Real Time VLBI Plan in the Key Stone Project

Michito Imae, Hitoshi Kiuchi, Shin'ichi Hama,Taizoh Yoshino
Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184, Japan

Tadahiro Goto,Yukio Takahashi
Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314, Japan

Hiroyuki Hara, Hisao Uose

1. Introduction

CRL has been constructing the precise crustal deformation measuring system in the Tokyo Metropolitan Area using the space geodetic technique, such as VLBI and SLR, which is called "Key Stone Project" (KSP) from 1993.

The main target of this project is monitoring the crustal deformation in mm order level daily for the study of earthquakes.

For this purpose the real time VLBI, real time received data transmission and quick data processing, is being planned in cooperation with NTT (Nippon Telephone and Telegraph Co.), with high rate digital data transmission links among the KSP stations.

This report describes the concept of the real time VLBI which is now under construction.

2. Limit of Recorder Base VLBI and Real Time VLBI

Development of the VLBI system was made possible by progress in highly stable frequency standards, such as the hydrogen maser, and high speed digital data recorder. But due mainly to the recording rate of the data recorder, there are several limitations in the VLBI system.

Some of them are

On the other hand, nowadays high rate digital data transmission links using optical fiber cable are available or under development which have the transmission rate of several hundred Mbps or exceed 1 Gbps.

The real time VLBI using such a high rate digital communication link will be one of the "breakthroughs" of VLBI to improve the performance and to shorten the duration to output the final data.

3. Real Time VLBI Plan in the KSP

Figure 1 shows the block diagram of the real time VLBI in the KSP project.

The received signal is digitized by the acquisition system at each station and transmitted to the correlation station by using high rate ATM (Asynchronous Transfer Mode) link through the ATM transmitter.

The transmitted data from each station are combined at the ATM cross-connector at NTT Mitaka and transmitted to the correlation center located at CRL Koganei. The ATM receiver placed at the correlation center absorbs the jitter of the ATM cells and transmission delays using memories.

Each ATM link has the ability of 2.4Gbps transmission rate, but we use the VLBI sampling rate of 256Mbps to keep the compatibility to the recorder base VLBI experiments.

The correlator which is used in the real time VLBI is basically a same as the tape-based one, and it has the processing rate up to 512Mbps. After the real time VLBI using 256Mbps sampling rate technology will be confirmed, we will challenge higher rate data sampling for the next step.

4. The Future of Real Time VLBI

The real time VLBI will be a very powerful solution to improve the VLBI's capabilities. But it is not the all-round tool, because So it is important to consider the real time VLBI's application field.

5. Conclusion

The real time VLBI facilities are now under construction for the KSP project, and its first fringes will be detected early in 1996.

But the regular base real time VLBI operations will be started after careful comparison with the recorder base VLBI experiments. The preliminary data will be presented in the next TDC's news.

Updated on November 2, 1995.
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