• Journal of Electrical Engineering and Technology
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• v.7 no.3
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• pp.429-435
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• 2012

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.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.3
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• pp.225-231
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• 2011

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.

• The Journal of the Korea institute of electronic communication sciences
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• v.7 no.2
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• pp.327-332
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• 2012

In this paper, we analyzed the influence of different observation stations distributions on satellite clock offset estimation based on the PANDA software. The result shows that, when the distance between stations is shorter than 200km, the correlation of troposphere parameter and satellite clock offset parameter is strong, the accuracy of satellite clock offset estimation will be up to 0.8ns; when the distance between stations is up to 500km, as the correction of troposphere parameter and satellite clock offset parameter is significantly reduced, and the two kinds of parameters can be distinguished.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.28 no.6
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• pp.563-571
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• 2010

The GPS satellite has three or four atomic clocks that consist of cesiums and rubidiums and the NANU messages can be used to identify the kind of the onboard atomic clock because they classify the clock type on a daily basis. In this study, for long-term analysis of the GPS satellite clock behavior, we extracted satellite clock errors for every PRN from years 2001 through 2009 using the SP3 files that are provided by the IGS. As a result, the cesium clock offsets usually have a linear trend of drifting. On the other hand, rubidium offsets show curvilinear variations in general, even though they cannot be represented as anyone specific polynomial function. For short-term analysis, we extracted satellite clock errors for each PRN for a week-long period using the CLK files that are also provided by the IGS and curve-fitted them with first-order and second-order polynomial functions. In cases of cesium clock errors, they were well-represented by first-order polynomial functions and rubidium clock errors were similar with second-order polynomials. However, some of rubidium clock errors could not be represented as any polynomial fitting function. To analyze the characteristic of GPS satellite by each block and atomic clock, we applied Modified Allan Deviation criterion to the dataset from years 2007 and 2010. We found that the Modified Allan Deviation characteristics changed significantly according the block and atomic clock type.

• Journal of Advanced Navigation Technology
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• v.13 no.2
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• pp.178-186
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• 2009

SBAS(Satellite Based Augmentation System) provides GNSS satellite orbit and clock corrections for positioning accuracy improvement of GNSS users. In this paper, the accuracy of SBAS satellite orbit and clock corrections were analyzed by comparing with the IGS(International GNSS Service) precise ephemeris. The GPS antenna phase center offsets and the P1-C1 bias are considered for the analysis. The correction data of the US WAAS and the Japanese MSAS were analyzed. The analysis results showed that the SBAS satellite orbit and clock corrections are highly correlated. The correction data accuracy depends on the SBAS ground network size and orbit trajectories.

• Journal of the Institute of Convergence Signal Processing
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• v.6 no.2
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• pp.95-99
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• 2005

This paper presents an accurate recovery method of the reference clock which is needed for network synchronization in two-way satellite multimedia systems compliant with DVB-RCS specification and which use closed loop method for burst synchronization. In these systems, the remote station transmits TDMA burst via return link. For burst synchronization, it obtains reference clock from program clock reference (PCR) defined by MPEG-2 system specification. The PCR is generated periodically at the hub system by sampling system clock which runs at 27MHz $\pm$ 30ppm. Since the reference clock is recovered by means of digital PLL(DPLL) using imprecise PCR values due to variable random jitter, the recovered clock frequency of remote station doesn't exactly match reference clock of hub station. We propose a robust recovery method of reference clock against random delay jitter The simulation results show that the recovery error is remarkably decreased from 5 clocks to 1 clock of 27MHz relative to the general DPLL recovery method.

• Journal of Satellite, Information and Communications
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• v.10 no.2
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• pp.102-108
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• 2015

In general, two way satellite communication systems based on TDMA(Time Division Multiple Access) require network clock synchronization between hub station and remote terminals. This paper describes basic concepts for network clock synchronization based on NCR(Network Clock Reference) clock recovery scheme as suggested in DVB-S2/ RCS2 international standards. in addition, a new NCR insertion method has been proposed and evaluated in terms of supporting CCM mode as well as ACM mode which optimizes throughput by changing code rates and modulation types ranging from QPSK to 32-APSK.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.12B
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• pp.1037-1041
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• 2004

In this paper we propose a new conceptual slave clock system in which remotely located clock is synchronized to the reference clock by intermediation of the satellite time, show a probability of adoption to real network by experiments. This new proposed method has lots of structural advantages over the existing methods because all of the node clocks can be maintained with the same hierarchical quality. The measurement results show that the accuracy of the experimental slave clock system can be kept within a few parts in 1012 and that the MTIE (Maximum Time Interval Error) meets the ITU-T Recommendation G.811 for the primary reference clock A prototype system having fully automatic operational functions has been realized, and it is expected to be commercially used as a node clock for synchronization in the digital communication network in the near future.

• Proceedings of the Korean Institute of Navigation and Port Research Conference
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• v.1
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• pp.309-314
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• 2006

In general DGPS system, the correction message is transferred to users by wireless modem. To cover wide area, many DGPS station should be needed. And DGPS users must have a wireless modem that is not necessary in standalone GPS. But SBAS users don't need a wireless modem to receive DGPS corrections because SBAS correction message is transmitted from the GEO satellite by L1 frequency band. SBAS signal is generated in the GUS(Geo Uplink Subsystem) and uplink to the GEO satellite. This uplink transmission process causes two problems that are not existed in GPS. The one is a time delay in the uplink signal. The other is an ionospheric problem on uplink signal, code delay and carrier phase advance. These two problems cause ranging error to user. Another critical ranging error factor is clock synchronization. SBAS reference clock must be synchronized with GPS clock for an accurate ranging service. The time delay can be removed by close loop control. We propose uplink ionospheric error correcting algorithm for C/A code and carrier. As a result, the ranging accuracy increased high. To synchronize SBAS reference clock with GPS clock, I reviewed synchronization algorithm. And I modified it because the algorithm didn't consider doppler that caused by satellites' dynamics. SBAS reference clock synchronized with GPS clock in high accuracy by modified algorithm. We think that this paper will contribute to basic research for constructing satellite based DGPS system.