Jeongho Cho

Improving prediction accuracy of GPS satellite clocks with periodic variation behaviour

The broadcast ephemeris and IGS ultra-rapid predicted (IGU-P) products are primarily available for use in real-time GPS applications. The IGU orbit precision has been remarkably improved since late 2007, but its clock products have not shown acceptably high-quality prediction performance. One reason for this fact is that satellite atomic clocks in space can be easily influenced by various factors such as temperature and environment and this leads to complicated aspects like periodic variations, which are not sufficiently described by conventional models. A more reliable prediction model is thus proposed in this paper in order to be utilized particularly in describing the periodic variation behaviour satisfactorily. The proposed prediction model for satellite clocks adds cyclic terms to overcome the periodic effects and adopts delay coordinate embedding, which offers the possibility of accessing linear or nonlinear coupling characteristics like satellite behaviour. The simulation results have shown that the proposed prediction model outperforms the IGU-P solutions at least on a daily basis.

Automated determination of fault detection thresholds for integrity monitoring algorithms of GNSS augmentation systems

The reference stations of GNSS augmentation systems including GBAS (Ground Based Augmentation System) and SBAS (Space Based Augmentation System) are designed to provide correction and integrity information for airborne users so that they can estimate the positions accurately and reliably using GNSS signal measurements. Therefore the integrity monitoring algorithms of the reference stations should detect and exclude GNSS signals with faults or anomalies which could cause user position errors. Normally, the detection thresholds are determined based on the nominal data collected during the site assessment and installation process assuming that there will be no critical environmental changes that impact on the test statistics of integrity monitoring algorithms. However, the characteristics of the test statistics could change in case of new constructions or RF interference nearby, which could induce higher continuity and integrity risk than allocated to the monitor. In order to prevent it and establish updated thresholds, this paper proposes an automated fault detection threshold determination procedure and environment change indicators. Examples with real GPS data show that the proposed procedure detects the environment change and determines new thresholds successfully.

An Approach for GPS Clock Jump Detection Using Carrier Phase Measurements in Real-Time

In this study, a real-time architecture for the detection of clock jumps in the GPS clock behavior is proposed. GPS satellite atomic clocks have characteristics of a second order polynomial in the long term showing sudden jumps occasionally. As satellite clock anomalies influence on GPS measurements which could deliver wrong position information to users as a result, it is required to develop a real time technique for the detection of the clock anomalies especially on the real-time GPS applications such as aviation. The proposed strategy is based on Teager Energy operator, which can be immediately detect any changes in the satellite clock bias estimated from GPS carrier phase measurements. The verification results under numerous cases in the presence of clock jumps are demonstrated.

An Implementation Study on a Low-cost High-accuracy Position Monitoring System for Unmanned Flying Robots

Abstract: Recently, UFRs (Unmanned Flying Robots) have begun to be utilized in various areas for civilian and military applications. Due to this increased utilization, accidents involving UFRsare also increasing. To prevent or monitor accidents caused by UFRs, high-accuracy positioning information is one of the most important technical elements. This paper proposes an efficient UFR monitoring system which provides accurate UFR positioning information with low-cost onboard elements; a small ARM module based on an embedded Linux operating system, a low-cost single frequency GPS receiver with a cheap patch antenna, and a versatile wireless network interface module. The ground monitoring system employs a dual frequency GPS receiver to generate exact UFR coordinates with cm-level accuracy. By processing the UFR measurements based on the Inverse RTK (Real Time Kinematic) method, the ground monitoring system determines the cm-level accurate coordinates of the UFR. The feasibility of the proposed UFR monitoring system was evaluated by three experiments in terms of data loss and accuracy.Keywords: URF, embedded linux, GPS, inverse RTK, monitoring system

GBAS Reference Receiver Clock Adjustment Effects on Continuity and Integrity Performance

Ground-based augmentation system (GBAS) reference stations provide pseudo-range corrections (PRCs) and range rate corrections (RRCs) for visible GPS satellites along with the standard deviations of the PRC errors (σpr_gnd) and B-values for the reference receivers. When generating PRCs, a GBAS should adjust the reference receiver clock bias to limit the PRC values within a certain range specified by international standards. Since σpr_gnd should reflect the PRC errors accurately for system integrity, it must consider the effect of the clock adjustment procedure. Nevertheless, this has hardly been researched, since it is well known that the clock adjustment does not impact on the user position solution. This paper notices that σpr_gnd is also used for GBAS integrity monitoring functions which are implemented in range domain, not position domain. Thus, the work presented here investigates continuity and integrity performance risk in range domain in case a GBAS estimates σpr_gnd ignoring the clock adjustment procedure and uses it for integrity monitors. The work presented here analyzes the risk by a simulation and proposes strategies for σpr_gnd establishment considering the clock adjustment effects to prevent system performance risk.

Statistical Test for Performance Evaluation of Code Carrier Divergence Detection at a GBAS Reference Station

In order to provide precision approach service based on GBAS, air navigation service providers should validate the GBAS system performance against international standard requirements and receive approval. The GBAS manufacturer should define integrity threat, analyze the integrity monitors and evaluate performance of the implemented monitors to verify integrity performance which is critical to the aircraft safety. This paper describes a statistical test result to evaluate performance of the code carrier divergence monitors implemented in KARI Integrity Monitor System software. The paper introduces analysis and test procedure which is developed for the performance evaluation and describes the analysis, the test scenario generation and the test results. The results show that the implemented monitors can detect the expected minimum detectable errors with satisfying the probability of missed detection requirements.

Verification Test for GBAS Correction Information of KARI IMT

Korea Aerospace Research Institute (KARI) has implemented an integrity monitor testbed (IMT) to provide archived GPS data and test results for integrity monitoring algorithm development. To verify that the system is implemented based on international standard requirements, this paper represents the basic functional verification test results of the implemented testbed as a GBAS reference station. It compares the IMT generated GBAS message fields with those of PEGASUS, which is a baseline toolset accepted by international GBAS experts, to show the validity of the correction information. It also verifies the integrity and availability of the system through analysis on GBAS user data in the range and position domain.

Improving Estimation Accuracy of Satellite Clock Error for GPS Satellite Clock Anomaly Detection

ABSTRACT The satellite clock anomalies, one of the abnormal signal factors of the GPS satellites, can have a significant impact on the GPS measurements. However, it can be difficult to detect the anomalies of the satellites clock before the range of the satellites clock error becomes bigger than the range of the other factors, due to the measurement including error of the orbit, ionosphere delay, troposphere delay, multipath and receiver clock. In order to perform quick and accurate detection by minimization of critical range in anomalies of the satellites clock, this paper suggested a solution to detect precise anomalies of the satellites clock after application of carrier smoothing filter from measurement by dual-frequency and adjustment of errors which can be occurred by other factor and the receiver clock errors. The performance of the proposed method was confirmed by comparing to the satellite clock biases which are provided by IGS. 초 록 GPS 위성 이상 신호의 발생 요인 중 위성 시계의 이상 현상은 GPS 측정치에 매우 큰 영향을 미칠 수 있으나, 측정치에는 궤도 오차, 이온층 지연 오차, 대류층 지연 오차, 다중경로 오차, 수신기 시계 오차 등의 성분들이 포함되어 있어 위성 시계의 오차 범위가 다른 요소에 의한 오차보다 커지기 전에는 위성 시계의 이상 현상을 검출하기 어려운 문제가 있다. 위성 시계에 이상 현상이 발생하였을 때 이상 판별의 임계 범위를 최소화 하여 빠르고 정확하게 검출을 수행할 수 있도록, 본 논문에서는 이중 주파수 측정치로부터 반송파 스무딩 필터를 적용하고 수신기 시계 오차 및 다른 여러 가지 요인에 의한 오차를 보정한 후 정확한 위성 시계 오차를 추정하는 방법을 제시하였고 IGS 기관에서 제공하고 있는 위성 시계 정보와 비교를 통해 제시한 방법의 성능을 확인하였다.Key Words : GPS Satellite Clock(GPS 위성시계), Clock Anomaly(시계 이상), Carrier Phase(반송파 위상), Smoothing Filter(스무딩 필터)

Real-Time GPS Clock Monitoring with IGS Ultra-Rapid Products

The objective of this study is to develop a real-time strategy that results in higher precision than any real-time solutions currently available for GPS satellite clock monitoring. A real-time time transfer methodology was employed for satellite clock monitoring, composed of carrier phase smoothed code measurements and IGS ultra-rapid products to obtain precise satellite positions. The performance of the time transfer method was assessed by comparison with the results based on the all-in-view method using the broadcasting ephemeredes. The results showed that the stability of satellite clock monitoring for a short-term period was improved by the proposed method.