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 Demerits 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] 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.

Development Schedule




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