Geodetic VLBI Observations Using the Giga-bit VLBI System

Yasuhiro Koyama1(koyama(AT)nict.go.jp), Tetsuro Kondo1, Junichi Nakajima1, Mamoru Sekido1, Ryuichi Ichikawa1, Eiji Kawai1, Hiroshi Okubo1, Hiro Osaki1, Hiroshi Takaba2, Minoru Yoshida2, and Ken-ichi Wakamatsu2

1 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2 Gifu University
1-1 Yanagido, Gifu 501-1193, Japan

Abstract: A series of the geodetic VLBI experiments have been performed with the giga-bit VLBI system using a baseline between Kashima and Koganei stations in the Key Stone Project VLBI Network, and a baseline between 34-m antenna station at Kashima and a mobile VLBI station at Gifu University. The baseline vector was successfully estimated from the first test experiment on October 19, 1999. The results are still preliminary, but the challenge became the first success of the geodetic VLBI experiment at the recording speed of 1024 Mbps. Details of the giga-bit VLBI system and the geodetic VLBI experiments using the system are reported in this paper.

1. Introduction

Communications Research Laboratory has been developing the giga-bit VLBI system which has a capability to perform VLBI observations with a continuous bandwidth of 512 MHz from the baseband. The primary purpose of developing the system is to observe weak radio sources by the means of VLBI since the wide bandwidth of the giga-bit VLBI system can improve the observation sensitivity. Since the bandwidth of the giga-bit VLBI system is four times wider than the currently used other operational systems, an improvement of the signal-to-noise ratio is a factor of two. This unique characteristic is considered to be most effectively utilized in the radio astronomical applications. Several observation sessions were already performed and are planned with the baseline between 34-m antenna station at Kashima and either 45-m antenna station at Nobeyama or 64-m antenna station at Usuda. Geodetic VLBI experiments are usually performed with multiple channels and these channels are allocated to X-band and S-band frequency bands. It is because the precise time delay is obtained by using the bandwidth synthesis technique and the ionospheric propagation delay have to be corrected by performing observations in two frequency bands. The single-channel architecture of the giga-bit VLBI system prevents to apply both the bandwidth synthesis technique and the ionospheric delay correction. However, on the other hand, the giga-bit VLBI system has a potential to improve the observation sensitivity so that small aperture antennas can be used for geodetic VLBI observations. In addition, the system does not require base-band converter units and phase-calibration tone signals, and has a possibility to simplify the geodetic VLBI observation system. By considering these advantages, we determined to try to apply the giga-bit VLBI system for the geodetic VLBI observation.

2. The Giga-bit VLBI System

The giga-bit VLBI system consists of 6 components, i.e. sampler units, data recorder units, a correlator unit and three kinds of interface units. The entire view of the giga-bit VLBI system is shown in the Figure~1. The sampler unit has been developed based on a commercially available digital oscilloscope products (Tektronix TDS784/TDS580). The oscilloscope unit has a high speed analog-digital sampler chip which operates at the speed of 1024 Mbps (bit-per-second) with a quantization level of 4 bits for each sample. One of the 4 quantization bits is extracted from the digital oscilloscope and is connected to the sampler interface unit. The sampler interface unit demultiplex the 1024 Mbps of serial data stream to 32 parallel lines. The parallel data are then formated by a time control unit (DRA1000) and the data are recorded by the data recorder unit (Toshiba GBR1000). The time control unit uses the track set ID counts in the data recorder unit to control the precise timing so that recorded data can be precisely reproduced with the recorded time. The data recorder unit records digital data in the D6 standard format. The recording speed of the data recorder unit was increased so that it can record the input data stream at 1024 Mbps. The correlator interface unit (DRA2000) is used in the data correlation processing and it absorbs the large time delay which can not be absorbed in the correlator unit (Giga-bit Correlator : GICO). The correlator interface unit also multiplexes and demultiplexes 32 parallel data into 64 parallel data since the correlator unit requires the 64 parallel data lines for the input data. In the correlation processing, two DRA1000 units are connected to a common 1 PPS signal and each unit synchronizes the reproduced data by controlling the GBR1000 data recorder. The correlator system was first developed at Nobeyama Radio Observatory for the Nobeyama Millimeter Array. It has a capability to correlate data stream of 1 GHz of 2 bits data and only half of the processing speed is used for the giga-bit VLBI system.


Figure 1. Entire view of the giga-bit VLBI system.

In the observations, the GBR1000 data recorder is controlled by a notebook PC using a PCMCIA GP-IB interface card. The observation program has been originally developed to control K-4 VLBI system and a 3-m mobile antenna system using a basic command interpreter. In the data correlation processing, the GICO and GBR1000 units are controlled by a Unix workstation over the GP-IB interface. The correlated data are saved in a file on the workstation and the time delay and its rate of change are calculated. For the geodetic VLBI data analysis, a set of programs have been developed to create Mark-III database files from the output file generated by the correlator control program.

3. Experiments

The first geodetic VLBI experiment by using the giga-bit VLBI system was performed for about 6 hours on October 19, 1999 with the KSP VLBI stations at Kashima and Koganei. Several softwares have been developed to process the correlator outputs using the actual data obtained in the test experiment. Following the test experiment, two full-day geodetic VLBI experiments were performed with a baseline between a mobile VLBI station at Gifu University and the 34-m antenna station at Kashima on January 18 and on February 29, 2000. The mobile VLBI system with a 3-m VLBI antenna was transported to the campus of the Gifu University in November, 1999 for the experiments. Figure 2 shows the 3-m transportable antenna and the observation shelter of the mobile VLBI system installed at Gifu University. Figure 3 shows the geographic locations of the observation stations.


Figure 2. 3-m mobile antenna and VLBI observation shelter system at the campus of Gifu University.


Figure 3. Geographical locations of the observation sites in the Key Stone Project VLBI Network and the new site in the campus of Gifu University.

The mobile VLBI system was developed in 1987, and has been used in geodetic VLBI experiments at Kashima, Koganei, Wakkanai, Okinawa, and Minamidaito island. Minamidaito island is located on the Philippine Sea Plate and the motion of the plate with respect to the North American Plate was detected by the means of geodetic VLBI technique for the first time by using the mobile VLBI system [Amagai et al., 1990; Kondo et al., 1992]. The 3-m antenna system does not have a receiver for S-band, but instead, it has two X-band receivers to expand the frequency bandwidth within the frequency band. Although the small aperture of the antenna degrades the sensitivity of the observations, the wide frequency bandwidth of the receiver helps to improve the precision of time delay measurements. The maximum slewing speed of the antenna is 3 degrees per second for both elevation and azimuth angles, and the fast slewing capability increases the number of observations within certain length of time, which also contribute to improve the results.

The giga-bit VLBI system improves the observation sensitivity by a factor of two compared with the conventional VLBI recording system with the recording speed of 256 Mbps. The high sensitivity of the giga-bit VLBI system is considered to be most effectively demonstrated in the VLBI experiments with a small aperture antenna like the 3-m antenna system. In this scope, it was decided to conduct two geodetic VLBI experiments by using the 3-m antenna and the giga-bit VLBI system. Since the giga-bit VLBI system allows to sample baseband signal up to the frequency of 512 MHz, phase calibration signals and baseband converter units are not required and the VLBI observation system can be greatly simplified.

During the first test experiment and two full day experiments, the K-4 VLBI system was also used for the observations in addition to the giga-bit VLBI system for the comparison between the results from two independent systems. The observation tapes recorded during the two full day experiments have not been processed yet, but the observation tapes from the first test experiments have been processed and the preliminary results are compared in the Table 1.

Table 1. Comparison with analysis results from the KSP system.

K-4 system giga-bit VLBI system
Baseline Length 109099666.04+/-3.69 mm 109099667.87+/-13.14 mm
RMS Delay Residual 48 psec. 183 psec.

Although the estimated baseline lengths are in good agreement, both the estimated error of the baseline lengths and the root-mean-squared of the residual time delays obtained from the observations with the giga-bit VLBI system were worse than the results obtained with the K-4 VLBI system. This fact suggests that there remains some problem either in the data processing software or the hardware system. In either case, we are planning to continue our efforts to eliminates these problems.

4. Conclusions and Future Plans

The giga-bit VLBI system was used for three geodetic VLBI experiments and we succeeded to estimate the baseline vector for the first time with the unprecedented speed of data recording at 1024 Mbps. Although the results are still preliminary, the high sensitivity of the system will give us many potential possibilities for the innovation in the technical developments in the field of geodetic and astronomical VLBI observations.

The observation tapes recorded in the two full-day experiments will be correlated as soon as the GBR1000 and related systems returns to the Kashima Space Research Center from Nobeyama Radio Observatory after an astronomical VLBI experiment which was sjust finished on March 12, 2000. The improvements of the data processing softwares and the hardware systems will be continued by using the actual data taken in these experiments.

We are also planning to develop a new data transmission system for the real-time VLBI observations based on the Internet Protocol. The 3-m antenna mobile VLBI station at Gifu University and the 34-m antenna station at Kashima will be used for the technical developments and test observations using the high speed network connection which will become available in the near future between Gifu University and the Communications Research Laboratory. We are looking forward to expand the real-time VLBI network to the international baselines by using the Internet Protocol and are hoping to make another innovation in the technical developments for the geodetic and astronomical VLBI observations.

References

Amagai, J., H. Kiuchi, A. Kaneko, and Y. Sugimoto, "Geodetic experiments using the highly transportable VLBI station", J. Commun. Res. Lab., Vol. 37, p. 63, 1990.

Kondo, T., J. Amagai, and Y. Koyama, "Data Analysis of Geodetic VLBI Organized by the Communications Research Laboratory", published by Commun. Res. Lab., October 1992.

Updated on June 2, 2000. Return to CONTENTS