1 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan
4CosmoResearch Co. Ltd.
| Data transportation|
|Human operation is inevitable at work of Magnetic tape transportation and tape mount on recorders||Real-time data transfer by ATM enables automatic operation.|
|Log information||Observation information and Clock parameter is provided by log file.||Information is collected by computer network just before the observation.|
|Trouble recovery||Trouble at data processing can be recovered by re-processing of recorded data.||Stop of data processing due to troubles leads to data loss directly.|
|Rapidity of output||Analysis result comes out 1-2 days after observation.||Analysis result comes out just 10-20 minutes after observation.|
Real-time VLBI is advanced from the standpoint of automatic operation and rapidity of output, but interruption of correlation processing due to any problems leads to direct loss of data. Therefore, non-stop operability is strongly requested of real-time correlation processing software.
Except for hardware errors and software hang ups, most probable cause of data loss would be wrong clock parameters. Wrong clock parameters will move fringes out of the lag window of the correlator, and such data cannot be used for baseline analysis. Also, fringe monitoring and adjustment of clock parameters by an operator is difficult, because VLBI experiments may start at midnight and the operators are not supposed to be familiar with VLBI. Therefore we implemented "Dynamic Clock Adjustment(DCA)" and "Automatic Fringe Search(AFS)" functions in RKATS. By using these functions, fully automatic real-time daily VLBI observation has realized.
Figure 1 shows the clock parameters are adjusted by AFS and they are kept almost within +/-0.1 usec by DCA function. AFS and DCA works as follows:
Data processed in "Fringe Search Mode (FSM)" is unusable for baseline analysis. The frequency of transition to FSM must be as small as possible. For stabilization of AFS and DCA, RKATS has several parameters. They are listed in Table 2.
By optimizing these parameters empirically, RKATS can be tuned to operate as reliably as possible.
|Fringe Search Mode transition threshold||If ratio of fringe detection (#detected/#total) become lower than this value, correlation mode is changed to FSM.|
|Averaging Number||Averaging number for fringe detection ratio.|
|SNR threshold||SNR threshold to judge fringe detection.|
|Source list for DCA||radio source list, on which fringe is expected to be detected certainly.|