With recent advances in location-based services, satellite navigation systems that guarantee high precision and high integrity have become increasingly important. The Japanese Quasi-Zenith Satellite System (QZSS) provides the Centimeter-Level Augmentation Service (CLAS) via the L6 signal. However, unlike standard QZSS navigation messages such as LNAV and CNAV, CLAS augmentation messages currently lack an authentication mechanism and are consequently vulnerable to spoofing attacks. Furthermore, since CLAS is already an operational service, incorporating authentication by modifying the existing message specification is not possible. To address this issue, in 2024 Jeon et al. proposed an authentication method that uses the reserved field in the navigation messages of the L62 signal. However, this method requires significant Time To Authentication (TTA) to deal with multiple fragments of an Elliptic Curve Digital Signature Algorithm (ECDSA) signature transmitted over multiple frames. In this paper, we propose a novel authentication protocol to mitigate this issue. The proposed protocol exploits the property of the ECDSA signature pair (r, s), where the r component can be generated independently of the message content. By transmitting r concurrently with the data, the receiver can initiate signature verification earlier. Analytical results demonstrate that the proposed protocol reduces TTA by up to 25% compared to the previous method. Experimental results on both a PC and a Raspberry Pi confirm that the computational overhead of the proposed scheme is negligible.

This paper proposes a recursive batch positioning algorithm to provide efficient integer ambiguity resolution and precise positioning for real-time Precise Point Positioning-Real Time Kinematic (PPP-RTK) receivers. The performance of the algorithm is validated using real measurement data. Because PPP-RTK measurements generally contain larger unmodeled errors than RTK measurements, an epoch-by-epoch approach often yields degraded integer ambiguity resolution performance. To address this issue, a batch method improves integer ambiguity resolution by using all measurements accumulated up to the current epoch. For real-time receiver implementation, the batch method is reformulated within a recursive framework. Experimental results using dual-frequency Global Positioning System (GPS) and Galileo measurements under a Single-Difference (SD) model show that ambiguities were resolved from the first epoch. Even with single-frequency data, ambiguities were successfully resolved within two minutes.

Low Earth Orbit Positioning, Navigation, and Timing (LEO-PNT) systems offer higher signal strength and faster convergence time over Medium Earth Orbit (MEO) Global Navigation Satellite System (GNSS). However, uncertainties in orbit determination persist due to orbital perturbations. Conventional Receiver Autonomous Integrity Monitoring (RAIM)-based Fault Detection and Exclusion (FDE) approaches suffer from a structural limitation where faulty satellite exclusion reduces navigation availability. Furthermore, in severely occluded environments such as urban canyons, fault-induced Geometric Dilution of Precision (GDOP) spikes can effectively suspend navigation services. This makes it difficult to meet the operational requirements of high-reliability applications such as autonomous driving and Urban Air Mobility (UAM). To overcome these limitations, this paper presents a conceptual study on ground station Phased Array Antenna (PAA)-based active fault remediation to enhance the availability and resilience of LEO-PNT systems. The proposed architecture consists of four core functions: spatiotemporal consistency assurance via digital beamforming-based PAA, real-time bias correction and transfer based on the nominal satellite constellation, precise state estimation combining an Extended Kalman Filter (EKF) with an orbital dynamics model and warm start mechanism, and an active feedback loop with correction data uplink within one second. Through a worst-case geometric scenario analysis, the proposed architecture confirms the feasibility of maintaining satellite geometry and enhancing positioning precision through resilient recovery, without excluding faulty satellites, even in critical environments where conventional FDE approaches render the service unavailable. This study proposes a conceptual design for ground station-centric active integrity management, and a quantitative performance validation is planned in subsequent research.

Global Positioning System (GPS) L1 C/A is a Gold code-based Code Division Multiple Access (CDMA) signal. Although the autocorrelation of the same Pseudo-Random Noise (PRN) code is symmetric, the cross-correlation between different PRN codes is nonzero, which results in Multiple Access Interference (MAI). When multiple satellite signals are simultaneously present, this interference can distort the code phase discriminator of the code tracking loop. In this study, we consider a scenario in which multiple GPS L1 C/A signals with equal power coexist, rather than the conventional near-far condition. In particular, we focus on Geostationary Earth Orbit (GEO) satellite environments characterized by low relative Doppler and quasi-static relative code phase/delay, where the time-averaging effect of cross-correlation terms may be limited. Based on this assumption, (1) the mechanism by which cross-correlation induces asymmetry in the Delay Lock Loop (DLL) discriminator is analyzed mathematically, (2) a first-order approximation of the resulting zero-crossing shift, corresponding to a steady-state code tracking bias, is derived, and (3) MATLAB simulations are performed for all combinations of GPS L1 C/A PRN 1 to 32 to evaluate correlation characteristics and tracking distortion metrics.

Reliable symbol demodulation is essential in Eurofix-based eLoran Loran Data Channel (LDC) reception because Reed-Solomon coding RS(30,10) can tolerate only a limited number of symbol errors. However, in noisy environments, pulse position estimation errors can lead to invalid modulation pattern decisions and consequently increase the symbol error rate before RS decoding. To address this problem, this paper proposes a front-end demodulation algorithm that combines Correlation Difference Correction (CDC) and Minimum Distance Decision (MDD). The proposed algorithm reconstructs candidate pulse patterns using correlation differences and, when necessary, determines the final symbol by minimumdistance matching to valid Eurofix modulation patterns. Simulation results in an AWGN channel show that the proposed algorithm outperforms the baseline demodulator in Pulse Error Rate (PER), BER, and SER, providing approximately 4 dB gain at BER = 10^-3 and about 1 dB improvement at the RS(30,10) correction threshold of SER = 1/3. Under skywave interference (SWI) conditions, the proposed algorithm reaches SER ≤ 1/3 at SNR ≈ +12 dB, achieving at least 4 dB SNR gain over the baseline demodulator, which fails to reach the threshold within the evaluated SNR range. The proposed method therefore improves symbol decision reliability and increases the likelihood of successful Eurofix LDC message decoding.

Regional Navigation Satellite Systems (RNSSs) employing Inclined Geosynchronous Orbit (IGSO) and Geostationary Orbit (GEO) satellites exhibit orbit and clock estimation behavior distinct from that of conventional Medium Earth Orbit (MEO)-based systems, owing to their limited ground geometry and unique orbital features. Accordingly, various orbit and clock determination strategies have been extensively investigated in such environments. In this study, an orbit and clock estimation strategy based on an Extended Kalman Filter (EKF) using smoothed code pseudorange measurements is proposed. A self-developed algorithm is employed to evaluate the orbit and clock estimation performance of the proposed strategy. The estimation filter, along with the associated dynamic and observation models, is described, with particular emphasis on a solar radiation pressure model suitable for IGSO and GEO satellites. The orbits and clocks of Quazi-Zenith Satellite System (QZSS) IGSO and GEO satellites are estimated, and the performance is evaluated under QZSS-only scenarios with different numbers of satellites.. The experimental results show that, when four QZSS satellites are used, the average 1D orbit RMS and clock RMS values are 4.85 m and 1.74 m, respectively. As the number of satellites decreases, the performance degrades to 5.62 m / 2.02 m (3 satellites), 7.08 m / 2.00 m (2 satellites), and 21.12 m / 12.23 m (1 satellite). The primary cause of this degradation is reduced observability due to the limited number of observations. In particular, in the single-satellite case, observability is significantly degraded, resulting in substantially poorer orbit and clock estimation performance compared to the other cases.

This study investigates the influence of GLONASS receiver Inter-Frequency Bias (IFB) on the estimation of the ionospheric Total Electron Content (TEC). To estimate and validate GLONASS IFB, we utilized GNSS observations obtained from a dense regional network of 19 stations operated by the National Geographic Information Institute (NGII) for the period of DOY 32-36, 2026. Our results reveal that the Vertical TEC (VTEC) values exhibit significant fluctuations, ranging from approximately -60 to 80 TEC units (TECU), when the GLONASS IFB corrections are not applied. On the other hand, after the IFB mitigation, the VTEC variations became remarkably consistent at all stations. The accuracy of the derived GLONASS VTEC is assessed through a comparative analysis with the CODE Global Ionospheric Map (GIM). The results exhibit a remarkable correlation between the two datasets, with a mean bias of 0.04 TECU and a root-mean-square (RMS) value of 4.37 TECU over the five days. The findings of this study suggests that the proposed method can accurately determine receiver-specific IFBs and improve local ionospheric TEC monitoring.

In order to estimate the six-degree-of-freedom (6-DOF) motion using GF-IMU and GR-IMU, the relationship between accelerometer outputs and rotational motion variables is expressed in terms of differential and algebraic equations in this paper. Based on this relationship, the required number of accelerometers for estimating rotational motion and their placement conditions are presented. Examples of accelerometer placement also are provided. It is shown that the obtainable differential and algebraic equations and estimated variables related to the rotational motion vary depending on the accelerometer placement. In particular, it is shown that the angular velocity can be estimated using a Kalman filter when differential equations and algebraic equations are available simultaneously. Furthermore, the limitation of Hanson's performance index, which does not account for quadratic angular velocity terms, should be compensated. To address this issue, new extended performance indices incorporating quadratic angular velocity terms are proposed. Using these indices, the 6-DOF motion estimation performance obtained with various accelerometer placements is quantitatively evaluated through simulations. The results demonstrate that the proposed indices effectively reflect both error amplification and numerical stability and thereby enable clear discrimination of performance differences that cannot otherwise be identified.

This study analyzes the effect of pre-outage maneuver strategies on Inertial Navigation System (INS) / Doppler Velocity Log (DVL) integrated navigation performance under DVL-loss conditions. Maneuvering motion can improve the excitation of navigation error states and parameter estimation performance in integrated navigation systems. However, improved parameter estimation does not necessarily translate into enhanced navigation accuracy during the subsequent dead-reckoning phase under realistic underwater operating conditions. Three representative maneuver patterns, namely straight, zigzag, and single-turn trajectories, are comparatively evaluated under identical operating conditions through Monte Carlo simulations using a horizontal-plane 3-DOF INS/DVL framework that estimates DVL yaw misalignment and yaw-rate gyro bias. A phasewise evaluation framework is adopted to separately quantify state estimation gains during the maneuver phase and error propagation behavior during the DVL-outage phase. The results show that maneuvering patterns with heading changes improve DVL yaw misalignment estimation, while they can also increase maneuver-induced position-error cost. In particular, the single-turn maneuver shows DVL yaw misalignment estimation improvement comparable to the zigzag maneuver, but its post-outage navigation benefit remains limited. These findings indicate that pre-outage maneuver strategies should not be evaluated solely by estimation-error reduction. Instead, maneuver strategy design for INS/DVL navigation under DVL-loss conditions requires a cost-benefit perspective that jointly considers estimation improvement, maneuver-induced positionerror cost, and post-outage navigation benefit.

A wide-area differential regional navigation satellite system (WAD-RNSS) integrates wide-area differential navigation technology with a regional navigation satellite system (RNSS) to provide navigation and correction signals as a unified service. This paper presents the modeling and design of system- and device-level operation state and mode for the operation of the WAD-RNSS central processing facility (CPF) using a finite state machine (FSM). The proposed FSM-based approach focuses on operational modeling of the system. The operation mode and operation state of the CPF were identified and designed for system operations.

Information related to the development of military Global Positioning System (GPS) receivers is largely restricted for security reasons. For this reason it is difficult to systematically analyze technological evolution and acquisition policy changes. This study systematically investigates the development trends of U.S. military GPS M-code receivers by analyzing publicly available policy documents, technical materials, and GPS modernization reports published by the U.S. Government Accountability Office (GAO) from 2021 to 2024. In particular, receiver systems based on P(Y)-code and M-code are compared not only from a technical perspective but also in terms of acquisition strategy and program structures. The results show that the evolution of military GPS receivers is not merely a transition in signal technology but also is closely associated with a broader shift from a security-controlled Precise Positioning Service (PPS) framework to a platform-integrated and modernization-oriented Military GPS User Equipment (MGUE) framework. Furthermore, key issues identified in the MGUE program, including insufficient technology maturity, semiconductor supply chain challenges, and limitations in integrated program management, are analyzed and corresponding lessons learned are derived. This study contributes to the field of military GPS receivers by providing a comprehensive analysis of military GPS receiver development trends based on open-source data and by integrating both technical and policy perspectives.