In urban areas, signals can be blocked and reflected by buildings, reducing the reliability of global navigation satellite systems (GNSS). To address this, the zonotope shadow matching (ZSM) algorithm has been proposed to estimate the set-valued receiver position by calculating the GNSS shadow based on the zonotope. However, the existing study only analyzed the performance of ZSM in dense urban areas where GNSS shadows occur frequently, and the performance analysis in various urban environments was insufficient. Therefore, in this paper, we analyzed the performance of the ZSM algorithm in four urban environments with different characteristics. The results showed that the receiver position estimation performance of ZSM was relatively poor in environments where buildings were not densely populated, and the performance of ZSM was shown to be effective in urban environments with narrow roads and tall buildings.

Errors included in Global Navigation Satellite System (GNSS) measurements degrade the performance of user position estimation but can be mitigated by spatial correlation properties. Augmentation systems providing correction data can be broadly categorized into State Space Representation (SSR) and Observation Space Representation (OSR) methods. The satellite-based cm-level augmentation service based on the SSR broadcasts correction data via satellite signals, unlike the traditional Real-Time Kinematic (RTK) and Network RTK methods, which use OSR. To provide a large amount of correction data via the limited bandwidth of the satellite communication, efficient message structure design considering service area, correction generation, and broadcast intervals is necessary. For systematic message design, it is necessary to analyze the influence of error components included in GNSS measurements. In this study, errors in satellite orbits, satellite clocks for GPS, Galileo, BeiDou, and QZSS satellite constellations ionospheric and tropospheric delays over one year were analyzed, and their spatial decorrelations and linear modeling characteristics were examined.

An Attitude and Heading Reference System (AHRS) provides the attitude of a vehicle with a constant velocity using an Inertial Measurement Unit (IMU) and a magnetometer. In this case, in order to avoid the disadvantage of a gyroscope, an AHRS using a Gyro-Free IMU (GF-IMU) that is composed of only accelerometers may be considered instead of the gyroscopes. In this paper, a design method of an AHRS using a GF-IMU is proposed. The proposed AHRS consists of roll and pitch calculation, yaw calculation, angular acceleration and angular velocity calculation, attitude calculation, and a Kalman filter. In particular, since the angular velocity cannot be measured from a gyroscope, the angular acceleration is obtained from the accelerometer output, and the angular velocity is calculated by integrating it. In order to show the usefulness of the proposed method, a performance evaluation was carried out. The performance evaluation results show that the attitude estimation performance of the proposed AHRS is similar to that of the conventional AHRS.

Following the introduction of civilian navigation, the commercial Global Navigation Satellite System (GNSS) receivers' market has been expanding in various fields such as autonomous driving and smart cities. With improved receiver performance and widespread use of GNSS, the configurations of base and rover receivers are becoming more complex. As a result, user must consider combinations of base stations with different qualities, costs, and performances. To address these issues, we conducted zero-baseline tests to analyze the double-differenced code-pseudorange noise of various receiver combinations, ranging from low- to high-cost. The results showed that the noise varied depending on the receiver combination. Notably, receivers from the same manufacturer exhibited similar noise and positioning errors despite significant price differences. We also found that the double-differenced noise of all receiver combinations was correlated with the Carrier-to-Noise Density Ratio (C/N0), the satellite elevation angle, and the Doppler shift, and it did not perfectly follow a normal distribution. Further analysis based on Modified Allan Deviation (MDEV) showed that different types of noise were observed for each receiver combination and the double-differenced noise and positioning errors have similar statistical characteristics. From this study, the importance of receiver combinations and their various characteristics can be better understood.

The Korea Augmentation Satellite System (KASS) satellite was successfully launched and service is being started. By receiving messages transmitted from the KASS satellite, users can employ the messages to improve positioning accuracy or to verify the integrity of Global Positioning System (GPS). In this paper, we propose a test method for a beam-formed GPS receiver developed to improve the survivability of KASS augmented messages that can enhance positioning accuracy even in an environment with jamming or interfering signals. Through the test method proposed in this paper, quantitative verification is performed for a beam-formed GPS receiver aimed at maintaining the augmented navigation solution to which KASS augmented messages are applied by tracking the KASS signal as much as possible under conditions where the jamming signal strength is gradually increasing. In addition, the proposed test method includes three conditions; first, a static lab test method for repeated verification of functions under the same conditions; second, a static outdoor test method for performance verification in an operating environment of a platform equipped with the beam-formed GPS receiver; and finally, a dynamic lab test method for performance verification of a moving platform equipped with the beam-formed GPS receiver toward a jammer. In this paper, we propose a method for simulating the jamming signal incident direction through the phase delay of an RF cable designed to prevent unintentional jamming signal emission in both lab and outdoor tests, and a method of applying test software for injecting a jamming signal to compare the survivability performance consistently according to the presence or absence of beamforming signal processing. Through the proposed test method, it was verified that the augmented navigation solution could be output for a longer time period when the beamforming signal processing was applied to the KASS satellite signal in the beam-formed GPS receiver.

On May 11, 2024, there was a strong solar flare explosion. A powerful geomagnetic storm triggered by a solar flare caused a major ionospheric disturbance over the Korean Peninsula. When a geomagnetic storm occurred, an abnormal change in vertical total electron content (VTEC) values was detected at all Global Navigation Satellite System (GNSS) stations in the Korean Peninsula. In addition, we performed GNSS precise point positioning (PPP) processing using observations from the SBAO and MKPO stations. We found that the up-directional position error increased significantly in both stations at around 17:00 UT on the day of year (DOY) 132, 2024. At that point, the root mean square (RMS) values for all position errors (East, North, and Up) increased compared to other dates. Due to very high noise, the L1 signal-to-noise ratio (SNR) values of QZSS pseudo-random noise (PRN) 07 dropped to about 25 dB. As a result, we suggest that the strong geomagnetic storm increased the GNSS PPP position errors in the Korean Peninsula.

In satellite systems, efficient communication and observation require identifying of specific signal arrival points using onboard antenna systems. When utilizing massive array antennas to estimate the angle of arrival (AOA) of signals, traditional high-performance AOA estimation algorithms such as Multiple Signal Classification (MUSIC) encounter extremely high complexity due to the numerous individual antenna elements. Although, in order to improve this computational complexity problem, the cascade AOA estimation algorithm with CAPON and beamspace-MUSIC was recently proposed, the comparison of the computational complexity of the proposed algorithm across different massive array antenna configurations has not yet been conducted. In this paper, we provide the analyzed results of the computational complexity of the proposed cascade algorithm based on various massive array antennas, and determine an optimal antenna configuration for the efficient AOA estimation in satellite systems.

In the Pedestrian Dead Reckoning (PDR)/Global Positioning System (GPS)/Wi-Fi-integrated navigation system for indoor/outdoor continuous positioning of pedestrians, the process of detecting outliers in measurements is very important. When accurate location information from measurements is used, reliable correction data can be generated during the fusion filtering process. However, abnormal measurements may occur in certain situations, such as indoor/outdoor transitions, which can degrade filter performance and lead to significant errors in the estimated position. To address this issue, this paper proposes a method for detecting outliers in measurements based on Rényi Divergence (RD). When the deviation of the RD value is large, the measurements are considered outliers, and positioning is performed using only pure PDR. Based on experiments conducted with real data, it was confirmed that outliers were effectively detected for abnormal measurements, leading to an improvement in the performance of pedestrian navigation.

In this paper, we propose a round trip time (RTT)-enabled Doppler-based positioning method considering the low earth orbit (LEO) satellite visibility restriction. Doppler-based positioning typically requires visibility to at least eight satellites, which is often unfeasible due to the limited coverage of LEO satellites, as beamforming technique is applied to current LEO satellites. To solve this problem, we utilize the RTT measurements, assuming that a communication link exists between the user equipment (UE) and LEO satellites. We employ the Newton-Raphson method to estimate the UE position with RTT and Doppler measurements. We analyze the positioning performance of the considered framework via simulation, demonstrating its performance in 3D positioning errors under varying satellite numbers and measurement errors.

Satellite and receiver Inter-Frequency Biases (IFBs) should be estimated or calibrated by pre-defined values for generating precise navigation messages and augmentation data in satellite navigation systems or the augmentation system. In this paper, a Kalman filter is designed and implemented to estimate the ionospheric delay and satellite/receiver IFBs using a regional receiver network. First, an ionospheric model and its filter parameter is defined based on previous studies. Second, a measurement model for estimating the relative satellite/receiver IFBs without any constraints is proposed. Third, a procedure for ensuring the continuity of estimation is proposed in this paper. To verify the performance of the designed filter, six Continuously Operating Reference Stations (CORSs) are selected. Finally, the stability and accuracy of satellite/receiver IFB estimation are analyzed.

Various structures of navigation messages in Global Navigation Satellite System (GNSS) signals can be categorized into fixed, packetized, and mixed types, each with different broadcast intervals and sequences. In the fixed format, the broadcast intervals and sequences of navigation messages of Global Positioning System (GPS) L1 C/A and BDS B1I are strictly defined in the Interface Control Document (ICD). Navigation messages of GPS L2C, L5, Quasi-Zenith Satellite System (QZSS) L2C, L5, and BeiDou Navigation Satellite System (BDS) B1C are in a packetized format, and their broadcast sequence is not explicitly specified in the ICD; only the maximum broadcast intervals are described. In the mixed format, signals such as GPS L1C, QZSS L1 C/A, L1C, and BDS B2a exhibit both fixed and packetized patterns. Sequences of certain parts of the message are not defined. Therefore, in this study, experimental-based investigations were additionally conducted alongside an ICD investigation to analyze the current broadcast intervals and sequences of GNSS navigation messages.

Reliable and precise indoor localization is crucial for personal navigation, emergency rescue, and monitoring workers indoors. To use this technology in different applications, it is important to make it less dependent on infrastructure and to keep the error as small as possible. Fingerprinting stands out as a popular choice for indoor positioning because it leverages existing infrastructure and works with just a smartphone. However, its accuracy heavily relies on the quality of that infrastructure. For instance, having too few access points or beacons can greatly reduce its effectiveness. To reduce dependence on RF infrastructure, we have developed surface correlation (SC) using accumulated Received Signal Strength Indicator (RSSI) signals This approach constructs a user mask for radio map comparisons using an accumulated RSSI vector and the trajectory of the user, which is estimated through PDR. The location with the highest correlation is considered as the user's position after comparison. Through a simulation, the performance of short RSSI vector-based technology and SC is analyzed, and future directions for the development of SC are discussed.

This paper presents the design results of a Multi-band Global Positioning System (GPS)/Korean Positioning System (KPS) Real-Time Kinematics (RTK) precise positioning simulator and evaluate its functionalities. The designed simulator consists of a trajectory generation module, a Radio Frequency (RF) signal generation module, a RF signal reception module, a coarse positioning module, a precise positioning module, and an error statistics reporting module. Simulations in realistic scenarios confirm that the proposed baseband simulator works appropriately. The developed simulator can adjust the type, number, band, and Pseudo Random Noise (PRN) code type of the satellite constellation in various ways, and the practical positioning performance can be tested. It can also reflect the channel influence at the actual RF stage and the influence during the initial synchronization and tracking process. Considering such advantages, the proposed simulator would be useful in future researches and developments related to KPS.

Time synchronization is crucial for ensuring the reliable operation of modern economic and social infrastructures. Techniques such as Global Navigation Satellite System (GNSS)-based methods and Two-Way Satellite Time and Frequency Transfer (TWSTFT) play key roles in precise time comparison and synchronization. TWSTFT, in particular, is recognized for its ability to achieve sub-nanosecond accuracy in time transfer, making it indispensable in fields such as satellite navigation. This paper proposes a comprehensive performance evaluation method for TWSTFT modems, emphasizing pre-validation in controlled environments to mitigate operational challenges. Using the proposed evaluation method, the study presents the standard deviation of RTT according to C/N₀ and compares it with the datasheet of a commercial TWSTFT modem. Through this approach, the aim of this study is to enhance the reliability and accuracy of TWSTFT-based time synchronization across diverse applications.