Jongsun Ahn

Weighted DOP With Consideration on Elevation-Dependent Range Errors of GNSS Satellites

This paper proposes the weighted dilution of precision (WDOP) with consideration of the satellite elevation angle in order to improve the performance of dilution of precision (DOP), which is a standard tool to quantify the positional precision of the Global Navigation Satellite System (GNSS). The WDOP is calculated by assigning different weights to visible GNSS satellites depending on their elevation angles. In order to demonstrate the effectiveness of WDOP, the conventional DOP and WDOP were mathematically analyzed and a comparative analysis was conducted using actual Global Positioning System data. Results showed that WDOP represents the position error trends more accurate than the conventional DOP, particularly when low-elevation measurements were used for positioning calculation. Therefore, the WDOP could be a promising replacement of DOP as a tool for representing and quantifying errors in GNSS positioning.

An efficient GPS parameter prediction method using GPS ephemeris patterns for Self-Assisted GPS

Ephemeris parameters in navigation message data which are updated every 2 hours has irregular patterns, But in this works, we discovered regular patterns in 24h interval some ephemeris parameters (Semi-Major Axis, Eccentricity, Mean Motion, Inclination Angle, Rate of Inclination Angle, Harmonic Perturbation Terms). In this paper, we evaluate the ephemeris parameter estimation algorithms for SA-GPS using 24h interval GPS ephemeris patterns. Using 24 hours ephemeris parameter regular patterns, we propose simple estimation methods (Polynomial method, Sine estimation method) for estimation of current ephemeris parameters.

Orbit Ephemeris Failure Detection in a GNSS Regional Application

To satisfy civil aviation requirements using the Global Navigation Satellite System (GNSS), it is important to guarantee system integrity. In this work, we propose a fault detection algorithm for GNSS ephemeris anomalies. The basic principle concerns baseline length estimation with GNSS measurements (pseudorange, broadcasted ephemerides). The estimated baseline length is subtracted from the true baseline length, computed using the exact surveyed ground antenna positions. If this subtracted value differs by more than a given threshold, this indicates that an ephemeris anomaly has been detected. This algorithm is suitable for detecting Type A ephemeris failure, and more advantageous for use with multiple stations with various long baseline vectors. The principles of the algorithm, sensitivity analysis, minimum detectable error (MDE), and protection level derivation are described and we verify the sensitivity analysis and algorithm availability based on real GPS data in Korea. Consequently, this algorithm is appropriate for GNSS regional implementation.

GPS Integrity Monitoring Method Using Auxiliary Nonlinear Filters with Log Likelihood Ratio Test Approach

Reliability is an essential factor in a navigation system. Therefore, an integrity monitoring system is considered one of the most important parts in an avionic navigation system. A fault due to systematic malfunctioning definitely requires integrity reinforcement through systematic analysis. In this paper, we propose a method to detect faults of the GPS signal by using a distributed nonlinear filter based probability test. In order to detect faults, consistency is examined through a likelihood ratio between the main and auxiliary particle filters (PFs). Specifically, the main PF which includes all the measurements and the auxiliary PFs which only do partial measurements are used in the process of consistency testing. Through GPS measurement and the application of the autonomous integrity monitoring system, the current study illustrates the performance of the proposed fault detection algorithm

GNSS Carrier Phase Anomaly Detection and Validation for Precise Land Vehicle Positioning

A carrier phase anomaly detection and validation method is proposed for precise vehicle positioning in dynamic environments. Given that carrier phase measurement is affected by satellite and user dynamics as well as unexpected anomalies, we propose four sequential processes to detect and identify an anomaly: 1) dynamics separation; 2) anomaly detection; 3) validation; and 4) position domain test. First, terms related to the satellite and user dynamics are estimated individually and removed from the carrier phase measurement to yield an error term, which possibly includes an anomaly and should be detected. The error term is examined with respect to a threshold level, and the measurement values are divided into anomaly candidates and normal candidates. Anomaly candidates are reexamined and validated one-by-one using a normal measurement set and the anomaly is determined. Finally, the position domain is evaluated so that position errors can be used as additional criteria for detecting the anomaly attributed to satellite deployment. The proposed algorithm is verified by simulation analyses and an experimental test. The proposed methods can be effective in detecting anomalies and increasing the reliability of precise vehicle positioning.

Performance Improvement of Inertial Navigation System by Using Magnetometer with Vehicle Dynamic Constraints

A navigation algorithm is proposed to increase the inertial navigation performance of a ground vehicle using magnetic measurements and dynamic constraints. The navigation solutions are estimated based on inertial measurements such as acceleration and angular velocity measurements. To improve the inertial navigation performance, a three-axis magnetometer is used to provide the heading angle, and nonholonomic constraints (NHCs) are introduced to increase the correlation between the velocity and the attitude equation. The NHCs provide a velocity feedback to the attitude, which makes the navigation solution more robust. Additionally, an acceleration-based roll and pitch estimation is applied to decrease the drift when the acceleration is within certain boundaries. The magnetometer and NHCs are combined with an extended Kalman filter. An experimental test was conducted to verify the proposed method, and a comprehensive analysis of the performance in terms of the position, velocity, and attitude showed that the navigation performance could be improved by using the magnetometer and NHCs. Moreover, the proposed method could improve the estimation performance for the position, velocity, and attitude without any additional hardware except an inertial sensor and magnetometer. Therefore, this method would be effective for ground vehicles, indoor navigation, mobile robots, vehicle navigation in urban canyons, or navigation in any global navigation satellite system-denied environment.

Research on Algorithm and Operation Boundary for Fault Detection of Onboard GNSS Receiver

In this paper, we proposed a algorithm and an operation boundary for fault detection of a onboard GNSS receiver. After aircraft exchange corrections computed by an aircraft receiver, a faulty aircraft receiver is detected by checking consistency of correction. For this purpose, PRC residual is used as the test statistic for fault detection of the onboard GNSS receiver. And operation boundaries are set by using DGPS position error increase with respect to the distance from a reference station. If the fault detection is performed by using aircraft only in operation boundary, the more accurate fault detection can be possible.

Performance Comparison of GPS Fault Detection and Isolation via Pseudorange Prediction Model based Test Statistics

Fault detection and isolation (FDI) algorithms provide fault monitoring methods in GPS measurement to isolate abnormal signals from the GPS satellites or the acquired signal in receiver. In order to monitor the occurred faults, FDI generates test statistics and decides the case that is beyond a designed threshold as a fault. For such problem of fault detection and isolation, this paper presents and evaluates position domain integrity monitoring methods by formulating various pseudorange prediction methods and investigating the resulting test statistics. In particular, precise measurements like carrier phase and Doppler rate are employed under the assumption of fault free carrier signal. The presented position domain algorithm contains the following process; first a common pseudorange prediction formula is defined with the proposed variations in pseudorange differential update. Next, a threshold computation is proposed with the test statistics distribution considering the elevation angle. Then, by examining the test statistics, fault detection and isolation is done for each satellite channel. To verify the performance, simulations using the presented fault detection methods are done for an ideal and real fault case, respectively.

Analysis of GBAS Availability and Requirement with respected to Protection Level at Jeju International Airport

ABSTRACT This paper presents the simulation results of GBAS availability and requirement (with respected to Vertical Protection Level) using simulated data a t CAT I, CAT II/III DH point (Decision Height), which are generated using Jeju international GNSS reference position, aircraft horizontal velocity and reference/aircraft GNSS antenna performance index and so on. Two kinds of protection levels are presented, one is from a null hypothesis (H0) and other is from a alternative hypothesis (H1). These protection l evels are compared with AL (Alert Limit), and we analyse the GBAS availability and require ment for CAT I and CAT II/III at the airport. 초 록 본 논문에서는 제주 국제 공항에서 실시된 항공우주연구원의 비행 실험 데이터를 기반으로 항공기 착륙 상황을 시뮬레이션하고, Protection Level 관점에서 제주 국제 공항 GBAS 시스템의 가용성 및 성능 향상을 위한 요구조건을 분석하였다. 요구조건을 분석하기 위해, CAT I, CAT II/III의 결심고도에서 항공기 착륙 상황 (기준국 정밀좌표, 항공기 착륙 속도, 기준국 및 항공기 안테나 성능지표 등)을 Protection Level 계산에 적용하였다. 이 때 귀무가설 / 대립가설 (H0 / H1) Protection Level을 한계치 (Alert Limit)와 비교 분석하여, 현재 CAT I, CAT II/III의 가용도 및 개선 방향을 도출하였다.Key Words : Ground Based Augmentation System(지역기반 위성항법 보강시스템), Decision Height(결심고도), Protection Level(보호수준), Jeju International Airport(제주국제공항)

The Abnormal Increasing Pseudorange Satellite Detection Method Using Comparison of Residual Horizontal Projection

This paper can be used for GPS air navigation study on integrity monitoring algorithm as, the projected horizontal plane using GPS pseudorange residuals for fault detection satellites were suspected. Failure to remove the detected suspicious satellite, compare with threshold which is calculated using satellite deployment (PDOP) and determine whether the failure is presented. The theory that horizontal projection of the satellite failure residuals greater than residual of normal satellite is proved mathematically. Comparison with horizontal projection residuals are likely to malfunction in the satellite were presented. To evaulate the proposed algorithm, bias fault insert into GPS pseudorange, and compare with conventional parity space method about fault detection and isolation capability.