Xiaomi Mi8 with a Broadcom BCM47755 chip, an Android smartphone that supports multi-constellation (GPS, GLONASS, Galileo, BeiDou, and QZSS) and dual-frequency (L1/E1 and L5/E5), was launched in May 2018. Unlike previously released smartphones, it was technically expected to provide robust precise positioning with a fast ambiguity resolution, which led many researchers to be overly optimistic about the applicability of high-accuracy techniques such as real-time kinematic (RTK) systems and precise point positioning (PPP) of smartphones. The global navigation satellite system (GNSS) raw measurement quality of Android smartphones is, however, inherently far lower than that of general GNSS receivers due to their structure, which accordingly makes it difficult for them to be realized. Considering inherent limitations of smartphones such as low-quality antenna, frequent cycle slips, and the duty cycle, a practical strategy including L5 measurements, pseudo-range corrections for L5, and a weighting method is proposed in this paper. The results show that the proposed methods of L5 differential GNSS (DGNSS) and Doppler-based filtering can guarantee a positioning accuracy of 1.75 m horizontally and 4.56 m vertically in an Android device, which is comparable to the performance of commercial low-cost receivers.

This letter introduces a GNSS/altimeter tightly coupled (TC) integration algorithm that combines barometric altitude measurements with GNSS observations to determine 3-D positions. By transforming barometric altitude measurements into Earth-centered Earth-fixed (ECEF) coordinates, a novel TC integration equation is derived. This equation enables accurate positioning with only three GNSS satellites and one altitude measurement, overcoming the limitations of conventional GNSS positioning techniques. A semi-indoor experiment for various smartphones was conducted to validate the proposed method, comparing it against conventional positioning techniques and the National Marine Electronics Association (NMEA) location provided by smartphone chipsets. The results demonstrated that the GNSS/BARO TC integration significantly reduced errors and improved accuracy. Notably, the mean error of 4.74 m horizontally and 2.43 m vertically improved the NMEA results by 63.31% and 89.18%, respectively, even under challenging indoor conditions. The proposed method met FCC-required accuracy levels, making it suitable for enhancing the accuracy and availability of wireless 911 calls, aligning with indoor positioning mandates.