Jeongrae Kim

Error Analysis of a Low-Low Satellite-to-Satellite Tracking Mission

The Gravity Recovery and Climate Experiment is a dedicated spaceborne mission whose objective is to map the gravity e eld with unprecedented accuracy. It consists of two satellites coorbiting in a nearly polar orbit at low altitude. Primary measurements are a series of measured range changes between the two satellites using a dual one-way microwave ranging system. These measurements are combined with accelerometer and global positioning system measurements. In this study, comprehensive simulation models for these major instruments have been developed to recover the gravity information. Effects of the primary error sources on the orbit and gravity estimation are analyzed through a series of numerical simulations.

Protecting Signal Integrity Against Atomic Clock Anomalies on Board GNSS Satellites

This paper introduces a method of detecting atomic clock anomalies for Global Navigation Satellites System (GNSS) satellites in real-time mode and presents some results obtained by monitoring actual GNSS satellite clocks. The method employs a Kalman filter formulation which facilitates the real-time processing of GNSS carrier-phase measurements. Judgment regarding clock anomalies is made using the prediction interval approach and actual distribution of predicted phase and frequency error is replaced by overbounding Gaussian distribution. Results show that the method could be a suitable method to detect GNSS clock events.

Selective Integration of GNSS, Vision Sensor, and INS Using Weighted DOP Under GNSS-Challenged Environments

Accurate and precise navigation solution can be obtained by integrating multiple sensors such as global navigation satellite system (GNSS), vision sensor, and inertial navigation system (INS). However, accuracy of position solutions under GNSS-challenged environment occasionally degrades due to poor distributions of GNSS satellites and feature points from vision sensors. This paper proposes a selective integration method, which improves positioning accuracy under GNSS-challenged environments when applied to the multiple navigation sensors such as GNSS, a vision sensor, and INS. A performance index is introduced to recognize poor environments where navigation errors increase when measurements are added. The weighted least squares method was applied to derive the performance index, which measures the goodness of geometrical distributions of the satellites and feature points. It was also used to predict the position errors and the effects of the integration, and as a criterion to select the navigation sensors to be integrated. The feasibility of the proposed method was verified through a simulation and an experimental test. The performance index was examined by checking its correlation with the positional error covariance, and the performance of the selective navigation was verified by comparing its solution with the reference position. The results show that the selective integration of multiple sensors improves the positioning accuracy compared with nonselective integration when applied under GNSS-challenged environments. It is especially effective when satellites and feature points are posed in certain directions and have poor geometry.

Performance evaluation of real-time carrier-phase GPS time transfer

A real-time GPS time transfer system was developed to measure the phase difference between clocks at the sub-nanosecond level. The relative performance of the system was assessed with other contemporary time transfer methods and several issues regarding real-time GPS time transfer were addressed, including the use of a sequential data processor and establishment of a real-time data link. By use of the Kalman estimator, a two-state clock model is incorporated into system dynamics and the classical client?server model is utilized to establish GPS data links via TCP/IP communication. To reduce part of the time transfer errors related to satellite position and clock biases, IGS ultra-rapid products are used. Validation of the real-time GPS time transfer system was performed in two ways, through the use of a common-clock signal and through direct measurement of the phase difference with a time interval counter. Finally, a 5 month long time transfer session was conducted among four national timing laboratories and the results were compared against those obtained with TWSTFT and Circular T issued by the Bureau International des Poids et Mesures.

Optimal Frequency Configuration for Dual One-Way Ranging Systems

Dual one-way ranging systems measure intersatellite distance with very high precision by combining the one-way range measurements from two satellites. This high precision is obtained by minimizing the oscillator noise effect, which is the main error source for microwave ranging systems. A key requirement for the dual one-way ranging system is the synchronization of the two one-way range measurement times, and this requirement makes it necessary to compute a high-accuracy clock solution. An optimal frequency configuration is proposed to mitigate the time-synchronization requirement. Numerical simulations demonstrate that the optimal frequency configuration becomes very effective when the clock offsets are not well known.

Estimation of non-gravitational acceleration difference between two co-orbiting satellites using single accelerometer data

Non-gravitational accelerations acting on two closely co-orbiting satellites are highly correlated, and one satellite’s non-gravitational accelerations, sensing by accelerometer, can be transferred for the other satellite. NASA/DLR GRACE mission has been suffering from intermittent single accelerometer situations due to power limitation beyond its design life time. To overcome this situation, three estimation methods to predict one satellite’s non-gravitational acceleration using a weighted moving average of another satellite’s data are proposed Differential non-gravitational acceleration projection along velocity direction is used to evaluate the three methods with the GRACE flight data. If no bias adjustment is performed during preprocessing, one of the new methods shows an accuracy improvement over the time-shift method, which utilizes a single epoch data from the other satellite. With a bias adjustment, the time-shift method is preferred for its simplicity. The annual variations of the differential acceleration projection and estimation errors are analyzed using long-term GRACE flight data. The differential acceleration magnitude is closely related to solar activity, and therefore large estimation errors occur in the geomagnetic equator around noon, local time.

Simultaneous estimation of ionospheric delays and receiver differential code bias by a single GPS station

In this paper, we propose an efficient Kalman filter algorithm for simultaneous estimation of absolute ionospheric delay and receiver differential code bias. It is well known that the two quantities are mixed in global positioning system (GPS) measurements causing a rank-deficiency problem. The proposed method requires only the broadcast navigation message and basic measurements provided by a single dual-frequency GPS receiver. Thus, it would be convenient to monitor local ionospheric activities autonomously in real time with minimum hardware. An observability analysis was performed in detail why the simultaneous estimation is possible in spite of the rank deficiency occurring in epoch-by-epoch measurements. Experimental results based on real GPS measurements demonstrate the feasibility of the proposed method.

ARMA Prediction of SBAS Ephemeris and Clock Corrections for Low Earth Orbiting Satellites

For low earth orbit (LEO) satellite GPS receivers, space-based augmentation system (SBAS) ephemeris/clock corrections can be applied to improve positioning accuracy in real time. The SBAS correction is only available within its service area, and the prediction of the SBAS corrections during the outage period can extend the coverage area. Two time series forecasting models, autoregressive moving average (ARMA) and autoregressive (AR), are proposed to predict the corrections outside the service area. A simulated GPS satellite visibility condition is applied to the WAAS correction data, and the prediction accuracy degradation, along with the time, is investigated. Prediction results using the SBAS rate of change information are compared, and the ARMA method yields a better accuracy than the rate method. The error reductions of the ephemeris and clock by the ARMA method over the rate method are 37.8% and 38.5%, respectively. The AR method shows a slightly better orbit accuracy than the rate method, but its clock accuracy is even worse than the rate method. If the SBAS correction is sufficiently accurate comparing with the required ephemeris accuracy of a real-time navigation filter, then the predicted SBAS correction may improve orbit determination accuracy.

Analysis of MSAS Correction Information and Performance in Korea

A GNSS software for processing the SBAS correction data is developed, and Japan MSAS correction data is analyzed. MSAS orbit correction data is analyzed and compared with WAAS data. MSAS ionosphere correction data is analyzed and the effect of the equatorial anomaly on the correction accuracy is discussed. Degradation due to receive delay of correction information and effect of the degradation on protection level analyzed using partial remove of MSAS correction information. Integrity and availability for precision approch using the MSAS system analyzed.A GNSS software for processing the SBAS correction data is developed, and Japan MSAS correction data is analyzed. MSAS orbit correction data is analyzed and compared with WAAS data. MSAS ionosphere correction data is analyzed and the effect of the equatorial anomaly on the correction accuracy is discussed. Degradation due to receive delay of correction information and effect of the degradation on protection level analyzed using partial remove of MSAS correction information. Integrity and availability for precision approch using the MSAS system analyzed.

Protecting signal integrity against atomic clock anomalies on board GNSS satellites

This paper introduces a method of detecting atomic clock anomalies of GNSS satellites and presents some results obtained from processing real GNSS data. The method employs a Kalman filter formulation which facilitates real-time processing of GNSS carrier-phase measurements. Decision on the onset of clock anomalies is made using the prediction interval approach, where actual distribution of relative phase difference data between satellite and ground clocks is replaced by a Gaussian overbounding distribution. Results obtained from processing real GNSS data shows that the method appears as a suitable tool for real-time detection and identification of GNSS clock anomalies.