Outline of the short term technical development plan
Technical Development at the TDC in Japan
by T. Yoshino
IERS activities have two aspects. One for the regular service. And the other
for introducing advanced technology for more precise observation. TDC
understands the both needs for technological improvement of observation and
practical use. We discussed the items of technical development in short term
range taking these needs into account.
K-4 SYSTEM by H. Kiuchi
In developing VLBI data acquisition system, it is impossible to ignore the
requirement of the multi-bit sampling and the wide-band data sampling.
Nowadays, each VLBI network becomes isolated due to the data acquisition and the
data compatibility. We are developing the next generation VLBI system, and
whose data acquisition mode include the VLBA (2-bit 16 MHz) and the VSOP (2-bit,
32 MHz) data mode. The new acquisition system has one bit sampling and two bit
sampling modes for VLBI, furthermore 4 and 8 bit sampling for general purpose
data acquisition. The total data rate is reached to 256 Mbps (DIR-1000 data
recorder).
It is supposed that the input video bandwidth is 32 MHz. The anti-aliasing
filtering is made in analog, and after sampling the 16 MHz, 8MHz, etc.
filtering are made by digital filter. It is possible to get the good filtering
and good phase characteristics by using digital filter. Number of video
channels can be selected (from 2 to 16 ch) for each station and each observation
purpose, geodesy or astronomy. An effort has been made to achieve an IRM (Image
Rejection Mixer) for 32MHz bandwidth .
K-4 Correlator by S. Hama
We decided to use the K-4 correlator not only for R&D experiments but for a
routine processing in some degree. It makes use of a custom LSI for the center
part. Its main features are; 2 bit sampled data processing, milli-second pulsar
gate, 16Mbps x 15ch, good for Earth-Moon VLBI. Software for this correlator is
also to be developed.
K-4 Supporting Software by Y. Koyama
K-4 back end terminals and K-4 data recorders have a GP-IB control capability.
To perform VLBI observations, it is needed to send command and request protocols
from a host computer. NKAOS, which has been developed on HP1000/A400 computer,
can interface with the 34m antenna system at Kashima Space Research Center and
organizes data acquisition with either K-3 and K-4 systems. MAOS software,
which has been developed on HP BASIC running on HP9000 series computers, is
convenient to control K-4 system in a various configurations.
Development of New Delay and Phase Calibrators by M. Imae
A new Delay and Phase Calibrators of the VLBI system are now under development
at CRL. This calibrators use a 100MHz reference signal obtained by making
frequency multiplication of the 10MHz reference signal of the Hydrogen Maser
frequency standard.
Namely the comb generator of the Phase Calibrator is driven by 100MHz signal and
it generates phase calibration signals of 100MHz frequency spacing. This 100MHz
phase calibration signals are fed to a micro-wave attenuator which has a gate
signal of 1MPPS obtained from the same 100MHz reference signal. The output
signal from this micro-wave attenuator has a impulse train of 1 micro-second
spacing in time domain and 1MHz frequency spacing in frequency domain. This
impulse train is used as the phase calibration signal.
The Delay Calibrator also uses the 100MHz band reference signal by measuring the
round trip time between the main unit placed at back-end site of VLBI equipments
and the antenna unit placed at the front-end site.
The new system also has a temperature compensation function to reduce the effect
of the environmental temperature perturbation.
Millimeter Wave VLBI for geodesy by H. Takaba
Merits
1. Precise delay time determination by the wide band receiving.
2. One frequency band geodesy VLBI (useless of the ionospheric
delay time correction).
Demerits
1. Poor system performance compared to the S/X bands (Low aperture
efficiency, tracking problem, and high receiver noise temperature).
2. Large atmospheric absorption (optical depth exceeds O.1 at the zenith
in case of rain).
3. Low correlation amplitude because of the intrinsic characteristics
and the resolving out of the radio sources.
4. Large coherence loss because of the atmospheric fluctuation and the
local's fluctuation.
System consideration
10-20m antennas' pair, 20 - 40 GHz band low noise receiver (super HEMT or SIS
?), IF band width of 2 GHz, 8 MHz x 16 ch or 16 MHz x 8 ch video conversion
system, and a 256 Mbps recorder.
Report of Water Vapor Radiometry by M. Sekido
One of the most important source of error in very-long-baseline interferometry
(VLBI) is propagation delay caused by water vapor. Water vapor radiometry (WVR)
is an instrument to estimate this wet delay by measuring the sky brightness
temperature with Water Vapor Radiometer. The typical uncertainty of VLBI group
delay data is within an order of 10mm. So, if we can estimate the wet delay
with an uncertainty much less than 10mm, it is a great improvement on VLBI
technology. G. Elgered et. al[1] have reported the comparison between the
propagation delay estimated by WVR and that obtained by Kalman filtering of the
VLBI data themselves. According to them, the repeatability obtained for
baseline length estimates shows comparable accuracies with both methods. On the
other hand, T.Tanaka et. al[2]. have observed wet component of the delay by
using water vapor radiometer at Uji in Kyoto. But the estimated delays are
different by several centimeters from one equipment to another. So the data are
not sufficient to imp rove the accuracy of base-line length estimates yet.
Present important problems in WVR are as follows;
1: biases in the measurements depending on the instruments
2: improperly
modeled height profiles of pressure, temperature and humidity
3: uncertainty of
attenuation coefficients of liquid water drops.
[1] G.Elgered, J.L.Davis, T.A.Herring, and I.I. Shapiro ``Geodesy by Radio
Interferometry : Water Vapor Radiometry for Estimation of the Wet Delay",
J.Geophys. Res., 96, 6541-6555, 1991.
[2] T.Tanaka, K.Nakamura, and K.Hirahara, ``PRELIMINARY RESULTS FROM AN
OBSERVATION OF WATER VAPOR IN THE TROPOSPHERE WITH TWO MICROWAVE RADIOMETER AT
UJI", Proceedings of the Japanese Symposium on Earth Rotation, Astronomy, and
Geodesy, 153-156, 1991.
