Optical Linked VLBI in Japan

Junichi Nakajima1(nakaji(AT)nict.go.jp), Yasuhiro Koyama1, Tetsuro Kondo1, Mamoru Sekido1, Yukio Takahashi2, Eiji Kawai1, Hiroshi Okubo1, Hitoshi Kiuchi2, Noriyuki Kawaguchi3, Moritaka Kimura3, Kenta Fujisawa3, and Hideyuki Kobayashi3

1 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2 Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan
2 National Astronomical Observatory
2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

1. Introduction

Optical linked VLBI is the key technology to change historical VLBI style in this decade. Not only achieve high sensitivity by the fast data transfer beyond magnetic tapes, but other features are also attractive for the VLBI operation. The optical linked data transfer without the media transport and its real-time correlation capability enables the quick look of observations. This means our global VLBI network will serve as a connected interferometer. Dynamic scheduling and real-time fringe check eliminate independent fringe test before observation and after standby. These feature of optical linked VLBI minimize observation failure usually known long after observation and maximize telescope resources. Currently operated optical VLBI network KSP(Key Stone Project), GALAXY (Giga-bit Astronomical Large Array Xross-connect) and other experimental based optical VLBI in Japan are briefly summarized in this report.

2. Currently Operated Optical Linked VLBI in Japan

2.1 Key-stone network

One of the long operated optical linked VLBI for geodetic data production is the KSP-VLBI around Tokyo metropolitan area. The 256Mbps VLBI data is transmitted via ATM (Transfer Protocol). The retrieved data from four telescopes are adjusted their data epoch at buffers in front of the Koganei correlator. The highly reliable KSP VLBI is operated every other day and additional R&D observations are scheduled between the geodetic regular schedule [Kiuchi et al., 1999].


Figure 1. Optical Connection of KSP and GALAXY domestic optical VLBI.

2.2 GALAXY

In 1998 geodetic KSP-network and astronomical VSOP/OLIVE network are mutually connected. The GALAXY is appeared under three different institutes. The network is consist from Usuda 64 m (ISAS), Nobeyama 45 m (NRO), Kashima 34 m (CRL) and four 11m in Kashima, Koganei, Miura, Tateyama [Kiuchi et al., 1999]. Correlation capability is four station maximum at Koganei correlator and there is additional limitation in telescope combinations. Possible observation frequency are listed in Table 1. The GALAXY network performed dynamical change of schedule when they found the HR1099 flare up stars variability during the observations. Mitaka VSOP-FX correlator is planned to share the processing.

Table 1. Two currently operated optical VLBI network in Japan.

KSP real time VLBI GALAXY real time VLBI
Telescopes 4 x 11m(KSP) Usuda 64m, Nobeyama 45m,
Kashima 34m, 4 x 11m (KSP)
Receiver Frequency 2/8 GHz 1.6/2/5/8/22GHz
Correlator (Terminal) Koganei (K4) Koganei (K4), Mitaka (VSOP-FX)
Baseline Maximum Length 134 km 208 km
Scheduling Automatic, Static Schedule Operator control, Dynamic schedule

3. Optical Linked VLBI with Experimental Result

3.1 Giga-bit ftp-VLBI

CRL has been developing the Giga-bit VLBI system which has a capability to perform 1024 Mbps VLBI observations. In this system, we realized ftp based data transfer which enables us the fringe check during observation sessions. Using large memory 1024 Mbit(=128 MByte) the unit freeze the stream data of assigned epoch. Beside the continuous tape recording, a connected PC start ftp transfer from a station to the other. After the ftp, immediately software correlation started and show the result. The software correlator has an advantage in their flexibility to extended lag windows. Although the delay resolution is less than 1-ns at 1024~Mbps/1~bit/1~ch system, we are able to know the precise clock offset before tape correlation. Except small telescope combinations, One second of the Giga-bit observations is enough to detect fringes from strong sources. Amount of ftp data is reduced when large telescopes are used. Figure 2 shows the ftp-VLBI 3C279 fringe between Nobeyama 45 m and Kashima 34 m from 12.8 MByte (0.1 sec) data. The processed lag window is expanded to about 50000 in this case. Start from ftp, it takes about 10 to 15 minutes to see the fringes when the observation is normal.


Figure 2. Gigabit ftp-VLBI fringe between Kashima 34m and Nobeyama 45m.

3.2 Giga-bit SDI-based optical system

There are several approach to establish over Giga-bit VLBI data transmission. One method is the high speed ATM and IP enhancement. The standardized transfer protocol promise commercial based extension in future. One the other hand, the interface employed in high speed consumer instruments are focused for local usage. In this case, the technology is concentrated into key devices and much cheaper than the ATM under its strict definition. SDI (Serial Data Interface) used in HDTV (High Definition TV) instrument connection support optical data transmission up to 1.5 Gbps and 20 km distance without optical repeater. We have developed the interface 1024 Mbps VLBI data and attribution data transfer Figure 3 [Nakajima et al., 1999]. Large digital delay is occurring at parallel to serial data transfer. To compensate this delay the concept of VSI (VLBI standard Interface, works important role. In the idea, 1 PPS tick label the data at DAS (Data Acquisition System). This will eliminate all digital delays introduced at latter digital/analog component and only the clock offset between the telescopes are remained in processing. There is no need to use cable counter when the DAS located near the receiver and the digital optical transmission is installed. We confirmed fringes and 1024 Mbps transmission performance by experimentally installing the unit in the correlation system as in Figure 4. Short distance interferometry experiment between Kashima 34 m and Kashima 26 m are planned.


Figure 3. SDI Gigabit optical transmission system.

3.3 Other optical-based VLBI experiment in Progress

The other optical based VLBI projects are follows. 155 Mbps STM-1 ATM transmitter is completed to provide real-time VLBI. The unit will support VLBI via an international optical link or a satellite link (Kiuchi, personal communication). Recently, National Astronomical Observatory carried out 2048 Mbps VLBI data transmission test which will fully utilize 2.5 Mbps ATM performance. They succeeded send-back more than hundreds kilometers(Kawaguchi, personal communication). The GALAXY network is expected to enhance the performance up to 1024 Mbps in future. NTT (Nippon Telegraph and Telephone Co. Ltd.) is developing high speed "IP over ATM" interface which will adapt the KSP system for future expansion. CRL is also developing IP-based VLBI system less than 1Mbps. The variable rate IP-VLBI is realized by PC computer based technology. Experimenting with a simple frequency standard, The system will provide possibility of preliminary VLBI experiment at many dishes never used for VLBI. This will give a experience and opportunity to join the VLBI network to new groups. In future, when we afford to use broadband optical link between remote place this will bring us standard bandwidth VLBI observations. All real-time VLBI progress brought about direct result never obtained by tape based VLBI. But it should be noted that the fringe finding method and observation procedure is difficult than tape based VLBI. Since most of the current VLBI system designed under tape based concept, future system should be improved to adapt automatic optical linked VLBI operation.

References

Kiuchi, H., T. Kondo, M. Sekido, Y. Koyama, M. Imae, T. Hoshino, and H Uose, "Real-time Data transfer and Correlation System", J. Commun. Res. Lab., Vol. 46, p. 1, 1999.

Kiuchi, H., Y. Takahashi, A. Kaneko, H. Uose, S. Iwamura, T. Hoashino, N. Kawaguchi, H. Kobayashi, L. Fujisawa, J. Amagai, J. Nakajima, T. Kondo, S. Iguchi T. Miyaji, K.Sorai, K. Sebata, T.Yoshino, and N. Kurihara, IECE Trans. Commun., VOL. E83-B. NO.2, FEB., 2000.

Nakajima, J., "Optival and Coaxial Serial Data Transmitter /Receiver for Giga-it VLBI and VSI", IVS TDC Center News.,Commun. Res. Lab., Ser. 15, p.26, 1999.

Updated on June 2, 2000. Return to CONTENTS