Noriyuki Kurihara

Two-way satellite time and frequency transfer networks in Pacific Rim region

A two-way satellite time and frequency transfer (TWSTFT) network in the Pacific Rim region is under construction to contribute to the calculation of the international atomic time (TAI). Four major time and frequency institutes in this region have been conducting long-term TWSTFT experiments. In addition to these institutes, several others in the region are planning to join the network. A new type of time transfer modem for TWSTFT is also described.

Automated geodetic Very Long Baseline Interferometry observation and data analysis system

A precise geodetic measurement network using three modern space geodetic techniques, i.e. Very Long Baseline Interferometry, Satellite Laser Ranging, and Global Positioning System, is being established around Tokyo, Japan by the Communications Research Laboratory. The Key Stone Project, which is the name of the project, was started to obtain precise relative positions of four stations using these three space geodetic techniques on a daily basis. The system was designed to make frequent observations possible with minimum human operations and to provide analyzed results as fast as possible. This paper describes various aspects of new features and the performance of the automatic geodetic Very Long Baseline Interferometry observation and data analysis system designed for the Key Stone Project. This automated design has allowed daily Very Long Baseline Interferometry experiments to be conducted since January 1995 and the results to be immediately made available for public use after each experiment.

Evaluation of repeatability of baseline lengths in the VLBI network around the Tokyo metropolitan area

Since 1995, VLBI measurements using fixed VLBI stations around the Tokyo metropolitan area have been continually producing data of station positions and baseline lengths. The accuracy of baseline length measurements is evaluated in terms of repeatability, conventionally defined as a standard deviation of those obtained by five continuous sessions. Continuous improvement both in system hardware and in the observation method have resulted in a remarkable improvement in measurement accuracy. Repeatability reaches about a 2-mm level in baseline length in our VLBI network.

Two-way satellite time transfer network in Pacific Rim region

A Two Way Satellite Time Transfer (TWSTT) network in the Pacific rim region is under construction to contribute to the International Atomic Time (TAI). Four major Time and Frequency Institutes in this region have been conducting long term TWSTT experiments. In addition to these institutes, several ones in this region are planning to join this TWSTT network.

Microwave flux density variations of compact radio sources monitored by real‐time very long baseline interferometry

Compact and strong radio sources were repeatedly observed in regular geodetic very long baseline interferometry experiments under the Key Stone Project. The flux densities of these sources in the S and X bands were estimated from a set of correlated amplitudes obtained through the correlation processing of the observed data. From ∼5 years of the observed data, irregular variations in the flux densities were detected for several radio sources using the source 2134+004 as the calibrator. The results showed the monitoring of flux density variations by means of very long baseline interferometry is feasible. Since the correlation processing of the observed data are performed real-time using a high-speed digital communication technique, this method gives a capability to monitor flux density variations of quasars and BL-Lac types of extragalactic radio sources.

Delay calibration method for precise and accurate two way satellite time and frequency transfer

The two way satellite time and frequency transfer (TWSTFT) is one of the most accurate and precise time transfer techniques. But there are possibilities of nanosecond level time transfer errors due to delay variations, mainly caused by temperature variations. This paper reports a measurement technique under development at the Communications Research Laboratory using a multi-channel time transfer modem.

VLBI phase-calibration system suitable for very-wide-band and ultra-high-frequency operation

The key factors determining the final performance in geodetic measurements made using very long baseline interferometry (VLBI) are the total instantaneous bandwidth and the actual value of the observation frequency. The first is strictly related to the uncertainty measurement of the group-delay, while the second is related to the ultimate resolution achievable with this measurement technique. The main advantages of using a higher frequency are a large improvement in the calibration of the ionospheric correction, the avoidance of interference from strong man-made radio signals, and the possibility of having a still wider receiving band. In contrast, implementing a scheme to fully calibrate the instrumental delay would be much more difficult. The authors have developed a new scheme, which expands the operation bandwidth and, in practice, does not have an upper limit on the frequency. It works by up-converting the comb pattern of the phase rails by a local generator, which can be in common with the main receiver or a standalone device. The results of the first VLBI experiment at 22 GHz by stations in Italy and Japan in 1995 demonstrated the effectiveness of this technique. It is currently used in the VLBI Space Observatory Program, which began in 1997.

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. >

Comparison of Site Velocities Measured by VLBI and GPS in the Key Stone Project Network

Communications Research Laboratory has established four space geodetic observation sites in and around Tokyo, Japan under the Key Stone Project (KSP) [KOYAMA et al, 1997]. At each of the four sites (i.e. Koganei, Kashima, Miura, and Tateyama), an 11-m antenna VLBI system, a 75-cm telescope SLR system, and a dual-frequency geodetic GPS receiver system are collocated closely with each other. The relative positions of the three reference points are repeatedly measured by ground survey measurements. The comparisons of the site coordinates and velocities estimated from three different space geodetic techniques are considered to be quite important to understand error sources of each measurement technique and to evaluate consistency of the coordinates and velocities defined in a conventional terrestrial reference frame. In this paper, site velocities estimated from VLBI and GPS observations are compared.