 2018 [73]. The additional use of the E5a/L5 frequencies is intended to reduce the Multipath effect (i.e., the reflection of satellite signals from trees, etc.) and provides the opportunity to make ionospheric-free linear combinations between observations of two frequencies to eliminate the ionosphere effect, thus improving accuracy [49; 74; 75]. The reception quality of the Xiaomi Mi 8, as the first dual-frequency smartphone, is particularly often studied [76; 77; 78; 79; 44; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79]. Moreover, a second-generation dual-frequency GNSS chip, the BCM47765, was released by Broadcom in May 2020. It can track new L5 signals, the new BeiDou-3 constellation B2a signal, and signals from the NAVIC constellation. This will increase the availability of dual-frequency signals on three different GNSS constellations instead of two [80; 81; 82]. Gradually, other important manufacturers also offered dual-frequency chips, including Qualcomm with the Snapdragon X24 LTE modem and HiSilicon with the Kirin 980 system-on-a-chip with high energy efficiency and a form factor for intelligent data processing and FinFET transistor design [82]. Therefore, the number of multi-frequency GNSS smartphones is increasing. Currently, numerous dual-frequency GNSS smartphones are already available. A search resulted in 211 smartphones from 14 different manufacturers [83]. Development is progressing, and there are currently 24 types of smartphones on the market that support three frequencies with BeiDou. In addition, there is progress towards the use of even more free frequencies. There are already five smartphones from Huawei on the market that can use four BeiDou frequencies (B1I + B1B + B1C + B2a) and seven smartphones that can use three Galileo frequencies (E1 + E5a + E2b) [83] (as of 27.12.2021). Therefore, it can be assumed that smartphones' positioning accuracy in forests will continue to increase, and that there is a need for further research in this field. This research aims to determine the relatively new multi-frequency multi-constellation smartphones' absolute GNSS accuracy and compare it with single-frequency multi-constellation smartphones as well as a differential corrected multi-frequency multi-constellation geodetic GNSS receiver (code and phase measurement) under the forest canopy. The position is to be determined based on the position supplied to the user in real time by the Android Location API of the smartphone.