Session B3: Precise GNSS Positioning Applications

Cycle Slip Detection and Antenna Calibration in Various Attitudes for RTK Positioning on Smartphones
Hyunwoo Park, Bu-Gyeom Kim, Hojoon Jeong, Changdon Kee, Department of Aerospace Engineering and the Institute of Advanced Machines and Design, Seoul National University; Beomju Shin, College of Information Science, Hallym University
Location: Beacon B

Smartphones are an essential part of modern life, and many studies have been conducted to calculate more precise user positions using the GNSS chipset embedded in smartphones. However, in the past, Android smartphones did not provide GNSS raw data and only offered the position results calculated internally, making it difficult to perform precise positioning. Since 2016, Android smartphones have provided access to raw GNSS measurements, leading to a significant increase in research utilizing these raw measurements across a wide range of smartphone models. At the same time, Google, the developer of the Android operating system, released the GnssLogger app, facilitating the collection and analysis of GNSS raw data from smartphones. One of the most significant advancements enabled by the availability of GNSS raw data is the utilization of carrier phase measurements, which allow for centimeter-level precision, thereby enabling the implementation of precise positioning algorithms on smartphones.
However, several challenges exist in implementing precise positioning on smartphones using centimeter-level positioning algorithms based on carrier phase measurements. These limitations include hardware-related factors as well as issues with the measurements themselves, which are closely interconnected and act as obstacles to achieve precise positioning on smartphones.
Among these factors, the influence of the embedded GNSS antenna in smartphones is analyzed as the primary obstacle to achieve precise positioning on smartphones. First, smartphones are typically equipped with linearly polarized GNSS antennas, which are easier to install in size-constrained devices and suitable for wireless communication. On the other hand, GPS satellites transmit signals using right-hand circular polarization (RHCP) to ensure easy reception from various angles. Due to this difference in polarization, when GPS signals transmitted with RHCP are received by a smartphone’s linearly polarized antenna, signal loss and degradation in measurement quality inevitably occur. Previous studies have shown that this is closely related to the frequent occurrence of cycle slips in carrier phase measurements. If cycle slips caused by changes in integer ambiguity are not detected and recovered, centimeter-level positioning accuracy cannot be maintained. Therefore, an algorithm to resolve cycle slips is essential for precise smartphone positioning. Another issue caused by the antenna is the quality of GPS L5 frequency measurements. Although more smartphones now support L5 frequency, there are limitations in providing high-quality measurements suitable for precise positioning algorithms. As a result, current research on precise positioning using smartphones primarily focuses on applying algorithms that use single frequency measurements.
Secondly, to perform precise positioning, antenna calibration must be conducted using the radio frequency characteristics of the antenna, such as phase center offset (PCO) and phase center variation (PCV). For commercial GNSS antennas, PCO and PCV information is provided by the manufacturer, and organizations like the International GNSS Service (IGS) offer PCO and PCV data for various antennas, enabling antenna calibration. Among these, PCV information is crucial for resolving integer ambiguity. However, since smartphones do not provide PCV information for their GNSS antennas, resolving integer ambiguity becomes challenging, which in turn makes accurate positioning difficult. Additionally, standard GNSS antennas are typically installed in the zenith direction, and for reference stations, their fixed position and orientation make it easy to apply PCV data. In contrast, due to the constantly changing orientation of smartphones, an algorithm is needed to properly apply PCV data.
In previous research, our research group experimentally verified that centimeter-level precise positioning could be achieved by applying Real Time Kinematics (RTK) algorithm to carrier phase measurements collected through a smartphone GNSS chipset using an external GNSS antenna and an RF shield box. Additionally, antenna calibration was performed for the use of the smartphone’s embedded antenna. As a result, by utilizing the PCO/PCV information obtained through antenna calibration, we confirmed that RTK can be applied to a smartphone with a fixed orientation.
However, using an external GNSS antenna attached to a smartphone to solve antenna-related issues is not a common practice. Additionally, it is unnatural for users to maintain a single posture while holding and moving with their smartphones. For these reasons, this study aims to implement precise positioning on smartphones using only GPS L1 signal measurements without relying on an external antenna. The goal is to perform precise positioning on smartphones by applying methods that address the limitations of the embedded antenna. In particular, the goal of this study is to implement all algorithms in real-time within the smartphone itself, rather than transferring the collected measurements to a PC for processing.
In this study, the experiment was conducted using a Galaxy S22+ smartphone equipped with the Samsung Exynos 2200 processor. For GNSS measurements, raw data were collected using the GnssLogger software, and the analysis was performed using only GPS L1 measurements. Then, the smartphone’s internal IMU sensors were used to calculate the smartphone's orientation, which was applied to compute the elevation and azimuth angles of the satellites relative to the smartphone's body frame. Based on this, antenna calibration was performed using the PCV values estimated by our research group, improving the probability of resolving integer ambiguity. In addition, time-differenced carrier phase (TDCP) measurements were utilized to detect cycle slips in the carrier phase measurements. By combining TDCP and IMU data, we detected cycle slips and performed recovery, maintaining integer ambiguity in the smartphone’s measurements and enabling continuous precise positioning. Finally, RTK algorithm was implemented to allow dynamic users to perform real-time precise positioning. Network communication was used to receive corrections, and the user’s position was calculated within the smartphone. The goal of this study is to achieve real-time centimeter-level precise positioning on smartphones.
As a result of preliminary analysis, it was confirmed that performing antenna calibration on the smartphone increases the probability of resolving integer ambiguity. Additionally, through carrier-phase differential GPS (CDGPS) analysis, it was verified that even using only GPS L1 measurements from the smartphone, centimeter-level precise positioning can be achieved once the integer ambiguity is resolved. Moreover, it was demonstrated that GPS measurements generated by the smartphone can be processed in real-time within the device, allowing for real-time application of the RTK algorithm. Therefore, with the addition of a cycle slip detection algorithm using IMU data, real-time RTK positioning utilizing GPS L1 measurements from the smartphone is expected to be achievable.