# Real-time Precise Point Positioning with a Xiaomi MI 8 Android Smartphone
## 1 Introduction
They considered that the carrier phase was not affected by duty cycling, and the raw observations quality of L5/E5 frequency are better than L1/E1 frequency. Subsequently, the same team conducted further dynamic positioning experiments on Xiaomi 8 smartphones. The horizontal RMS positioning errors of 1.17 and 2.23 m was achieved using the RTK (real-time kinematic) and precise point positioning (PPP) methods, respectively. [PERSON] et al. [12] also performed a carrier phase differential positioning test on a Xiaomi MI 8 smartphone (using single frequency data) and achieved horizontal RMS positioning errors of 1.02 and 1.95 m at low and high multipath sites. In December 2018, [PERSON] et al. [13] collected a whole day's observations with a Xiaomi MI 8 smartphone and conducted the PPP positioning test. The experiments were performed using dual-frequency data (ionosphere-free combination of code and carrier phase) and single-frequency data. The dual-frequency model RMS positioning errors of E, N, and U directions are 21.8, 4.1, and 11.0 cm, respectively, but the result shows that the time for three-dimensional positioning errors to converge to 1 m is as long as 102 min. For single-frequency PPP, the time required for positioning errors in N and E directions to be under 1 m is more than 100 min. At the same time, the orbit and clock files used in this experiment are WHU (Wuhan University) final product, which means that the positioning result cannot represent the real-time PPP positioning performance. According to this published research, before the advent of the Xiaomi MI 8 smartphone, the accuracy of mobile phones positioning was on the level of several meters [6; 7; 8]. After Xiaomi MI 8 entered the market, its excellent GNSS raw observations quality quickly gained people's attention, many scholars used it to conduct positioning tests. Research shows that the RTK and PPP positioning methods can effectively improve its positioning performance. Using a static RTK method, it is possible to obtain a horizontal positioning accuracy of approximately 1 m [11; 12]. The experiment by [PERSON] et al. shows that using the dual-frequency PPP method, after a long-time measurement, the positioning accuracy of the horizontal RMS less than 30 cm can be achieved [13]. The aim of their work is similar to ours: to obtain higher-precision positioning results on mobile phone without reference stations. But their study has some drawbacks: using the final orbit and clock errors products and cannot obtain high-precision positioning results in short time. This has resulted in their method not being able to be performed on the phone in real-time, which is obviously inconsistent with most mobile application scenarios. In addition, no one has made improvements in positioning strategy for mobile phones. Our experiment found that the differences between pseudorange observations and carrier phase observations of smartphones are not fixed (this phenomenon exists in all the three smartphones tested). We believe that it is necessary to modify the positioning strategy accordingly, and the experimental results verify our thought. In this work, we use a single-frequency PPP strategy that estimates two clock biases of smartphone, a real-time high precision smartphone positioning is achieved. It should be noted that the definition of PPP (Precise Point Positioning) is a method for obtaining the absolute position of a single GNSS receiver using pseudorange, carrier phase observations, and high-precision IGS (International GNSS Service) products [14]. The accuracy of PPP should be centimeter-level, which is hard to achieve on a mobile phone until now. Because the method we used is same with PPP method, we also call it precise point positioning. The experimental setup adopted is described in Section 2. The preliminary analysis helps us determine the specific positioning methodology are reported in Section 3. The PPP methodology followed to conduct our experiment are elaborated in Section 4. The experimental results are reported in Section 5.