Sung-Hoon Yang

A BLUE Estimator for Passive Localization by TDOA Method

In this paper, we derived a closed-form equation of a Best Linear Unbiased Estimator (BLUE) and its Crammer-Rao Lower Bound (CRLB) for the estimation of the position of the emitter based on the Time Difference of Arrival (TDOA) teclmique. The BLUE and CRLB were derived for the case of estimating 2 dimensional position of the emitter with 3 base stations or sensors, and for this purpose, we nsed an approximated equation of the TDOA hyperbola equation obtained from the first order Taylor-series after setting the reference points of the position. The derived equation can be used for any kind of noises which are uncorrelated in each other in the TOA measurement noises and for a white Gaussian noise also.

Analysis of a clock-aided global navigation satellite system (GNSS)

This paper presents a dilution of precision (DOP) analysis of a clock-aided global navigation satellite system (GNSS). Firstly, the analysis shows that a best linear unbiased estimator should be used in the clock-aided GNSS to get the best improvements in accuracy. Secondly, the influence of the clock on vertical DOP (VDOP) and horizontal DOP (HDOP) is analysed by using the clock characteristics as a weighting matrix. The analysis highlights quantitative improvements in the VDOP and HDOP and explains why improvements in the VDOP are superior to those of the HDOP in the clock-aided GNSS. Finally, the paper analyses how incorrectly known a priori clock variances degrade the level of accuracy and offers guidelines for determining a weighting matrix in the cases where the clock characteristics are not clearly known.

Impact of the transponder configuration on the Asia-Europe TWSTFT network

Two-way satellite time and frequency transfer between Asia and Europe has been performed for many years first by a two-transponder and later by a one-transponder configuration on the IS-4 and the AM-2 satellites, respectively. A diurnal phase instability was observed in links established through IS-4, while such instability has not been observed when AM-2 was used. It could be shown that the diurnal variation are site-independent. An on-board path asymmetry due to the use of two transponders seems thus to cause it.

TWSTFT results by using software-defined receiver data

The precision of two-way satellite time and frequency transfer (TWSTFT) is currently limited due to instabilities of signal arrival time. We use a software-defined receiver to measure the arrival time of code signal transmitted by SATRE modem. The SDR systems have been successfully installed and performed at TL, NICT and KRISS. The SDR results show excellent suppression on the diurnals. For data of the KRISS-TL link, the TDEV of the SATRE modem shows a peak of 135 ps at 8 hours; where the TDEV of SDR is only 38 ps.

Estimation of GPS Holdover Performance with Ladder Algorithm Used for an UFIR Filter

In this paper, we described the simulation results of the phase offset performance of a clock in holdover mode which was normally operated in GPS Disciplined Oscillator (GPSDO). In the TIE model, we included the time error term caused by environmental temperature variation because one of the most important parameters of clock phase error is the frequency offset and drift caused by the variation of temperature. For the simulation, we employed Maximum Time Interval Error (MTIE) for the performance evaluation when the frequency offset and drift are estimated by using an Unbiased Finite Impulse Response (UFIR) filter with ladder algorithm. We assumed that the noise in the GPS measurement is white Gaussian with zero mean and 1 ns standard deviation, and temperature linearly varies with a slope of per hour. From the simulation results, the followings were observed. First, with the estimation error of temperature of less than 3 % and the temperature compensation period of less than 900 seconds, the requirement of CDMA2000 phase synchronization under 10 us could be achieved for more than 40,000 seconds holdover time if we employ an OCXO (Oven Controlled Crystal Oscillator) clock. Second, in order to achieve the requirement of LTE-TDD under 1.5 us for more than 10,000 seconds holdover time, below 3 % estimation error and 500 seconds should be retained if a Rubidium clock is adopted.

Evaluation of Synchronization Performance with PTP

In this paper, we described the investigated theoretical time synchronization performances and experiment results obtained by commercially provided PTP (Precise Time Protocol) modules when the time of a slave clock is synchronized to the master clock. In the case of the theoretical performance analysis, we investigated 3 types of clock levels such as Crystal Oscillator (XO), TCXO (Temperature Compensated XO) and OCXO (Oven Controlled XO). From the analysis, it was observed that the synchronization performance is greatly influenced by the synchronization period and the required performance under 1 us can be achieved by using XO level clocks when the synchronization period is less than 2 seconds and the uncertainty of the propagation delay is under 100 ns. For the experiments using commercial PTP modules, the synchronization performance was investigated for direct, through 1 hub and through 2 hubs connections between the master clock and the slave clock. From the experiment results, we observed that time synchronization under 90 ns with 1,000 seconds observation interval can be achieved in the case of direct connection.

Measurement of Reference Phase Offset for the Loran-C Transmitting Signal of Pohang

In order to establish eLoran (enhanced Long Range Navigation) system, it needs the advancement of receiver, transmitter, data channel addition for Loran information, differential Loran sites for compensating Loran-c signal and ASFs (Additional Secondary Factors) database, etc. In addition, the precise synchronization of transmitting station to the UTC (Coordinated Universal Time) is essential if Loran delivers the high absolute accuracy of navigation demanded for maritime harbor entrance. For better timing synchronization to the UTC among transmitting stations, it is necessary to measure and monitor the transmission delay of the station, and the correction information of the transmitting station should be provided to the users receivers. In this paper we presented the measurement method of absolute delay of Pohang Loran transmitting station and developed a time delay measurement system and a phase monitoring system for Loran station. We achieved -2.23 us as a result of the absolute phase delay of Pohang station and the drift of Loran pulse of the station was measured about 0.3 us for a month period. Therefore it is necessary to measure the delay offset of transmitting station and to compensate the drift of the Loran signal for the high accuracy application of PNT (Positioning, Navigation and Timing).

Introduction of software-defined receivers in two-way satellite time and frequency transfer

The precision of two-way satellite time and frequency transfer (TWSTFT) is currently limited due to unstable measurement of the signal arrival time. To improve the TWSTFT precision without interrupting the main system, we introduce the software-defined receiver (SDR) to measure the arrival time of the code signal. The SDRs have been successfully installed among TL, NICT and KRISS, and time-difference results for more than 20 days were obtained and compared. From the results, the SDR has better precision and less diurnal, even when an earth station suffered from a bad condition.

International Time Comparison by TWSTFT and GPS at KRISS

In this paper, time comparison is performed with standardization institution in Japan using a Two-Way Satellite Time and Frequency Transfer (TWSTFT) technique as one of the methods for high precision time comparison. To analyze the performance of time comparison in the TWSTFT method, time comparison results via the Global Positioning System (GPS) code and carrier wave are analyzed. Through the time comparison performance, frequency stability is analyzed using modified Allan deviation and by this result, characteristics of time comparison of the TWSTFT that is utilized in international time comparison are presented.

Evaluation of Daily Jump Compensation Methods for GPS Carrier Phase Data

In this paper, we described the timing-offset comparison results between various daily jump compensation methods for GPS carrier phase measurement data. For the performance comparison, we used about 70 days GPS measurement data obtained from two GPS timing receivers which share the reference 1 PPS and RF signals and closely located in each other within a few meters. From the experiment results, it is observed that clock bias can be occurred in the case of using a simple compensation method accumulating daily jumps with non-perfectly estimated jump values.