Mark 5 Newsletter
MIT Haystack Observatory
The Mark 5 Newsletter is issued from time to time to keep users informed regarding features, plans, problems, solutions and workarounds. All back issues of the Mark 5 Newsletter are available at the Mark 5 web site at http://wwww.haystack.mit.edu/tech/vlbi/mark5/index.html. We invite input from anyone on subjects we should discuss or questions that need answers; please send them to email@example.com.
On Saturday June 28 at UTC 012:59:00 colleagues at Kashima and Haystack Observatories completed the process of estimating UT1-TAI from VLBI data less than 24 hours after the start of data acquisition. Data were collected at Kashima on the K5 system and at Westford on the Mark 5A system. The first fringes were detected 53 minutes after the completion of the 2-hour experiment. Approximately 83GB of data were transferred between the two sites in under 3 hours at a maximum sustained rate of 107 Mbps . After some initial difficulties, correlation was completed at the Haystack Mark 4 correlator 15 hours after the start of the experiment and UT1 analysis was completed at NASA/GSFC ~7 hours later, for a total turnaround time of ~22 hours.
With improved coordination and a little practice, we believe that future experiments should easily reduce the total time from the start of data-taking to final UT1 results in less than 8 hours; further experiments are planned.
Figure 1: Screen shot of Beppe Maccaferri's Mark 5A monitor/control interface
At Haystack we have observed that unterminated output ECL signals on the Mark 5A I/O Panel sometimes tend to create sufficient cross-talk to somewhat elevate observed error rates on an attached Mark 4 decoder even though the data on the disk are perfectly good. This is only a problem at stations where the four Mark 5A output connectors are not used. To solve this problem, we have fabricated small terminator PCB boards that can be attached to these connectors. If you need some of these terminator boards for your system, please drop us an e‑mail at firstname.lastname@example.org and we will be glad to send some to you.
Hitachi Global Storage Technologies, which purchased the IBM disk division some months ago, has just announced the Deskstar 7K250 250GB disk; this unit uses the ramp load/unload technology that appears to be more durable than Contact Start-Stop method used by some other manufacturers. The unit is expected to be available in August 2003 at an estimated $325/unit. Additional information available at http://www.hgst.com/hdd/desk/7k250.html.
Each Mark 3 or Mark 4 DAS rack is assigned a ‘rack ID’ by jumpering on one of the connectors which mates to the Mark 3 or Mark 4 formatter. When recording data in ‘parity-stripped’ mode (‘mode=mark4:xx’), the rack-ID must be an even number; in fact the Mark 5A will refuse to record in Mark 4 parity-strip mode unless the rack ID is even.
The reason for this requirement is a little obscure, but let us try to explain. In the Mark 3/Mark 4 track format, the rack-ID field is contained in the byte just preceding the track-frame sync word. When parity is stripped, the sync word becomes ‘ffffffff’; an odd-numbered rack-ID adds at least one additional ‘1’ immediately before the sync word, considerably complicating the job of reconstructing proper synchronization on playback. It would be possible for the Mark 5A to force the bit immediately prior to the sync word to be a ‘0’, but that would invalidate the CRC character embedded in the Mark 4 header and force it to be recomputed on the fly (different for every track). It is simply much easier to ensure that the rack-ID is an even number. The VLBA format already prescribes a ‘0’ immediately preceding the sync word, so there is no problem with VLBA rack IDs.
Forcing the rack ID to an even number is a simple procedure involving grounding one pin on the relevant Mark 4 DAS connector. Contact Dan Smythe for instructions.