Real-time VLBI system

Hitoshi Kiuchi (kiuchi(AT)nict.go.jp)

Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan

Abstract: 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 (http://ksp.nict.go.jp).

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.

2. Results

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.

References

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. Return to CONTENTS