# Precise Point Positioning Using Dual-Frequency GNSS Observations on Smartphone
## 1 Introduction  
Precise Point Positioning (PPP) is a method for obtaining the absolute position of a single GNSS receiver using carrier phase and pseudorange observations with high-precision IGS (International GNSS Service) products to achieve centimeter-level accuracy [1; 2]. To achieve this accuracy, a geodetic GNSS (Global Navigation Satellite System) receiver and antenna relatively expensive are usually required. Smartphone with positioning function is a kind of low-cost receiver widely used in transportation, agriculture, geological and environmental monitoring due to its convenience [3; 4; 5].  
RTKLIB is an open source software package for GNSS standard and precise positioning which supports multiple satellite systems, positioning modes and data formats [6; 7]. RTKLIB is widely used in navigation; for example, it can be used for automated machine guidance (AMG) and automated machine control (AMC) of mining vehicles [8], determination of position or trajectory of aircrafts [9], and navigation of marine vessels with IMU (Inertial measurement unit) [10]. RTKLIB's PPP module can be applied to produce daily solutions for GPS time series analysis [11], and to evaluate the seismic waveforms and the co-seismic displacements derived by an earthquake [12]. Secondary development based on RTKLIB, GAMP utilizes the PPP algorithm to process multi-GNSS undifferenced and uncombined observations [13].  
Previous Android smartphones merely supported the collection of single-frequency GNSS observations. Several studies had attempted to improve the accuracy of single-frequency smartphone by using different post-processing algorithms. The pseudorange double-difference algorithm can achieve the positioning accuracy within 5 m based on carrier phase smoothing of the single-frequency smartphone observations [14]. Using relative positioning method to process the data set in a fast static mode, the decimeter-level accuracy can be achieved without solving carrier phase ambiguity [15].  
The single-frequency uncombined PPP algorithm had an accuracy of 0.37 m for the horizontal component and 0.51 m for the vertical component [16]. The combination of pseudorange, carrier phase and doppler observations reached a positioning accuracy of 0.6 m in horizontal direction and 1.4 m in vertical direction, which were processed using the time series differential algorithm with the SNR-dependent weighting method [17].  
Currently, the application of the dual-frequency GNSS chips embedded in smartphones allows the smartphone to record dual-frequency GNSS observations, which can eliminate the first-order ionospheric delay through ionosphere-free combination. In this paper, a Xiaomi Mi 8 dual-frequency smartphone and a Hi-target geodetic receiver were applied to conduct experiment in static and kinematic modes, aiming at investigating the PPP accuracy of such dual-frequency smartphone. The dual-frequency observations of smartphone and receiver were processed by the GPS and Galileo dual-frequency PPP algorithm modified from RTKLIB [7] and GAMP [13], and single-frequency observations were directly processed by RTKLIB PPP-static mode.