Session B3: Precise GNSS Positioning Applications

Design Strategies for RTK Systems for Universal Application in Various Fields
Yongrae Jo and Byungwoon Park, Sejong University
Location: Beacon B

Real-Time Kinematics (RTK) is a technology that provides high-accuracy coordinates at the centimeter to millimeter level based on differential techniques using GNSS carrier phase measurements. It has been widely used, particularly in the surveying field. Traditionally, the RTK method has been employed primarily in high-cost survey-grade GNSS receivers and antennas. However, the equipment costs, which range from thousands to tens of thousands of dollars, have restricted its widespread application across various fields. Recently, there has been an increase in attempts to overcome these limitations with the introduction of low-cost GNSS receivers and antennas capable of receiving multi-frequency signals. Nevertheless, several issues remain to be resolved in each component of the augmentation system -specifically the reference station, the user, and the communication channel for providing correction messages- for the universal use of RTK systems in various fields, even with the adoption of low-cost equipment.
First, there is the issue of Antenna Phase Center (APC) errors at the reference station. In an RTK system consisting of a reference station and a user, APC errors cannot be eliminated through the double-difference method, necessitating additional calibration processes. Failure to correct these errors may result in vertical positioning inaccuracies of several centimeters. While high-cost receivers used in the surveying field automatically correct for APC errors, low-cost receivers either do not do so or require the user to input commands manually; some devices lack APC error calibration functionality altogether. Consequently, the accuracy of RTK corrections becomes dependent on the performance of the user’s receiver and the user’s expertise.
Second, there is a lack of APC information for low-cost antennas at the user end. Institutions such as the International GNSS Service (IGS) provide calibration information for surveying antennas, but such information is often lacking or provided only in a rough manner for low-cost antennas. This can lead to inadequate correction of APC errors in user antennas, thereby degrading positioning accuracy.
Third, there is the issue of communication load for corrections. Existing RTK systems provide reference station measurements at a rate of 1 Hz, requiring a bandwidth of approximately 4800 to 9800 bps. This results in a significant communication cost burden for service providers supporting multiple users, posing constraints on the widespread use of RTK systems across various fields.
This study proposes several strategies to address these issues. First, a method is suggested for integrating the APC errors of the reference station antenna into the RTK base station measurements to provide uniform correction accuracy, regardless of user expertise or receiver performance. Second, a relative calibration method utilizing short-range double-difference techniques is introduced to model the APC errors of low-cost antennas. This method is simpler compared to absolute calibration techniques, which require additional equipment, and allows for reliable positioning performance with low-cost antennas. Third, a method is proposed to reduce communication costs while maintaining the accuracy of corrections through message scheduling.
The proposed methods aim to reduce the costs of RTK systems while maintaining performance, thereby facilitating the universal application of RTK systems across various fields.