# Real-time Precise Point Positioning with a Xiaomi MI 8 Android Smartphone
## 1 Introduction  
Smartphones are an indispensable tool in people's lives today, and the GNSS services provided by smartphones have greatly improved modern human life. In the development of GNSS navigation and positioning technology, accuracy has always been a key issue that restricts its further application to human production and life, and it is also suitable for the GNSS navigation and positioning on smartphones. In May 2016, Google provided an interface for accessing GNSS raw observations on the mobile devices with Android N operating system. Since then, researchers have begun to evaluate the quality of GNSS raw observations collected by mobile terminals and analyze the positioning performance. However, compared to the geodetic receivers, the assessment of smartphone GNSS raw observations and the analysis of smartphone positioning performance are still lacking. Soon after, Google [1] released an app named GnssLogger which can easily capture and record GNSS observations. The source code of this app was also made open to the public. This app can save GNSS raw observations as text files, which greatly facilitates scholars' research in this area all over the world. The next year, the European GNSS agency [2] released a white paper about using GNSS raw measurements on Android devices. This white paper not only shows the current research status in this respect, but also provides developers with detailed development instructions. Early GNSS positioning experiments on smart devices mostly use the Nexus 9 tablet. The experiment of [PERSON] et al. [3] is a pseudorange positioning test using the Nexus 9 tablet, and the result shows that the RMS positioning errors in horizontal and three-dimensional were 3.05 and 3.82 m. [PERSON] et al. [4] carried out a single-frequency carrier phase double-difference positioning experiment using the tablet and several base stations, and the positioning accuracy better than 20 cm was achieved within 20 minutes. [PERSON] et al. [5] also used a Nexus 9 tablet for GNSS raw observations quality analysis and positioning test. The results show that the carrier-to-noise density ratio (C/N0) value of GNSS raw observations collected by mobile devices is 10 dB-Hz lower than the representative values obtained from a geodetic-quality antenna and receiver. Adopt TD (time-differenced) filtering method, achieved static positioning with horizontal and vertical RMS positioning errors less than 0.6 and 1.4 m, respectively. These studies have important implications for subsequent experiments using smartphones, however, the positioning of ordinary smartphones is not comparable to this tablet. In September 2017, [PERSON] et al. [6] performed a pseudorange point positioning test under a good observation environment with a Huawei P10 smartphone, and the RMS positioning errors were about 10 m in the N and E directions and about 20 m in the U direction. In November 2017 [PERSON] et al. [7] conducted a NRKT (network real-time kinematic) positioning test using a Samsung S8+ smartphone and a Huawei P10 plus smartphone. The best positioning performance is the average horizontal positioning error of about 60 cm. In addition, [PERSON] et al. [8] used six Samsung GALAXY smartphones for maritime differential positioning test. The results show that the horizontal positioning accuracy below 10 m can be achieved, which meeting most of the maritime navigation accuracy requirements. This situation was improved to some extent in May 2018, when Xiaomi launched the world's first dual-frequency GNSS smartphone with the Broadcom BCM47755 chipset which is a dual-frequency (L1/E1+L5/E5) GNSS chip [9]. This experiment shows that the GNSS raw measurements quality of each frequency is better than ordinary single-frequency smartphones (which is reported in Section 5). The NSL's FLAMINGO (Nottingham Scientific Limited's fulfilling enhanced location accuracy in the mass-market through Initial Galileo Services) Team [10; 11] assessed the quality of the GNSS raw measurements from Xiaomi MI 8 smartphones in July 2018.