## 4 Equipment and Surveying Methodology  
The static observation sessions with use of the Huawei P30 Pro smartphone were conducted on three different days at the same reference point located in the open field area at the University of Warmia and Mazury in Olsztyn. All surveying session data was collected in the end of January 2020 on three separate days, starting at the same local time. Although we presented the results of static GPS positioning based on three 1 h measurement sessions, we actually performed longer additional tests before, also focusing on short baseline solutions, where the accuracy results were very similar to the presented ones. Before we started the final tests, we tried out different mobile RINEX loggers to check the quality of data. Nowadays, the most popular available applications are GEO++ RINEX Logger (Version 2.1.6, Geo++ GmbH, Garbsen, Germany, 2020) and RINEX ON (Nottingham Scientific Ltd, Nottingham, United Kingdom, 2020). Since data collected by two of the same Huawei P30 pro devices using GEO++ RINEX logger was poor quality after data and signal analysis using Topcon Tools commercial software (Version 7.5, Topcon Corporation, Tokyo, Japan, 2009) (Figure 2), we decided to use the RINEX ON application for further GNSS raw data measurements. The Huawei P30 Pro smartphone has a possibility to collect data from GPS, GLONASS, BEIDOU and GALILEO positioning systems. However, due to the Topcon Tools v.7.5 software used, it was possible to process GPS plus GLONASS, and GPS-only observations. GPS plus GLONASS calculations were made, but no better results were obtained than with GPS-only. Therefore, we decided to present GPS-only positioning results. The latest commercial software packages enable static and kinematic data processing with the integration of all GNSS systems. Therefore, this topic will be the subject of further research by the authors. For comparison purposes, we firstly occupied a fixed control point in the research area with a GNSS Javad Alpha geodetic receiver (3 h session) to obtain accurate reference coordinates. Baseline post-processing was performed using Topcon Tools v.7.5 with reference to the local OPNT (Olsztynski Park Naukowo-Technologiczny) permanent GNSS station (ASG EUPOS), located 4 km away. The first surveying to assess the usefulness of the Huawei P30 Pro was conducted on 15 January 2020. Data was collected over 60 min and the mobile phone was mounted in vertical position and centered over a reference point (Figure 3). To obtain centimeter level of accuracy using carrier-phase observations, there is a need for precise information on average phase center position of internal antenna and possible phase center variations. Since the specification of the used smartphone does not include any information on GNSS antenna location, we did some research on the internet and contacted technical services and learnt that the antenna phase center is located at the top of device, in the line of the upper camera. This information was confirmed later in our research on phase center, which was determined by us very precisely (millimeter level of accuracy). We conducted three separate experiments (2 h sessions) for this purpose, where the mobile phone was placed on the base made of an aluminum beam with centrally positioned mandrel that allows for mounting it on the levelling head, which is centered over the reference point. At the same time, when the central device is positioned above the point being determined, the other external receivers may be set at both the edge sides at a constant distance of 0.50 m from the central one. The central Huawei P30 pro smartphone was fixed in three various positions on the aluminum base. Firstly, mounted vertically, then, lying along the beam (parallel) and for the last option--perpendicularly to it. The base was equipped with a professional sensitive compass allowing the positioning of the base along the north-south line.