Real-time VLBI system
Hitoshi Kiuchi (kiuchi(AT)nict.go.jp)
Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan
We have developed a real-time VLBI system that
uses a high-speed ATM (asynchronous transfer mode) network. In this real-time
system, observed 256-Mbps/station VLBI data is transmitted through a 2.488-Gbps
ATM communication network [STM-16] instead of being recorded onto magnetic tape.
The system was specially designed for the Key Stone Project (KSP), which is
concerned with measuring crustal deformation using four stations in the Tokyo
metropolitan area in Japan. Cross-correlation processing and data observation
are carried out simultaneously, and it takes about two hours to analyze crustal
deformation data after the VLBI observation is completed. In regular geodetic
VLBI experiments every other day lasting 24 hours, horizontal position
uncertainty of about 1 mm and vertical position uncertainty of about 10 mm have
been achieved. With the real-time VLBI system, one operator can handle both the
observation and the correlation processing. The system was designed to enable
automated operation throughout the entire process, and the obtained results are
open to the public via the Internet
1. The real-time VLBI system
We have two VLBI systems in operation: one tape-based and the other a real-time
system. In the KSP tape-based VLBI system, the observed data is recorded on
magnetic tape at the observing site, and the tapes are sent to the correlation
site by mail. The analysis is done the next day, so it takes at least one day
to obtain a measured value of crustal deformation. This delay was eliminated by
using an ATM network [Sato et al., 1990]) to send the VLBI data, which has a
transmission capability of up to 2.488 Gbps through an optical fiber link. The
four VLBI stations of the KSP are connected by this ATM network, and the data
transmitted from these remote observing stations to the correlation site in
Tokyo is processed in real time.
Figure 1. Block diagram of the real-time VLBI system.
A block diagram of the real-time VLBI is shown in Figure 1. The ATM network
transmits information in fixed-length packets called cells. A cell (AAL type 1)
is composed of 53 bytes of data in total, a 5-byte header and a 48-byte payload
but 1-byte is used as a sequence number to check for cell loss and mis-delivery.
The signals input to the ATM transmitter are written in the payload of the ATM
cell in arrival order. The header of the cell, which shows its destination, is
attached when the payload becomes full, and the cell is output to the 2.488-Gbps
transmission path [STM-16]. In the KSP real-time VLBI system, the signals from
the four stations can be transmitted along one transmission path, and a
cross-connect switch connects the multiple transmission paths. In the receiver,
the multiplexed signal (cells) is separated into the data for each station and
cells are disassembled and restored as digital signals after the destination in
the header data is checked. The real-time VLBI system also has functions that
compensate for delay, absorb cell-delay fluctuations in the transmission system,
and compensate for mistaken cell delivery and cell loss in the receiver. In
real-time VLBI, the data from each station is data synchronized in the receiver
(Figure 2). To absorb the transmission-path delay, the signal begins to
accumulate in the buffer memory from the time the time stamp (every 64 Mbit) is
received. This time code is generated by the input-interface [Kiuchi et al.,
1997]. The readout for the buffer memory starts immediately after the time
stamps from all observation stations have arrived allowing the timing to be
synchronized. Because the data is output to the correlator after the timing has
been synchronized, the output data for each station is correct up to the time on
the time stamp. The KSP correlation processor is an XF type that uses field
programmable gate arrays. The maximum processing data rate is up to 512 Mbps.
Figure 2. Block diagram of the data synchronization and cross-correlation processing
in the real-time VLBI system.
The data for an identical observation time taken at different stations differs in
its arrival time due to the difference in the transmission-path length. To absorb
the transmission-path delay, the signal begins to accumulate in the buffer memory
from the time the time stamp is received. The accumulated period in the buffer memory
is long when the transmission path is short, and vice versa. The readout from the
buffer memory starts immediately after the time stamps from all observation stations
have arrived. Thus the timing can be synchronized.
The KSP system has two data transfer systems: a tape-based system and a
real-time system. The systems were designed to be fully automatic throughout
the entire process, and only one operator is needed for observation and
correlation arrangement. The two systems can be operated simultaneously. Using
the tape-based system, we carried out daily 5-hour experiments starting in
January, 1995. The real-time VLBI system has been used instead of the
tape-based system since April, 1997. After a continuous 120-hour 256-Mbps test
session (from July 28 to August 1, 1997), a 24-hour experiment was done every
other day starting on September 30, 1997. In these regular geodetic VLBI
experiments, a horizontal position uncertainty of about 1 mm and a vertical
position uncertainty of about 10 mm, in terms of the internal estimation error
represented by one sigma of standard deviation, have been achieved. All
required observation and data analysis processes are fully automated, and the
obtained results are available via the Internet
(http://ksp.nict.go.jp). CRL has
also developed an lower bits rate ATM system (STM-1: 155.6 Mbps) for
international real-time VLBI experiments. We carried out real-time VLBI
experiments between Koganei and Kashima using the STM-1 ATM network, good
fringes were obtained.
3. View of the real-time correlation processing system
A photograph in Fig 3 shows the real-time correlation processing system. Left
side is ATM receiver and other two racks are correlation processor racks. The
correlators are lower three units in each correlation processor rack.
Figure 3. View of the real-time correlation processing system.
Kiuchi, H., J. Amagai, S. Hama, and M. Imae, K-4 VLBI data-acquisition system,
Publ. Astron. Soc. Japan, 49, 699--708, 1997.
Sato, K., S. Ohta, I. Takizawa, Broad-band ATM network architecture based on virtual
paths, IEEE Trans. Commun., 38, 1211--1222, 1990.
Updated on June 2, 1998.
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