Design Strategies for RTK Systems for Universal Application in Various Fields

Real-Time Kinematics (RTK) is a technology that provides high-accuracy coordinates from centimeters to millimeters employing differential techniques to 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. Previously , the high cost of equipment, ranging from thousands to tens of thousands of dollars, limited RTK technology primarily to specific fields such as land surveying and structure deformation monitoring where such investment could be justified. However, recent advancements and significant reductions in the cost of GNSS receivers and antenna, which are now capable of receiving multi-frequency signals, have enabled its widespread application across various industries. This shift has transformed RTK from a specialized tool into a universal applicable technology, enhancing its accessibility and utility in numerous applications. 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, most low-cost receivers do not have a function to correct antenna phase center (APC) errors at the reference station, requiring manual intervention, which can compromise position accuracy. 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 1Hz, requiring a bandwidth of approximately 4800 to 9600 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 innovative strategies to overcome these obstacles. First, a method is suggested for integrating the APC errors of the reference station antenna into the RTK reference station measurements to provide uniform correction performance, regardless of user expertise or receiver specifications. Second, we introduce a relative calibration method utilizing short-baseline double-difference method, simplifying the calibration process and making it more accessible for low-cost antenna users. 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.