Fujinobu Takahashi

The relative movement of the North American and Pacific plates in 1984-1985, detected by the Pacific VLBI network

Heki, K., Takahashi, Y.. Kondo, T., Kawaguchi. N., Takahashi. F. and Kawano. N.. 1987. The relative movement of the North American and Pacific plates in 1984-1985, detected by the Pacific VLBI network. In: R.L. Wesson. Mechanics of Earthquake Faulting. Tectonophysics. 144: 151-158. Very long basline interferomet~ (VLBI) experiments from stations on the North American and Pacific plates in the period 1984-1985 revealed that the interplate baselines have undergone changes by several centimeters which are consistent with the values expected from the model of present plate kinematics. It was also clarified that the intraplate baseline lengths are almost stationary in this period. These results demonstrate that the relative movement of these two plates in the last two years is similar to the plate motion observed on a geological time scale. say a few millions of years.

Development of a GPS codeless receiver for ionospheric calibration and time transfer

A Global Positioning System (GPS) dual-frequency codeless receiver, which has a design similar to that proposed by M. Imae et al. (1988), is developed for measurement of the total electron content in the ionosphere. The authors built a simulator to calibrate the codeless receiver and made calibrations with an accuracy of about 2-3 ns. Ionospheric measurements are made along the line of sight of the scheduled GPS satellites and time comparisons between KRISS (Korea Research Institute of Standards and Science) and CRL (Communications Research Laboratory) are carried out using the measured ionospheric delays at both sites under the common-view schedule of BIPM. The precision of time comparison is improved by using the measured ionospheric data. >

The first comparative experiment of VLBI and two-way time transfer with better than 1 nsec precision

Time transfer technique comparison between very long baseline interferometry (VLBI) and two way satellite method was firstly performed between Koganei (Tokyo) and Kashima (Ibaragi) over 110 km baseline in November, 1992. Clock variation in two stations are compared by both techniques with a precision of better than 1 nsec. Obtained clock variation by both techniques behaves in the same manner. However, significant scale difference was found in the results. The most likely explanation is that the VLBI clock parameters are coupled with the other adjusted parameters.<<ETX>>

Calibration of the delay time in the GMS/GPS time transfer receivers using portable reference receivers

The Communications Research Laboratory (CRL) of Japan and the National Measurement Laboratory (NML) of Australia have performed international time transfer using the Geostationary Meteorological Satellite (GMS) and the Global Positioning System (GPS) since 1986. The precision of the time transfer using GPS is 10 ns, while that using GMS is 20 ns. However, there has been a bias of 200 ns between the time transfer results of the two time links, which is mainly due to the estimation error of the delay time in the time transfer receivers. To improve this bias error, CRL developed a portable reference receiver (PRX) for the GMS time transfer to calibrate the delay time in the time transfer receivers. In November and December 1987, a calibration experiment was carried out by the PRX method for GMS and GPS between CRL and NML. As the result of the experiment, the bias was reduced from 200 to 75 ns and the accuracy of the time transfer between CRL and NML was significantly improved. >

VLBI, SLR and GPS observations in the Key Stone Project

A space geodetic observation network has been established around Tokyo, Japan under a project name of Key Stone Project by Communications Research Laboratory. Three space geodetic methods, i.e. Very Long Baseline Interferometry, Satellite Laser Ranging, and Global Positioning System, are involved in the project. As of September, 1997, VLBI and GPS observation facilities at all four stations are operational, whereas developments of SLR observation facilities are in course of final alignment procedures. Daily VLBI observations began in January 1995 with a single baseline between Koganei and Kashima, and the full network observations with four stations began in September 1996. Observations and data analysis of VLBI measurements are fully automated and the analysis results are produced shortly after all observations of an experiment session finished. GPS observations at four sites began in July 1997 and the automatic data collection and analysis system are under developments.

Millimeter accuracy of geodetic VLBI measurements achieved on a 54-km baseline

Millimeter accuracy of geodetic VLBI (very-long-baseline interferometry) measurements was demonstrated in experiments repeated five times from 1984 to 1988 on a 54-km baseline between a 26-m antenna at Kashima and a 5-m transportable antenna at Tsukuba. The average formal errors of the five measurements were 4.6 mm in length, and 4.4 and 6.7 mm in the east and north directions. In the error analysis, effects of atmospheric turbulence and thermal receiver noise were investigated and quantitatively evaluated. As a result, it was found that the stability of the atmosphere was on the order of 10/sup -14/ in the Allan standard deviation, which agrees well with other results obtained in interferometric and radiometric measurements. It was also found that the atmospheric turbulence was a major cause of an error on delay rate observations, whereas the receiver noise is a principal source of error in delay observations. >