## 1 Introduction  
At the May 2016 Google I/O developer conference, it was announced that general developers would be provided with the raw global navigation satellite system (GNSS) measurements of smartphones and tablets with Android N (\"Nougat\" = version 7) version operating system [1], which enables the raw GNSS observations of smartphone to output directly in REINX format with application (APP). This makes it extremely convenient to obtain the GNSS pseudorange, doppler, signal strength, and carrier phase measurements from an Android smartphone for postprocessing [2]. Moreover, in September 2017, Broadcom announced the world's first dual frequency GNSS chip GCM47755 [3], and other producers quickly followed suit with new dual frequency chips. Since then, the GNSS positioning method of smartphones has evolved from single frequency to dual frequency, which provides more ideas for smartphones to achieve high-precision positioning. In fact, prior to the Google I/O developer conference, [PERSON] et al. [4] had discussed the feasibility of centimeter-level positioning via the smartphone's antenna and GNSS chip. The feasibility study showed that the smartphone pseudorange measurements are of high quality and conform to expected conventions, but the carrier phase measurements suffer from five anomalies. One of the anomalies that cannot be remedied is an error in the phase measurement that appears to grow linearly with time. Nevertheless, the authors still believe that the smartphone appears fully capable of supporting centimeter-level carrier phase differential GNSS positioning. In recent years, as more smartphones are equipped with a dual frequency GNSS chip, in addition to differential positioning method [5; 6; 7; 8; 9] the precise point positioning (PPP) method is also used in GNSS high-precision positioning of smartphones [10; 11; 12]. PPP is a precision absolute point positioning technology based on state space domain correction information with international GNSS service (IGS) products [13; 14]. The GPS/Galileo pseudorange and carrier phase observations L1/L5 and E1/E5 of Xiaomi Mi 8 were used in the ionosphere-free combined PPP mathematical model [11], and the experimental results showed that the dual frequency GNSS smartphone is capable of achieving decimeter-level positioning accuracy. [PERSON] et al. [12] compared Xiaomi Mi 8 and geodetic receiver from the perspective of single frequency and dual frequency PPP models, the experimental results showed that the positioning accuracy of dual frequency PPP with ionosphere-free combination on Xiaomi Mi 8 is similar to the geodetic receivers in single frequency PPP mode. However, the scarcity of GPS satellites with L5 band and the poor geometric distribution of Galileo satellites in the Asia-Pacific region make it difficult to conduct dual frequency PPP using smartphone measurements. [PERSON] et al. [12] recorded that Xiaomi Mi 8 received more than four satellites with L5/E5 band for about 13 h in 24-h collection. This makes it difficult to meet the requirement of dual frequency PPP based on GPS/Galileo for a smartphone during the whole day. Therefore, in order to improve the applicability and stability of multi-GNSS PPP on a smartphone, we also need to focus on the quality of B1 and G1 band observations of BeiDou (BDS) and GLONASS. Moreover, some characteristics of GNSS signals can adversely affect positioning performance on a smartphone, which should be further discussed. To support low power consumption and thus prevent battery drainage, duty cycle power saving techniques are widely used in the smartphone. It intermittently puts the receiving device into a sleep state, especially in static mode the sleep period can be set to the maximum [15]. This function will seriously affect the reception of GNSS carrier phase observations. [PERSON] et al. [12] conducted the duty cycle experiment on the Xiaomi Mi 8, and the rate of cycle slip increased from about 20% to 70% within 30 min after turning on the duty cycle. [PERSON] et al.