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Previous: Operational Matters
Up: EVN TWG/Friends Meeting
Next: Calibration
Previous Page: Scheduling
Next Page: Spares
Porcas discussed the type of problems we can expect once the VLBA (and the Socorro correlator) are in full swing. The logistics of tape flow and supply were discussed. Since the likely acceptable gap between observing and correlating is expected to be only a week or so, EVN stations may have to ship recorded tapes continually through the session - rather than the present policy in which tapes are shipped out at the end of the session. Porcas expected that all global experiments would eventually migrate to the Socorro correlator (until the EVN correlator came into operation) and that there would be fewer MKII experiments.
Schilizzi asked if the VLBA could correlate MkIII experiments. Porcas explained that it could but with only 8 channels, Mode B required 2 passes and Mode A four. Mode E could be accommodated in one pass. Hybrid experiments were not possible. Porcas noted that as global experiments were correlated in Socorro, the present 6 month backlog at the Bonn correlator would be reduced. There will be a net flow of tapes to Socorro with the thick tape being sent back to the EVN. Each station should investigate a rapid route for tapes to be sent to Socorro. Mantovani commented that for NASA supported geodesy experiments, tapes could be shipped both quickly (no customs) and without cost since NASA was a government agency.
Alef described a new program TRACK which is being used by NRAO to keep track of VLBI tapes. It will be installed and used at MPIfR for the next session. He urged others to do the same. The program can determine whether a given tape is in transit, at the correlator centre or missing. The program can be accessed remotely by logging into Socorro. The tape ID must be entered for each tape. A bar code reader can be used to avoid having to enter each ID by hand. Everyone agreed that each station should attempt to implement this system before the next session. Porcas noted that tapes which failed to arrive at NRAO on time would not be correlated.
Burgess asked if any VLBA experiments would come to the Bonn correlator. Porcas thought that there would be some cases ( e.g. EVN + one or two VLBA antennas, mm VLBI). These would require the return of thin tapes to NRAO within their cycle time. This would probably be shorter than the time to correlate in Europe (at least until the new EVN correlator came into operation), so for this reason we would need a ``buffer'' of thin tapes, purchased by us, to give some thin tapes back quickly. Porcas noted that since tape changes will have to be staggered for the VLBA, PI's will not be solely responsible for scheduling. We can expect NRAO to set down rules about this kind of thing. Another feature of the VLBA is that calibration and log data will be fed into a correlator database. The EVN will have to do the same but the database format has yet to be decided. Schilizzi felt that we ought to be in on the design stage of this new database format. Porcas reminded the group that the logs and calibration data would have to get there just as quickly as the tapes. Schilizzi enquired about the use of phase cals on the VLBA. Porcas explained that phase-cals were now being retro-fitted at each station. He suspected that phase-cal values from VLBA sites would appear in the users ``CL'' table and warned that if EVN stations did not send phase-cal values to the correlator (in some yet-to-be-agreed format), they would not appear in the ``CL'' table. The same was true for amplitude calibration data. EVN phase-cals would then have to be calculated manually. Mantovani noted that the HP field system could monitor phase-cals in real time - though the decoder has to be working. Alef wondered if we shouldn't try and copy the VLBA's use of phase-cal retrofits.
ACTION ITEM: Foley to investigate extracting phase-cal information from the Field System.
Schilizzi felt that several important issues had been raised and that some written communication with NRAO was called for. This should be well focussed and not wide ranging or too detailed. In addition we need to assess the number of thin tapes we will require for the pool and how much this is likely to cost. Porcas felt that it would be premature to buy any thin tapes at the moment in case NRAO's purchasing policy was to change.
ACTION ITEM: Schilizzi, Porcas & Burgess Write to NRAO in 6 months time to clarify the position on calibration, log data and phase cals.