Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314, Japan
Every morning, the observation tapes are replaced by human operators at each station during one hour from 9:00 o'clock by Japanese Standard Time (JST). After all the observation tapes are set in the Digital Mass Storage (DMS24) system, all tapes to be used in the next observation schedule are mounted into the DIR1000 data recorder and the quality of the tapes and whether the tapes are write-enabled are investigated, and in the end, all the tapes are rewound and set in place for the next experiment. This procedure is also performed automatically, and it tells system operator at Koganei station by e-mail if there is any problem. When the system operator receives an alarm message by e-mail, the cause of the problem is investigated and then resolved, if it is possible to do so from Koganei remotely. If an on-site repair is found to be necessary, related hardware vendors and CRL staffs are notified with detailed information. For example, a PC at Tateyama station which controls the Antenna Control Unit failed to keep the internal clock, although the PC clock is usually synchronized with UTC by using a GPS receiver signal. This problem caused data loss for the Tateyama station for three consecutive experiments but the system was recovered within three days. On the other hand, this case can be considered as a demonstration of the robustness of the KSP VLBI Network in the sense that an unpredictable failure at a remote site can be recovered within three days. In addition, the overall software system is being implemented so that the same type of problem can be resolved from central control system at Koganei or at Kashima.
These figures indicates the data quality of the data obtained in the daily KSP VLBI experiments. The most notable point is the evident improvement in the formal error and RMS residual delay for both of the baselines from February 22, 1997. Before that day, the formal errors of the baseline length estimates were around 4 mm but improved to approximately 2 mm. The RMS delay was improved from the level of 60 psec to about 20 psec. This was a result of the improved short time phase stability of the phase calibration signal at Kashima station. Before the improvements, the temperature inside the receiver unit located below the 11 m antenna dish was controlled in the range 25 +/- 1 oC. But it was found that this temperature control had been causing the short time periodic variation of the temperature inside the unit. The phase of the phase calibration signal is quite sensitive to the ambient temperature and the phase variation causes error in the observed delay. If the variation is slow, the error of the time delay can be removed as the clock difference between two stations through the VLBI data analysis. If the variation is faster than the interval of the clock parameter epochs which is 1 hour for the KSP data analysis, the delay error can not be removed and it degrades the estimated results and increases the residual delay. At present, the receiver units of all four stations are not temperature controlled and the observations are performed in the time range from 00:00 to 05:30 local time when the temperature is most stable during the day. An improvement of the temperature control system of the receiver unit with slow and smooth temperature control is considered and will be implemented in the future. The phase calibration signal unit will also be improved with a small temperature control system inside the case of the unit to stabilize the phase of the signal.
The other characteristic thing of concern in Figure 2 and 3 is the high percentage of invalid data to the total number of the available data for the Miura station. This is shown in Figure 3 (c) only for the Kashim11-Miura baseline, but it is also true for the other two baselines which include the Miura station. This is caused by weak phase calibration signals recorded in specific channels at Miura station. The band-width synthesis software automatically rejects a data channel if the amplitude of the phase calibration signal recorded in the channel is below 1% of the total power of the channel. At Miura station, the amplitude of the recorded phase calibration signal is not uniform for all channels. When the system noise temperature is high at low elevation angle or due to rainy conditions, up to four observation channels are removed from the band-width synthesis software. In this case, much data becomes invalid because of false detection of side-lobe of the delay resolution function. The cause of the weak phase calibration signal was diagnosed to be a problem in the Input Interface (DFC2100) of the KSP data acquisition system. At present, solutions are being considered and the problem will be solved in a near future.
First of all, real-time VLBI test observations were performed for four consecutive days starting April 7, 1997. The observations were done using the maximum data rate of 256 Mbps for 22 hours from 10:30 local time each day. The tests were nearly totally successful and the results obtained from the real-time VLBI data were consistent with the daily tape-based VLBI experiments performed at 64 MHz prior to April 6, 1997. At present, minor problems and software developments for the automated processing are in progress. The 256 Mbps real-time VLBI experiments will be routinely started near future.
In the data analysis, a program dbupdate was revised to obtain the nutation offsets from the IAU80 model provided in the IERS EOP Bulletins and include the offsets in the Mk-3 database. These offset values are now used in the data analysis.
Lastly, the phase calibration signal unit at Tatayema station was replaced on April 8, 1997. Before it was replaced, the phase stability of the unit was insufficient. Now, the phase stability has been improved and the formal errors of the baseline lengths of the three baselines towards Tatayema are at same level as the other three baselines. The power of the phase calibration signal at Miura station was increased and since then the ratio of invalid data to the total number of available data decreased. Although the Input Interface of the Miura station must still be repaired to make the phase calibration signal level uniform among the 16 observation channels, the data quality of the Miura station is now satisfactory.