# RTK with the Assistance of an IMU-Based Pedestrian Navigation Algorithm for Smartphones
## 1 Introduction
Carrier phase collected during several minutes after a cold start of a Huawei P10 was used to provide precise position. Results showed the smartphone could reach decimeter accuracy in static conditions and sub-meter when used in urban vehicle scenarios [18]. The Nexus 9 tablet with BCM4752 was one of few devices that had the duty cycling feature disabled before May 2018. Tests using the carrier phase collected by the Nexus 9 tablet for precise positioning proved that not integer phase ambiguities, but float solutions could be estimated. It also proved that precise positioning was feasible with a moderate level of multipath [20]. There were only chipsets supporting the single frequency of each constellation such as BCM4774 and BCM4752 mentioned above. These chipsets differed in supporting different kinds of constellations. A new revolution happened in May 2018 as the first dual-frequency GNSS smartphone produced by Xiaomi was launched. The Xiaomi Mi 8 could collect dual-frequency (L1/E1 + L5/E5a) raw GNSS measurements with the BCM47755 chipset. Meanwhile, Google introduced the Android P operating system that could disable duty cycling with the developer option \"Force full GNSS measurements\" [21]. These two changes made precise positioning on smartphones much more promising. [PERSON], the Principal Engineer at Google, pointed out that some phones disabled the duty cycling automatically when users requested raw measurements [22]. The NSL's FLAMINGO (Nottingham Scientific Limited's fulfilling enhanced location accuracy in the mass-market through Initial Galileo Services) Team found that the carrier phase collected by the Xiaomi Mi 8 was not affected by the duty cycling [23; 24]. Static differential positioning using the carrier phase of GPS and Galileo (L1/E1 + L5/E5a) on a very short baseline provided cm-level precision in the horizontal component and decimeter-level in the vertical component [19]. Observations and positioning performance were analyzed in different multipath conditions, indicating that the Xiaomi MI 8 was very promising to provide sufficiently accurate positioning results in urban areas [25]. With dual-frequency raw measurements, the Xiaomi Mi 8 could use the combination of L1 bands and L5 bands to eliminate most ionosphere errors, which helped obtain more accurate positioning results than smartphones like the Samsung S8 utilizing single-frequency measurements [26]. The carrier phase measured by the Xiaomi Mi 8 could also be used for precise point positioning (PPP) [27]. Users of smartphones are often pedestrians in urban areas. They are more likely to walk through an urban canyon, which is a narrow street with very tall buildings on both sides. They also often walk under overpasses. These overpasses and obscurations can cause the receiver to lose track of the signals, therefore leading to the loss of the carrier phase for many seconds [28]. During that time, RTK cannot provide valid positioning results. The outage of signals hinders all kinds of GNSS positioning methodologies. One common way to improve this is to integrate GNSS with the inertial navigation system (INS). INS is based on an IMU, which consists of gyroscopes and accelerometers, enabling computing velocity, attitude, and position. A hybrid IMU containing magnetometers can also be called a magnetic, angular rate, and gravity (MARG) sensor, which gives more accurate orientation [29]. Assisting GNSS with INS can reduce acquisition time and improve immunity to noise and interference [30]. When it comes to RTK, ambiguity resolution and cycle slip detection benefit from INS aiding [31; 32]. Single-frequency multi-constellation RTK tightly coupled with INS performs better than dual-frequency RTK without INS in urban areas [33]. We can couple RTK with INS to assist ambiguity resolution and resist noise, but no position output can be gained frequently in GNSS-challenged environments and GNSS-denied environments. The positioning results are provided by the INS during the GNSS outage [34].