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TAPE LENGTHS and PASS TIMES

Tapes for the Mark III and VLBA systems come in two nominal lengths, 9600 ft and 18000 ft. Actually, nearly all of the shorter tapes have now been removed from the system so most users will not encounter them. The lengths vary plus the usable length is shortened by the amount of leader needed at each reel. Experience has shown that it is best not to count on more than 17600 feet for the long ``thin'' tapes. Most short ``thick'' tapes are over 9000 feet, but there are a significant number as short as 8500. It is probably reasonable to assume that the short tapes will be 8800 ft, half the length of the long tapes, for scheduling purposes.

There are three basic recording speeds to consider, at least when using the VLBA correlator. The ``normal'' speed it that at which 4 million data bits per second are recorded on each track. Mark III and Mark IV format data at low density are recorded at 135 ips (inches per second). VLBA format data at low density are recorded at 133.33 ips. All formats at high density are recorded at 80 ips, which is now the standard for most observations. Recording rates of half and twice these values are used when the number of bits per second per track is half or twice the 4 Mbps. High density recordings can only be made on ``thin'' tapes. The VLBA correlator plays back all observations at a track bit rate of 8 Mbps, using 160 ips for high density recordings. Thus there is a speed up factor for recordings made at lower speeds -- a factor of 2 for 4 Mbps tracks and a factor of 4 for 2 Mbps tracks.

The following table gives the recording times per pass to expect with these tape speeds and recording rates:

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     RECORDING TIMES for 1 pass on VLBA and Mark III tapes.
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     Tape length   Format /        Bit rate per track
        (feet)      density     (2 Mbps)   (4 Mbps)   (8 Mbps)
 ------------------------------------------------------------------
        17600      All/High     1:28:00      44:00      22:00
        17600      VLBA/Low       52:48      26:24      13:12
        17600      Mark III/IV    52:08      26:04      13:02
         8800      VLBA/Low       26:24      13:12       6:36
         8800      Mark III/IV    26:04      13:02       6:31
 ------------------------------------------------------------------

When full automatic tape handling is not being used, scans should be scheduled so that they are in blocks that constitute a pass. This is not absolutely necessary, because, regardless of the scan times, SCHED will make sure that the scan will fit on the tape and, if not, will reverse the tape. If a scan cannot fit, SCHED will complain and die. Because of these actions, no data will be lost if the schedule is not in blocks of a tape pass, but some amount of tape will be wasted -- it is better to be aware of the pass lengths. In fact, it is not uncommon for a user to find that, when he/she deletes a scan, the amount of tape used increases. This happens when a schedule that went to the end of tape on each pass gets out-of-sync and has to reverse the tape before the end. Once this starts to happen, it often continues for the rest of the experiment. Note that it is typical to round down the pass times to, for example, 13 minutes for those cases where the full number is 13:02 or 13:13.

Scheduling in blocks of tape passes is more complicated if there is a mixture of thin, high density tape at some sites and thick, low density tapes at other sites. For Mark III, the longer tapes will hold 3.37 times as much data per pass as the shorter tapes. Probably the best way to deal with this is to schedule in blocks such that the 2 blocks fit per pass on the thick tapes and 7 blocks fit on the high density, thin tapes. The appropriate time for this for Mark III or VLBA formats is 6:17. Be careful with scans less than 30 seconds in such a schedule because the passes could get out of sync. Mixed thick and thin tape observations should be rare, especially since the VLBA correlator refuses to deal with thick tapes.

Note that, if there is a problem at a station and a tape does not start on time, or a scan is missed, the tape position will be behind that expected by SCHED. At the end of the pass, the tape will reverse when the schedule tells it to, which in this case will be some distance from the end. For this reason, the next pass is likely to run out of tape before it is finished and some data will be lost. The pass after that will begin at the expected place and there will be no further problems.

Users of PCFS controled stations (eg EVN) should be aware that during long scans no Tsys information is acquired. It is necessary to insert gaps to make sure Tsys measurements are made.