Session B4: GNSS Integrity and Augmentation

Time-Differenced Carrier Phase Based Integrity Monitoring in Urban Environment
HoJoon Jeong, Changdon Kee, Department of Aerospace Engineering, and the Institute of Advanced Machines and Design, Seoul National University; Junesol Song Department of Mechanical Engineering, University of Suwon
Location: Beacon B

Precise positioning is becoming increasingly important in urban environments. Accurate and reliable positioning is especially important as transportation becomes more advanced, such as autonomous vehicles and Urban Air Mobility (UAM). However, in urban areas, it is difficult to utilize satellite-based navigation systems. This is because tall buildings and other obstacles often reduce the visibility of the GNSS satellites, and multipath error due to reflections of the signals from these obstacles is extremely high. In particular, errors due to multipath in pseudorange signals are significant, and various studies are underway to mitigate these errors in order to perform precise urban navigation. Recently, there has been a lot of research into UAM using aircraft as a means of transportation in urban areas. In order to operate UAM in urban areas, it is a prerequisite to satisfy not only high-precision navigation but also highly reliable navigation. Reliable positioning is critical when operating aircraft in densely populated urban centers, as it is directly related to the safety of pedestrians as well as UAM passengers. Reliable navigation can be achieved through an integrity-assured navigation system, and in the past, we proposed a Time Differenced Carrier Phase (TDCP) based receiver autonomous integrity monitoring (RAIM) method.
Carrier phase measurement has a lower noise level compared to pseudorange measurement, which makes it widely used in precise navigation and less susceptible to multipath error. However, in order to use carrier phase measurements for navigation, such as RTK or PPP, the unknown integer ambiguity included in the carrier phase measurements must be resolved. The unknown integer ambiguity, once determined, remains constant over time. TDCP uses time differencing to eliminate the unknown integer ambiguity, leveraging the property that integer ambiguities in carrier phase measurement remains constant over time. Therefore, there is no need to estimate the integer ambiguity to utilize TDCP measurement. The conventional method that uses time differencing of carrier phase measurements is RRAIM. RRAIM utilizes the delta between a fixed initial time and the current time for positioning and integrity monitoring. The algorithm we proposed in the past, called TDCP RAIM, uses the delta between consecutive epochs and accumulates them. Using carrier phase measurements with time differencing results in the calculation of a relative position, thus requiring an initial position assured integrity.
There are several advantages to applying TDCP RAIM to UAM landing in urban areas. First, it is possible to obtain an initial position with assured integrity by utilizing the position calculated using SBAS correction while in open airspace before landing. SBAS is a representative augmentation system that provides correction message, including integrity information, via geostationary satellites, targeting pseudorange measurement users. However, in urban areas, SBAS signal can be blocked by buildings, and the quality of pseudorange measurements can be significantly degraded, making SBAS performance unreliable. By using TDCP RAIM, the initial SBAS performance can be maintained for a short period while the UAM is landing in urban areas. The second advantage is that there are more available satellites in urban environments, where the number of visible satellites fluctuates, compared to conventional RRAIM methods. Conventional RRAIM method have a fixed initial time epoch, which means that temporarily lost tracking satellites or newly risen satellites during coasting cannot be used. This is because the initial epoch is fixed, so there are no satellite measurements that can be time differenced to eliminate the integer ambiguity, or the value of integer ambiguity changes. However, in the case of TDCP RAIM, the two consecutive epochs are time differenced so that even a newly rising or temporarily blocked satellite during coasting can be used as a measurement. If a cycle slip occurs in carrier phase measurements between two consecutive epochs, the integer ambiguity cannot be eliminated using the time differencing method, making cycle slip detection very important. Previous research has shown that due to the short time differencing interval of TDCP, it is possible to detect cycle slip in TDCP measurement using a low-cost MEMS-grade IMU. The third advantage is that continuous position results and protection levels can be obtained even when the number of visible satellites decreases. In RRAIM, the temporal and spatial decorrelation error between two measurements used for time differencing increases as the coasting time increases. As a result, when the number of visible satellites decreases, a position error proportional to the error contained in the removed measurements occurs, and there is a rapid increase in the protection level. However, since TDCP RAIM is based on two carrier phase measurement which are only 1 epoch apart, the impact of reduced measurements remains small, allowing for continuous positioning and protection level results.
In the existing TDCP RAIM study, a study was conducted with simulation data. Urban scenarios were simulated using 3D building models, and performance analysis was conducted based on simulation data. In this study, the performance of TDCP RAIM was analyzed using real-world data. We conducted test in an actual urban environment, and the performance of the proposed algorithm was analyzed based on data recorded in this urban environment.