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.
