Akihiro Kaneko

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.

Development of GPS positioning system 'PRESTAR'

A precise relative positioning system using GPS, called PRESTAR, has been developed and is designed to give accurate relative position measurements, precise time synchronization, and precise orbit determinations. PRESTAR uses a high-gain beam antenna to obtain highly accurate range data through the high signal-to-noise (S/N) ratio of the received signal. It makes use of a beam antenna, necessitating a single channel sequential system. Its performance gives accurate relative position measurements, precise time synchronization, and precise orbit determinations have been demonstrated with a 55-km baseline experiment. Longer baseline positioning experiments and orbit determination experiments are being planned. The authors outline the PRESTAR system and some results of experiments. >

An application of GPS dual frequency codeless receiver for ionospheric delay to single frequenc band VLBI observation

A dual-frequency codeless Global Positioning System (GPS) receiver for measuring ionospheric delay, called GTR-2, has been developed. Results of a feasibility study of using GTR-2 for single-frequency very-long-baseline interferometry (VLBI) experiments are presented. GTR-2 uses a cross-correlation property of the P-code signals transmitted from GPS satellites to measure ionospheric total electron content (TEC) with a measurement uncertainty of about 2*10/sup 16/ electrons/m/sup 2/ along the line of site to the satellite. Ionospheric compensation of the signal from radio stars is studied using the TEC data obtained from GTR-2. >

Improvement of NTP time offset under the asymmetric network with double packets method

The NTP (Network Time Protocol) is widely used in various networks, such as from LAN to WAN. In the case of Internet, the time synchronization accuracy is degraded due to the property of the asymmetric structure of the network between the NTP server and the client. We propose a new technique to reduce the effect of this problem using double packets method.

Very long baseline connected interferometry via the ATM network

The Communications Research Laboratory (CRL), the National Astronomical Observatory (NAO), the Institute of Space and Astronautical Science (ISAS), and the Telecommunication Network Laboratory Group of Nippon Telegraph and Telephone Corporation have developed a real-time VLBI array, maximum baseline-length was 208 km. The very long baseline interferometry (VLBI) observed data is transmitted through a high-speed asynchronous transfer mode (ATM) network (2,488- Gbps [STM-16/OC-48] ATM network) instead of being recorded onto magnetic tapes. The system was composed of two real-time VLSI networks: the Keystone Project network of CRL (which is used for measuring crustal deformation in the Tokyo metropolitan area), and the OLIVE (optically linked VLBI experiment) network of NAO and ISAS which is used for astronomy (space-VLBI). The acquired VLBI data were corrected via the ATM network and the cross-correlation processing were done simultaneously. A radio flares on the weak radio source (HR1099) and weak radio sources were detected.

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.

Instrumental delay calibration for VLBI time comparison via short baseline interferometry

We have developed a short baseline interferometry (SBI) system with a directly controlled reference frequency for VLBI time comparison. SBI is a kind of portable clock method in order to calibrate the instrumental delay. Instrumental delay is an important factor in determining the accuracy of time-and-frequency comparisons.