• Journal of Positioning, Navigation, and Timing
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• 제7권3호
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• pp.175-181
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• 2018

The use of dual-frequency measurements from the Global Navigation Satellite System (GNSS) enables us to observe precise ionospheric total electron content (TEC). Currently, many GNSS reference stations in South Korea provide both GPS and GLONASS data. In the present study, we estimated the grid-based TEC values and relative receiver differential code biases (DCB) from a GNSS network operated by the Korea Astronomy and Space Science Institute. In addition, we compared the diurnal variations in a TEC time series from solutions of the GPS only, the GLONASS only, and combined GPS/GLONASS processing. A significant difference between the GPS only TEC and combined GPS/GLONASS TEC at a specific grid point over South Korea appeared near the solar terminator. It is noted that GLONASS measurements can contribute to observing a variation in ionospheric TEC over high latitude regions.

• Journal of Astronomy and Space Sciences
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• 제33권1호
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• pp.45-54
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• 2016

This study developed an approach for improving Carrier-phase Differential Global Positioning System (CDGPS) based realtime satellite relative navigation by applying laser baseline measurement data. The robustness against the space operational environment was considered, and a Synthetic Wavelength Interferometer (SWI) algorithm based on a femtosecond laser measurement model was developed. The phase differences between two laser wavelengths were combined to measure precise distance. Generated laser data were used to improve estimation accuracy for the float ambiguity of CDGPS data. Relative navigation simulations in real-time were performed using the extended Kalman filter algorithm. The GPS and laser-combined relative navigation accuracy was compared with GPS-only relative navigation solutions to determine the impact of laser data on relative navigation. In numerical simulations, the success rate of integer ambiguity resolution increased when laser data was added to GPS data. The relative navigational errors also improved five-fold and two-fold, relative to the GPS-only error, for 250 m and 5 km initial relative distances, respectively. The methodology developed in this study is suitable for application to future satellite formation-flying missions.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2000년도 한국우주과학회보 제9권2호
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• pp.28-28
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• 2000

• 대한전기학회:학술대회논문집
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• 대한전기학회 2001년도 하계학술대회 논문집 D
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• pp.1984-1986
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• 2001

In order to provided continuous solutions, latest developing navigation systems tend to integrate GPS receiver with INS or DR. Using the GPS carrier-phase measurements, an attitude GPS receiver with three antennas obtain the 3-dimensional attitude such as roll, pitch, and heading as well as position and velocity. With these angle measurements, in the attitude GPS/INS integrated system, attitude or gyro errors can be directly compensated. In this paper, we develop an integrated navigation system that combines attitude GPS receiver with INS. The performance of real-time integrated navigation system is determined by not only the implements of integration filter but also the synchronization of measurements. To meet these real-time requirements, the navigation software is implemented in multi-tasking structure in this paper. We also employ time-synchronization technique in the multi-sensor fusion. Experimental results show that the performance of the attitude GPS/INS integrated system is consistent even when cycle-slip occurs in carrier-phase measurements.

• 제어로봇시스템학회논문지
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• 제7권11호
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• pp.944-952
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• 2001

Inertial navigation system(INS) errors increase with time due to inertial sensor errors, and therefore it is desired to combine INS with external aids such as GPS. However GPS informations have a randomly abrupt jump due to a sudden corruption of the received satellite signals and environment, and moreover GPS can\`t provide navigation solutions. In this paper, smoothing and prediction schemes are proposed for GPS`s jump or unavailable GPS. The smoothing algorithm which is designed as a scalar adaptive filter, smooths abrupt jump. The prediction algorithm which is proved by Schuler error model of INS, estimates INS error in appropriate time. The outputs of proposed algorithm apply stable measurements to GPS aided INS Kalman filter. Simulations show that the proposed algorithm can effectively remove measurement jump and predict INS error.

• 한국항행학회논문지
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• 제27권6호
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• pp.841-848
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• 2023

위성 항법 장치 (GPS; global positioning system) 항법메시지에 포함된 Klobuchar 전리층모델은 L1 단주파수 사용자들에게 전리층 보정정보를 제공한다. 전리층모델 정확도는 항법해의 정확도에 큰영향을 끼치므로 이에 관한 분석이 필요하다. 본 연구에서는 1993년부터 2022년까지의 GPS 항법메시지를 조사하여 Klobuchar 모델의정확도 및 계수 존재 여부와 효용성 여부를 분석 하였다. 초기 GPS 항법메시지의 경우 전리층데이터를 포함하지 않는 경우가 많으며, 전리층모델을 포함되어 있더라도 정확도가 상당히 낮은 경우가 많이 존재하였다. 전리층모델의 정확도가 안정화된 2002년부터 2022년까지 전리층모델의 정확도 변화와 지자기 위도에 따른 정확도 차이를 IGS (International GNSS Service)에서 제공하는 전리층모델과 비교하는 방법으로 분석하였다.

• 한국항공우주학회지
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• 제33권7호
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• pp.92-97
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• 2005

본 논문에서는 6년째 운용되고 있는 아리랑 1호에 탑재된 GPS 수신기의 실제 궤도 상 성능을 분석하였다. 수신기의 환경변수 설정에 따라 지상에서 수행하는 궤도결정 정밀도 변화, GPS 위성 가시성들을 분석하였으며, 실제 궤도에서 운영 중 관측되는 문제점에 대해서 기술하였다. 분석 결과, 항행해를 구하는 Position Fix Algorithm을 ‘Best-of-4’ 로 설정한 상태에서 획득한 항행해를 이용한 지상 궤도결정 정밀도가 ‘N-in-View’ 설정 상태의 결과 보다 약간 높았으며, 이때 ‘Mask Angle'은 0도로 설정하는 것이 유리함을 알 수 있었다. 반면, ‘N-in-View’ 방식하에서는 GPS 위성에 대한 가시성이 조금 높았으며, 일시적인 3D Fix Loss 발생 빈도 수도 상대적으로 적은 것을 알 수 있었다.

• 한국항해항만학회:학술대회논문집
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• 한국항해항만학회 2006년도 International Symposium on GPS/GNSS Vol.1
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• pp.455-458
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• 2006

The Real Time Kinematic (RTK) technique is the most productive and accurate GPS positioning method today, as it can be determinate position within few centimeters instantly. This method is widely used for applications such as surveying, structure monitoring and machine guidance etc. In order to perform RTK processing for large scale systems (i.e. precise vehicle monitoring with many rovers), many expensive RTK receivers and same number of bidirectional communication units have to be installed to collect observation data communicate with the reference site and monitor its RTK solutions. Moreover, if applications require remote control or apply sensing instruments, we have to install computers at each rover. To limit expense and complexity of system management with a large number of rovers, we have developed server based RTK processing platform to share RTK function for all rovers. The system can be process many GPS stations with a single personal computer. we have also developed a specialized dual frequency GPS receiver unit without on-board RTK processing capability to reduce receiver cost in order to demonstrate the advantage of our server based RTK platform. This paper describes the concept of our server based RTK platform and specialized GPS receiver unit with existing applications in Japan.

• 한국항해항만학회:학술대회논문집
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• 한국항해항만학회 2006년도 International Symposium on GPS/GNSS Vol.1
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• pp.15-19
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• 2006

GPS has been increasingly exploited to provide positioning and navigation solutions for a variety of applications. In vessel berthing application, however, there are stringent requirements in terms of positioning accuracy, availability and integrity that cannot be satisfied by GPS alone. This is because the performance of satellite-based positioning and navigation systems are heavily dependent on both the number and the geometric distribution of satellite tracked by receivers. Due to the limited number of GPS satellites, a sufficient number of ‘visible’ satellites cannot be sometimes guaranteed. This paper discusses some issues associated with the implementation of ground-based pseudolite augmentation for vessel berthing. Pseudolite means small transmitter that transmits GPS-like signals in local area. Actually, pseudolite can play three different roles in GPS augmentation scheme, depending on the operational conditions. Firstly, in the case of kinematic GPS operation where there are no signal blockages, and more than five satellites are available, additional pseudolites strengthen the GPS satellite-pseudolite geometry, and more accurate and reliable positioning solution can be achieved. Secondly, in the case when there are adverse GPS operational environments in which the number of tracked satellites is less than four, pseudolites can complement the GPS signals. In the third case, GPS signals are completely unavailable, such as when operated indoor. In such cases the pseudolites can replace the satellite constellation. However, the first role will be considered in this paper, since more than four satellite signals can usually be tracked in most marine applications. This paper presents that the pseudolite-augmented precise positioning system can provides continuous centimeter-level positioning accuracy through comparison analysis of RDOP simulation result of the GPS satellite constellation and the pseudolite-augmented GPS satellite constellation.

• Journal of Astronomy and Space Sciences
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• 제26권1호
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• pp.59-74
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• 2009

Relative navigation system is presented using GPS measurements from a single-channel global positioning system (GPS) simulator. The objective of this study is to provide the real-time inter-satellite relative positions as well as absolute positions for two formation flying satellites in low earth orbit. To improve the navigation performance, the absolute states are estimated using ion-free GRAPHIC (group and phase ionospheric correction) pseudo-ranges and the relative states are determined using double differential carrier-phase data and singled-differential C/A code data based on the extended Kalman filter and the unscented Kalman filter. Furthermore, pseudo-relative dynamic model and modified relative measurement model are developed. This modified EKF method prevents non-linearity of the measurement model from degrading precision by applying linearization about absolute navigation solutions not about the priori estimates. The LAMBDA method also has been used to improve the relative navigation performance by fixing ambiguities to integers for precise relative navigation. The software-based simulation has been performed and the steady state accuracies of 1 m and 6 mm ($1{\sigma}$ of 3-dimensional difference errors) are achieved for the absolute and relative navigation using EKF for a short baseline leader/follower formation. In addition, the navigation performances are compared for the EKF and the UKF for 10 hours simulation, and relative position errors are mm-level for the two filters showing the similar trends.