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.
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.
