Characteristics Analysis of Raw Multi-GNSS Measurement from Xiaomi Mi 8 and Positioning Performance Improvement with L5/E5 Frequency in an Urban Environment  
[PERSON]  
School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China; [PERSON]IL_ADDRESS] (L.G. ); [EMAIL_ADDRESS] (J.S. ); [EMAIL_ADDRESS] (X.L. ); [EMAIL_ADDRESS] (X.G. ); [EMAIL_ADDRESS] (W.Z.) [PERSON]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +86-027-6875-8529  
[PERSON]  
School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China; [PERSON]IL_ADDRESS] (L.G. ); [EMAIL_ADDRESS] (J.S. ); [EMAIL_ADDRESS] (X.L. ); [EMAIL_ADDRESS] (X.G. ); [EMAIL_ADDRESS] (W.Z.) image-based navigation, and a smartphone-based photogrammetric unmanned aerial vehicle (UAV) system [4; 5; 6]. All these applications inspire the development of accurate and continuous navigation solutions based on smartphones. Prior to 2016, the GNSS chipsets in smartphones only outputted the position-velocity-time and limited satellite information such as the elevation and azimuth [7]. The positioning accuracy only reached 2-3 m, and even degraded to 10 m or worse in the case of adverse multipath conditions [8]. The revolution started in May 2016 when Google announced that raw GNSS measurements will be available to the public through the Android Nougat operating system (Android N). Subsequently, many studies attempted to refine the smartphone raw measurement error model and develop advanced algorithms to improve positioning accuracy. In the beginning, mainstream portable smart devices were only embedded with L1/E1 single-frequency GNSS chipsets. The quality assessments of raw GNSS measurements from smartphones showed that the C/N0 of smartphones was about 10 dBHz lower than geodetic receivers [2], and the noise of the L1/E1 pseudorange observation was about 10 times larger than that of a geodetic-quality receiver [9]. The quality analysis for the carrier phase measurements demonstrated that the noise of carrier phase observation was about 0.1 cycle [9], which might have the potential to provide decimeter-level or better positioning accuracy [10]. As a response to these expectations, high-precision GNSS positioning attempts were conducted on Android N. For example, a series of conventional real-time kinematic (RTK) tests were conducted with static and dynamic raw measurements under open environments, and a decimeter-level positioning accuracy was achieved [9; 11; 12]. As for the positioning performance under complex dynamic environments, [PERSON] et al. adopted a time-differenced filtering to process the raw Global Positioning System (GPS) L1 measurements from a Nexus 9 tablet in the GNSS-degraded environment, and the resulting position trajectory fluctuated seriously and obvious discontinuities existed [2]. Clearly, due to low-cost single frequency GNSS chipsets and linearly polarized planar inverted F (PIF) antenna embedded in smartphones, the quality of the raw single frequency GNSS observations is usually poor [13; 14]. The navigation performance is thus unsatisfactory especially in GNSS-degraded environments. Innovatively, in 2018, the world's first dual-frequency GNSS smartphone, Xiaomi Mi 8, fitted with Broadcom's BCM47755 chipset, was released [15; 16]. The newly added L5/E5 signals with binary phase-shift keying (BPSK(10)) and alternative binary offset carrier (AltBOC(15,10)) modulation were superior, especially in cities, where they are less prone to distortions from multipath reflections than L1/E1 [17; 18]. The quality assessments for static Xiaomi Mi 8 GNSS measurements showed that characteristics of the C/N0 and carrier phase noise are similar to that of previous single frequency smartphones [19]. The L1/E1 pseudorange observation noise was about 4-7 m for most GNSS observations, but the noise of L5/E5 pseudorange observations was only about 1.5 m. Moreover, the L5/E5 signals have a lower pseudorange multipath error [20]. The better quality of the L5/E5 measurements may be a key to improving the navigation performance of smartphones.