## 1 Introduction  
The Global Navigation Satellite System (GNSS) chipsets built into the latest mobile phones, along with their miniaturization and reduced production costs, are nowadays a main aim of the electronic mass-market. Satellite positioning became a common practice in spreading of location-based applications dedicated for mobile devices, such as mobile phones, tablets or portable navigation systems. When the first Android Nougat 7 was introduced in May 2016, the GNSS observations could be processed by Android-dedicated applications and recorded into a file. Since then, the scientific community has been paying special attention to GNSS raw observations derived from smartphones. These observations include not only code pseudorange information but also carrier phase and Doppler GNSS measurements using Application Programming Interface (API). Such an interface defines interactions between multiple software intermediaries running on an Android operating system. Considering this solution, the stored GNSS observations are available now for analysis in post-processing. Today, for navigational purposes, the typical accuracy obtained by a smartphone device is about a few meters, and under difficult conditions, may reach up to ten meters. It is caused by pseudoranges which are mainly used for low-precision real-time positioning [1]. To obtain precise positioning results, carrier-phase measurements are commonly used. Geodetic receivers with applied advanced positioning algorithms using multi-frequency GNSS observations may easily provide centimeter and sub-centimeter positioning results in real time or in post-processing mode. Obtaining high positioning accuracies using GNSS technology can be achieved with the carrier phase observations and the capability to fix their ambiguities to the correct integer values. A short while ago, GNSS chipsets built into mobile phones provided mainly code and phase measurements only on the single frequency L1/E1. At the same time, the first post-processing algorithms were applied trying to obtain submeter positioning results with smartphones [2; 3; 4]. Developers started extracting raw measurements in a format which was best suited for post surveying processing and analysis. In the field of geodesy, there is a well-known interchange format for raw satellite navigation system data called RINEX (Receiver Independent Exchange Format). Considering current mobile applications such as RINEX ON or GEO++ RINEX Logger, there is an opportunity to collect GNSS data and conduct numerous research on post-processing positioning performances and signal analysis. The next significant technological breakthrough in smartphone positioning was in the year 2017, when the Broadcom company released the first dual-frequency GNSS chipset (BCM47755). Such a step was followed by other producers like Qualcomm or U-blox, who further presented chipsets (Snapdragon X24, Teseo, U-blox F9) using multi-constellation and multi-frequency technology [5; 6]. The first smartphone equipped with a dual-frequency GNSS chipset was the Xiaomi Mi 8, introduced in June 2018. This device was the first equipped with the Broadcom BCM47755 GNSS dual-frequency chipset, designed by the Broadcom Limited company. The ability to use additional observations on a second frequency can increase signal availability and enables a higher level of positioning performances and accuracy by simplifying ambiguity fixing using wide-open techniques and corrections of most error sources, introduced mostly by ionospheric refraction. This strictly depends on the processing methodology and the current state of the atmosphere. The way to solve the carrier phase integer ambiguity enables introducing RTK (Real-Time Kinematic) or PPP (Precise Point Positioning) algorithms directly in mobile phones and the design of the additional L5/E5 signals help to recognize original signals from the ones reflected by near objects (multipath effect).