Feb 2, 2000 - SUMMARY. The VSOP terminal is a new data-acquisition system for the Very-Long-Baseline Interferometry (VLBI). This terminal was primarily ...
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PAPER
Development and Performance of the Terminal System for VLBI Space Observatory Programme (VSOP) Satoru IGUCHI† , Student Member, Noriyuki KAWAGUCHI†† , Member, Seiji KAMENO†† , Hideyuki KOBAYASHI††† , Nonmembers, and Hitoshi KIUCHI†††† , Member
SUMMARY The VSOP terminal is a new data-acquisition system for the Very-Long-Baseline Interferometry (VLBI). This terminal was primarily designed for ground telescopes in the VLBI Space Observatory Programme (VSOP). New technologies; higher-order sampling and digital filtering techniques, were introduced in the development. A cassette cart was also introduced, which supports 24-hour unattended operations at the maximum data rate of 256 Mbps. The higher-order sampling and digital filtering techniques achieve flat and constant phase response over bandwidth of 32 MHz without using expensive wide base-band converters. The digital filtering technique also enables a variety of observing modes defined on the VSOP terminal, even with a fixed sampling frequency in an A/D converter. The new terminals are installed at Nobeyama, Kashima, Usuda, Mizusawa, and Kagoshima radio observatories in Japan, and are being used in VSOP and other domestic VLBI observations. In this paper the key features of the VSOP terminal focusing on these advanced technologies are presented, and the results of performance tests are shown. key words: VLBI, VSOP, higher-order sampling, digital lter,
bandwidth, phase
1.
Introduction
The VLBI Space Observatory Programme, VSOP, enables VLBI observations on baselines up to three times larger than the Earth’s diameter. The space radio telescope, HALCA (the Highly Advanced Laboratory for Communications and Astronomy), was launched on February 12, 1997 and injected into an elliptical Earth orbit, with an apogee height of 21,000 km and a perigee height of 560 km [1]. The primary scientific goals of the VSOP mission are to clearly resolve individual components of jets associated with radio galaxies and to discern bends in jet even close to the core. These will improve our understandings on the physical nature and origin of jet components. In a radio astronomy observation Gaussian noise from a celestial radio source is received by a radio telescope (this is called a “signal”). In a conventional VLBI Manuscript received May 20, 1999. The author is with the Department of Electronic Engineering, The University of Electro-Communications, Chofushi, 182-8585 Japan. †† The authors are with National Astronomical Observatory, Mitaka-shi, 181-8588 Japan. ††† The author is with Institute of Space and Astronautical Science, Sagamihara-shi, 229-8510 Japan. †††† The author is with Communications Research Laboratory, Koganei-shi, 184-8795 Japan.
data-acquisition system, the signal in an analog form is converted in frequency to a base-band, then digitized and recorded on a magnetic tape, as shown in Fig. 1. Rogers [2] in 1971 first developed a base-band converter of 2-MHz bandwidth by using a 90-degree phase shifter in a wide base band as shown in Fig. 2. It is technically very difficult to achieve flat phase response over an entire band from 0 Hz to the band edge with a conventional phase shifter. It is more difficult to implement that on a base-band converter, especially if the band is very wide. Many narrow-band channels have been synthesized by local oscillators in previous VLBI terminals because it is very difficult to make a base-band converter of a wide single band. However, narrow multi-channel systems require much instrumentation and are therefore impossible to set on a space radio telescope because of the weight and size limitations on a space craft. For these reasons, one or two observing channels of very wide bandwidth were required to support VSOP ob-
Fig. 1
Conventional VLBI data acquired system.
†
Fig. 2 A functional block diagram of conventional base-band converter. ω0 , ωU and ωL are the angular frequency of the center band, Upper Side-Band (USB) and Lower Side-Band (LSB), respectively.
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Fig. 3 Difference in the frequency allocation of the HALCA data acquisition system and the ground VSOP terminal.
servations in a new data-acquisition system. A total bandwidth of 32 MHz was requested to support highsensitivity VSOP observations. It is only possible to achieve such a wide bandwidth in the data acquisition system by introducing new technologies; higher-order sampling and digital filtering techniques [3], or by accepting a loss in the base-band conversion of more than 40 dB. In the case of the space radio telescope, HALCA, the latter technical solution was taken because the new technologies had not been tested at the stage of the satellite planning and development. The primary aim of the new development presented here is to realize a bandwidth of 32 MHz first for ground applications and to have a flat and constant phase response over a wider bandwidth than had previously been. In this paper the key features of the VSOP terminal focusing on these advanced technologies are presented, and the results of performance tests are shown. 2.
Principles of VSOP Terminal Design
In VSOP observations the maximum bandwidth is 32 MHz, wider than had previously been achieved in VLBI observations. A “higher-order sampling technique” was used to realize the wide bandwidth of 32 MHz because it can avoid paying the heavy penalty of sensitivity loss [3]. The frequency allocations of the HALCA data acquisition system and the ground VSOP terminal are shown in Fig. 3. In a HALCA data acquisition system, signals on observing frequencies f1 and f2 are frequency converted to a base-band of 0–32 MHz, respectively, then sampled at the usual Nyquist rate of
Fig. 4 Frequency diagram representing the compatibility between the VLBA and the VSOP terminal in case of the 8-MHz bandwidth.
64 MHz. In the VSOP terminal, signals on the same observing frequencies are frequency converted to the 64–96 MHz band, not to a base-band, then sampled at a rate of 64 MHz. This sampling is called “1-st order sampling” [3]. The new sampling technique also performs the frequency conversion in parallel. The VSOP terminal has been designed with the 1-st order rather than n-th order sampling as the first step of development. In both terminals the observing channels are located at the same observing frequencies, so that the signals can be correlated with each other. A design has been also made to keep compatibility with other existing terminals, such as the VLBA (Very-Long-Baseline Array) terminal [4], for supporting ground VLBI observations. A “digital filtering technique” was utilized because it enables flexible and sharp selection of an arbitrary bandwidth. A frequency diagram representing the compatibility between the VSOP and VLBA terminals is shown in Fig. 4, in case of a sampling mode of two 8-MHz channels of Upper Side-Band (USB) and Lower Side-Band (LSB). Compatibility between the two terminals is achieved as follows. First,
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Fig. 5
VSOP terminal architecture.
observing channels are located at the same observing frequencies as the USB/LSB channels in the VLBA terminal (Fig. 4(a)). Then, the data in frequency channels from 72 to 88 MHz are frequency converted, and cut with an analog bandpass filter of a band of 64–96 MHz (Fig. 4(b)). The output signals from the analog filter are distributed to two sampler channels via an IF distributor (Figs. 4(b),(c)). The two signals are then digitized with each sampler (sampler 1,2) at a Nyquist rate of 64 MHz (Fig. 4(c)). The data in frequency channels from 72 to 88 MHz are folded in a band of 8–24 MHz. The two bandwidths of 8–16 MHz and 16–24 MHz are cut using a digital filter in each sampler, which correspond to LSB and USB channels in the VLBA terminal, respectively (Fig. 4(d)). Finally, by reducing the output rate to 16 MHz, the data in frequency channels from 8 to 16 MHz are folded (Fig. 4(e), left) and the data in frequency channels from 16 to 24 MHz are converted (Fig. 4(e), right) to frequencies ranging from 0 to 8 MHz, respectively. The data cut by digital filters are set on the same observing frequencies as the USB and LSB in the VLBA terminal. The signals acquired with the VSOP and VLBA terminals can then be correlated using a digital filtering technique. For other narrower band observations such as 4, 2, 1 MHz, observers can use the same method as for the 8-MHz case and set the digital filters to narrower bandwidths. 3.
VSOP Terminal Architecture
The VSOP terminal was developed for supporting VSOP observations, and for testing the new technologies of higher-order sampling and digital filtering. The
primary goal of the design is to realize a flat and constant phase response over the maximum bandwidth of 32 MHz. An overview of the instrumental architecture is shown in Fig. 5. The terminal consists of an analog processing unit (APU), a digital sampling unit (DSU), a timing control unit (TCU), a recording unit, and a control computer. 3.1 Sampling Architecture The APU consists of an IF distributor, two single sideband mixers (SSBM) and two analog filters. The IF distributor distributes an IF signal of 500–1000 MHz to two ports. The IF signals are transmitted to the SSBMs. The SSBM consists of an in-phase power divider, quadrature hybrids and two double-balanced mixers. In the mixers, a local frequency is tuned from 500 to 1000 MHz in 1-MHz steps. The frequency is generated in a synthesizer. The synthesizer is locked in phase to a standard signal of 10 MHz generated by a hydrogen maser. Signals are frequency-converted at the SSBMs and restricted in bandwidths by two analog filters in the APU. In the analog filter, a band of 64–96 MHz is cut out for the 1-st order sampling as noted in Sect. 2. The analog filter is a six-pole butterworth bandpass filter. A signal from the analog filter is distributed to two IF channels. A total of four channels are available in the DSU. The DSU consists of four 8-bit A/D converters at a fixed sampling frequency of 64 MHz, four FIR digital filters with 63 taps of a 10-bit word, a multiplexer, and a bit selector. The selector rounds the 18-bit word (8+10) down to either one bit per sample (two-level
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Relation between output data rate and clock rate.
Data Rate (Mbps) 256 128 64
Table 2
Clock (MHz) 32 16 8
Number of logical lines
3.3 Recording System
8 8 8
Sampling modes with the VSOP terminal.
Data Rate (Mbps) 256
Bandwidth (MHz) 32 32 16 128 32 32 16 16 8 64 32 16 16 8 4 ∗: VSOP mode, ∗∗: VLBA
∗ ∗ ∗, ∗∗ ∗∗
∗∗ ∗∗ mode.
Quantization (bit) 2 1 2 2 1 2 1 2 1 2 1 2 2
IFs (CH) 2 4 4 1 2 2 4 4 1 1 2 2 4
quantization) or two bits per sample (four-level quantization). In two-level quantization, the two-level word indicates that the signal voltage is positive or negative. In four-level quantization, the four-level word is assigned, in order of input level, 00, 01, 10, 11, from the lowest to the highest voltage. The code is the same as the VLBA terminal specification [4]. The DSU output has eight parallel lines, which consist of sixteen physical wires for complementary ECL data transmission. The output data rate is 64, 128, or 256 Mbps at a clock rate of 8, 16, or 32 MHz in a logical line, respectively (Table 1).
Data recorders of an ID-1 type cassette tape (American National Standard 19-mm Type ID-1 Instrumentation Digital Cassette Format) are now used in various fields. The ID-1 type cassette uses 13-µm thickness FeO tape. A tape length of 4500-feet has a data storage capacity of 920 Gbits. The recording system has a cassette cart (IDCC), which supports unattended tape changing. The data recorder (IDREC) and the IDCC were introduced for realizing unattended operation. The IDREC has rotary heads, which consist of eight recording heads and eight play back heads. Soon after recording, the data are played back with a time lag of 54.162 msec. A bit error rate of the IDREC can be monitored through a host computer. The bit error rate after the error correction is better than 10−10 . The IDREC has helical data tracks, two longitudinal annotation tracks and a control track. Data are recorded on the helical data tracks. The playing time of a tape is 4, 2, and 1 hours at recording data rates of 64, 128, and 256 Mbps, respectively. The IDCC consists of a bar-code reader, a tape changer, and a rack. The rack has a storage capability of 24 cassette tapes. Each cassette tape is identified with a bar-code labeled on the cassette container. The tape is changed by a mechanical arm. The IDCC supports unattended operation for 24 hours at the maximum rate of 256 Mbps. The continuous recording time without human operation is longer than 9 and 8.5 hours in the VLBA [4] and S2 [6] terminals, respectively, at a recording data rate of 128 Mbps. The storage capability of the IDCC is 22 Tbits in total. A recording system with the IDREC and the IDCC has also been introduced in the K-4 system [7].
3.2 Sampling Modes 3.4 Timing Control Unit A variety of observing modes is necessary to keep compatibility with the HALCA data acquisition system and other existing terminals. In the VSOP terminal, observing modes are changed with a digital filter and a bit selector implemented in the DSU. The observing modes are listed in Table 2. Data transmission from HALCA to ground tracking stations is made through a communication link at a bit rate of 128 Mbps [5]. The three modes marked by the character “∗” in Table 2 are actually used in VSOP observations. These are, one 32MHz channel of 2-bit quantization, two 32-MHz channels of 1-bit quantization and two 16-MHz channels of 2-bit quantization [1]. The new terminal is also compatible with the VLBA terminal [4]. The modes marked by the character “∗∗” in Table 2 are used. These are, two 16-MHz channels of 2-bit quantization (VSOP mode), four 8-MHz channels of 2-bit quantization, two 8-MHz channels of 2-bit quantization and four 4-MHz channels of 2-bit quantization.
The Mark III terminal has a sensitivity loss of 0.8% caused by writing a UTC time code on a data track [8]. Data are recorded on a helical track in the IDREC. The IDREC can not write a time code on the helical track. The TCU writes the time code on the control track in the IDREC. The standard signal in the TCU is locked to a UTC signal of 1 Hz generated by a hydrogen maser. The TCU translates a UTC time code to a time code called the Track Set Sync (TSS) ID code. The time code is represented by a 23-bit word. The 1 TSS ID code changes the state at 144432 clocks by ID-1 definition. As the result, the rotation rate of rotary heads in the IDREC is controlled with the TCU. Since time codes do not overwrite data on a data track in the VSOP terminal, no sensitivity loss is incurred.
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3.5 Field System VLBI observations are usually managed with a VLBI oriented operating system called “field system,” in which individual instruments are controlled under an observation plan. The observation plan is described in a schedule file. The schedule file (SCHD) defined in the Mark III terminal is now used in a number of VLBI observatories [8]. In the schedule file, only the control parameters of an antenna and its recorder are defined. The VSOP field system (CFS) was requested to control all instruments. A VSOP schedule file was introduced, using a format slightly modified from that of the SCHD. Observing parameters and the sampling mode are added to the SCHD. After VSOP observations, all observing and control parameters are written into log files (LOG). The LOG is utilized for correlation processing at the VSOP correlator in Mitaka [9]. Error or warning messages are displayed on the front panel of all instruments via the CFS. The CFS also displays error or warning messages of the instruments. The CFS provides a high-level interface to the VLBI operator, eliminating the need to worry about details of the underlying hardware. 4.
Observations with VSOP Terminal
4.1 Phase and Amplitude Responses in 16-MHz Bandwidth The sampling mode using two 16-MHz channels of 2bit quantization was tested with a VLBI observation on April 10, 1998. On a baseline between the Usuda 64-m and the Kashima 34-m radio telescopes, the celestial radio source 3C273 (IAU name 1226+023) was observed at 4930 MHz. The observed data were crosscorrelated with the VSOP correlator at Mitaka [9]. The correlator searches for a geometric delay of the baseline and a rate of the delay by correcting instrumental delays caused in individual instruments. The output data format is a complex cross-power spectrum. The output data include the residual delays and delay rates due to fluctuations of path length of the atmosphere and the ionosphere, etc. The residuals show a phase slope over an entire band from 0 Hz to the band edge. The phase slope is corrected with a calibration method, called fringe fitting. The fringe fitting was done using the Astronomical Image Processing System (AIPS) developed in the National Radio Astronomy Observatory. AIPS can also calibrate the amplitude of cross-power spectra. The amplitude calibration has been taken by applying antenna gain factors and system temperature measurements. A cross-power spectrum with an integration time of 300 seconds, after the above corrections for phase and amplitude effects, is shown in Fig. 6(a).
Fig. 6 Phase and amplitude responses of 16-MHz bandwidth at two IFs after amplitude calibration and fringe fitting in VSOP (a) and VLBA (b) terminals, respectively. The integration time is 300 seconds.
A true variance of phase with the cross-power spectrum is within 1 degree as shown in Fig. 6(a) (top). There is an inverse relation between the true variance of phase and a Signal-to-Noise Ratio (SNR) in VLBI [10]. The SNR is decided with flux density of a radio source, antenna gains and system temperatures. The system temperature includes the phase responses caused in the VSOP terminals. The true variance of phase within 1 degree corresponds to a sensitivity loss of 0.005 %. Therefore, using higher-order sampling and digital filter techniques, good phase response with a standard variance less than 1 degree was successfully achieved, that is, a sensitivity loss of less than 0.005 %. Slopes in the amplitude response appear in the low frequency band of 0–4 MHz and the high frequency band of 15–16 MHz (Fig. 6(a) (bottom)). The slope in the low band is caused by an analog filter in the APU. On the other hand, the slope in the high band is mainly
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caused by a digital filter in the DSU. The amplitude response shows that the digital filter can easily achieve sharper cut-off response than that of the analog filter. A cross-power spectrum obtained with a VLBA observation is shown in Fig. 6(b). With a baseline between the Owens Valley and the Pie Town 25-m radio telescopes, the celestial radio source OT081 (IAU name 1749+096) was observed at 4800 MHz on August 20, 1998. The observed data were cross-correlated with the VLBA correlator at Soccoro [4]. A data calibration was taken using the same analysis method as the above AIPS reduction. A sharp cut-off at about 13 MHz in phase response is shown in Fig. 6(b) (top). A dent response in frequency channels from 3 to 10 MHz is also shown in Fig. 6 (bottom). These responses are caused by a conventional base-band converter in the VLBA terminal. The responses, especially, are due to a wideband 90-degree phase shifter. In the VSOP terminal, higher-order sampling and digital filter techniques are substituted for the base-band converter, so that the responses are improved. The authors should note that the good phase response is successfully achieved in the difference between the Usuda and Kashima radio telescopes and not necessary in the individual instruments. This means that a higher-stability data-acquisition system has been achieved.
Fig. 7 Phase and amplitude responses of 32-MHz bandwidth at one IF on a baseline between Usuda and Kashima telescopes after amplitude calibration and fringe fitting. The integration time is 180 seconds.
4.2 Phase and Amplitude Responses in 32-MHz Bandwidth The sampling mode using one 32-MHz channel of 2-bit quantization was tested with a VLBI observation on March 2, 1999. On a baseline between the Usuda 64-m and the Kashima 34-m radio telescopes, the celestial radio source 3C84 (IAU name 0316+413) was observed at 8430 MHz. The observed data were cross-correlated with the VSOP correlator at Mitaka [9]. Data calibrations were made using the same analysis method as the AIPS reduction noted in Subsection 4.1. After correction for phase and amplitude effects, the derived cross-power spectrum with an integration time of 180 seconds is shown in Fig. 7. The phase response is shown in Fig. 7 (top), in which a ripple with a cycle of about 32 nsec is apparent. The authors believe that the ripple within five degrees arises from a standing wave in a waveguide which connects the receiver and the antenna feed at X-band in the Usuda 64-m telescope. Slopes in the amplitude response are apparent in the low frequency band of 0–4 MHz and the high frequency band of 24–32 MHz (Fig. 7 (bottom)). These slopes are caused by the 64–96 MHz analog bandpass filter in the APU. A bandwidth of 32 MHz was successfully achieved using a higher-order sampling technique, which is a wider bandwidth than that of previous ground terminals.
Fig. 8 First fringe results of the VSOP observation with the VSOP terminal. Radio source is PKS 1519-273, frequency 1648 MHz, the Usuda-HALCA baseline.
4.3 First VSOP Observation Result After the launch of the HALCA on February 12, 1997, VSOP observations have been carried out on baselines between HALCA and the Usuda 64-m telescope, in which the VSOP terminal is installed. The celestial radio source PKS 1519-273 was observed on this baseline at 1648 MHz on May 7, 1997. On May 13, 1997, first fringes were found with the VSOP correlator (Fig. 8). The fringes had a delay of −4.4 microseconds and a delay rate of 1.44 nanoseconds per second. The signalto-noise ratio was 14 with a 64 second coherent integration. These values were obtained despite the use of incomplete orbital data for HALCA. The bandwidth of 16 MHz supporting conventional VSOP observations thus was successfully achieved in the development of a new data acquisition system.
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5.
Fig. 9 Comparison between the power spectrum and the cross power spectrum (Iguchi and Kawaguchi [11]).
4.4 Folding Noise in VLBI In signal communications and processing, filter response is designed so as to exclude folding noise. The decrease of the folding noise in the phase rotation process on VLBI cross correlation was pointed out by Iguchi and Kawaguchi [11]. They reported a new optimum filtering method taking the process into consideration. To approve the decrease of the folding noise, a VLBI expermiment was made with the VSOP terminal. The aliasing phenomenon was demonstrated using a digital filter. The 63 tap coefficients of the digital filter can be easily changed in the VSOP terminal. Filter response was designed at the cut-off frequency of 12 MHz that is 1.5 times as wide as bandwidth. On a baseline between the Usuda 64-m and the Kashima 34-m radio telescopes, the radio source 3C273 was observed at 4930 MHz on April 10, 1998. The observational results are shown in Fig. 9. The power spectrum is represented by a geometric mean of power spectra in each telescope. The cross-power spectrum is obtained in VLBI correlation process. Folding noise is shown across the entire band in the power spectrum, but not in the cross-power spectrum. The folding noise shown in the band of 0–4 MHz in the power spectrum is provided with band response of an analog filter in the APU. The results show that the folding noise is reduced in the phase rotation process of the VLBI correlation. On the other hand, the fluctuation of amplitude in the band of 6–16 MHz is 2 times as large as that of other frequency channels in Fig. 9. This is caused by folding random noise in the band of 16–26 MHz, as random noise is not reduced in the VLBI correlation process. The decrease of the folding noise in the VLBI correlation process was successfully demonstrated with data from VSOP terminals.
Conclusion
The VSOP terminal has been developed with a bandwidth as wide as 32 MHz for supporting ground VLBI observations. In the development of the new terminal, the new technologies of higher-order sampling and digital filtering were introduced. The new techniques enable frequency conversion without a wideband 90-degree phase shifter. The flat and constant phase responses over the wide bandwidth of 32 MHz were successfully achieved using these new techniques. Moreover, the new techniques also enable 13 observing modes defined on the VSOP terminal, even with a fixed sampling frequency of the A/D converter. These techniques are important advances for the future development of VLBI observing instruments. The results of VSOP observations with VSOP terminals were displayed, and the discovery of the new aliasing phenomenon in VLBI cross correlation process was successfully described. The new terminals have been installed at Nobeyama, Kashima, Usuda, Mizusawa, and Kagoshima radio observatories in Japan, currently, and are being used for both VSOP and general VLBI observations. Acknowledgement The authors are grateful to the VSOP project team and the VLBI staffs in CRL. The authors express our sincere thanks to Dr. K. Fujisawa, Dr. K.M. Shibata, (NAO), Dr. T. Miyaji (NRO) and Dr. J. Nakajima (CRL) for help in setting up instruments for VLBI experiments. The authors thank Dr. T. Miki (UEC), Dr. P. Edwards (ISAS) and Dr. S. Enome (NAO) for useful comments. References [1] H. Hirabayashi, H. Hirosawa, H. Kobayashi, Y. Murata, P.G. Edwards, E.B. Fomalont, K. Fujisawa, T. Ichikawa, T. Kii, J.E.J. Lovell, G.A. Moellenbrock, R. Okayasu, M. Inoue, N. Kawaguchi, S. Kameno, K.M. Shibata, Y. Asaki, T. Bushimata, S. Enome, S. Horiuchi, T. Miyaji, T. Umemoto, V. Migenes, K. Wajima, J. Nakajima, M. Morimoto, J. Ellis, D.L. Meier, D.W. Murphy, J.G. Smith, S.J. Tingay, D.L. Traub, R.D. Wietfeldt, J.M. Benson, M.J. Claussen, C. Flatters, J.D. Romney, J.S. Ulvestad, L.R. D’Addario, G.I. Langston, A.H. Minter, B.R. Carlson, P.E. Dewdney, D.L. Jauncey, J.E. Reynolds, A.R. Taylor, P.M. McCulloch, W.H. Cannon, L.I. Gurvits, A.J. Mioduszewski, R.T. Schilizzi, and R.S. Booth, “Overview and initial results of the very long baseline interferometry space observatory programme,” Science, vol.281, pp.1825– 1829, 1998. [2] A.E.E. Rogers, “Broad-band passive 90◦ RC hybrid with low component sensitivity for use in the video range of frequencies,” Proc. IEEE, vol.59, no.11, pp.1617–1618, 1971. [3] N. Kawaguchi and S. Iguchi, “Higher-order sampling and digital filtering techniques for a VLBI data acquisition system,” IEICE Trans. Commun., to be submitted.
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[4] P.J. Napier, D.S. Bagri, B.G. Clark, A.E.E. Rogers, J.D. Romney, A.R. Thompson, and R.C. Walker, “The very long baseline array,” Proc. IEEE, vol.82, pp.658–672, 1994. [5] N. Kawaguchi, H. Kobayashi, T. Miyaji, H. Mikoshiba, A. Tojo, Z. Yamamoto, and H. Hirosawa, “Ground Supporting Facilities for VSOP Observations,” in VLBI Technology: Progress and Future Observational Possibilities, eds. T. Sasao, S. Manabe, O. Kameya, and M. Inoue, pp.26–33, Terra Scientific, Tokyo, 1994. [6] R.D. Wietfeldt, D. Baer, W.H. Cannon, G. Feil, R. Jakovina, P. Leone, P.S. Newby, and H. Tan, “The S2 very long baseline interferometry tape recorder,” IEEE Trans. Instrum. & Meas., vol.45, no.6, pp.923–929, 1996. [7] H. Kiuchi, J. Amagai, S. Hama, and M. Imae, “K-4 VLBI data-acquisition system,” Publ. Astron. Soc. Japan, vol.49, no.6, pp.699–708, 1997. [8] A.E.E. Rogers, R.J. Cappallo, H.F. Hinteregger, J.I. Levine, E.F. Nesman, J.C. Webber, A.R. Whitney, T.A. Clark, C. Ma, J. Ryan, B.E. Corey, C.C. Counselman, T.A. Herring, I.I. Shapiro, C.A. Knight, D.B. Shaffer, N.R. Vandenberg, R. Lacasse, R. Mauzy, B. Rayhrer, B.R. Schupler, and J.C. Pigg, “Very-long-baseline radio interferometry: The Mark III system for geodesy, astrometry, and aperture synthesis,” Science, vol.219, pp.51–54, 1983. [9] K.M. Shibata, “The status of Mitaka correlator,” Technical Workshop for APT and APSG, pp.192–194, 1996. [10] A.R. Thompson and J.M. Moran, and J.G.W. Swenson, Interferometry and Synthesis in Radio Astronomy, John Wiley & Sons, New York, U.S.A, 1986. [11] S. Iguchi and N. Kawaguchi, “The optimum filtering in very-long-baseline interferometry (VLBI),” IEICE Trans., vol.J82-B, no.3, pp.420–426, March 1999.
Satoru Iguchi received the B.E. and M.E. degrees in the Department of Electronic Engineering from the University of Electro-Communications, in 1995 and 1997, respectively. His M.E thesis was the development of a VLBI data acquition system and study of an optimum filtering method in VLBI. Then, he also developed a near real-time fringe detector as ground supporting facilities for the VLBI Space Observatory Programme (VSOP). He is currently working toward the D.E. degree at the University of Electro-Communications and the National Astronomical Observatory. He has engaged in research of Optical Linked VLBI techniques with an optical data transmission network of 2.5 Gbps for the purpose of higher-sensitibity VLBI observations.
Noriyuki Kawaguchi received the B.E. and M.E. degrees from the the Department of Electronic Engineering from the University of ElectroCommunications, in 1972 and 1975, respectively, and Ph.D. in department of astronomical science from the graduate university for advanced studies in 1998. He had worked on developments of VLBI systems in Radio Research Laboratories (now Communications Research Laboratory) since 1977 for use in geodetic applications. He has started the development of VLBI systems in the National Astronomical Observatory (NAO) since 1989 for the VLBI Space Observatory Programme (VSOP) and other astronomincal applications. Now he is a professor of NAO and is engaged in the VERA project as the project engineer.
Seiji Kameno received the B. Sc. degree from the Department of Astrophysics from the Kyoto University in 1990, and the M.Sc. degree department of astronomy from the University of Tokyo in 1992. He has worked on astrophysics of active galactic nuclei using VLBI observations in the National Astronomical Observatory (NAO) since 1994. Now he is a research associate of the NAO and is participating in the VLBI Space Observatory Programme (VSOP).
Hideyuki Kobayashi received the B.Sc. from the Department of Science of University of Nagoya in 1983 and M.Sc. and Ph.D. degrees from the Department of Science of Tokyo University in 1985 and 1989, respectively. He researched on system developments of millimeter wave interferometer at the Nobeyama Radio Observatory of the National Astronomical Observatory of Japan from 1983 to 1989. He studied H2 O maser regions in star forming regions to determine their positions accurately. Since 1989, he has belonged to the Institute of Space and Astronautical Science. He is working on the VLBI Space Observatory Programme (VSOP) and developed the onboard radio astronomy observing system.
Hitoshi Kiuchi received B.E. degree in Electrical Engineering from the University of Electro-Communications, Tokyo in 1982. He joined the Communications Research Laboratory in 1982. Since then, he has engaged in the research and development of VLBI data acquisition and correlation processing systems, the K-3, K-4 and Keystone-Project systems, and has been responsible for real-time VLBI via the ATM networks.
