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Head Positions and Track Assignments

This appendix describes the default track and head assignments used on the VLBA for projects scheduled using SCHED. These are gory details that most users should not need to be concerned about. Just let SCHED use its defaults and the observations should be ok.

The VLBA uses 14 head positions. Both the positions and the order in which they are used are the same for all projects. The head position for each index number are in the following table in microns.

             HEAD POSITIONS
--------------------------------------------
     Forward Passes      Reverse Passes
     Index   Offset      Index   Offset
--------------------------------------------
       1     -319          2       31
       3     -271          4       79
       5     -223          6      127
       7     -175          8      175
       9     -127         10      223
      11      -79         12      271
      13      -31         14      319
-------------------------------------------

All forward passes are to be written in the negative offset positions and all reverse passes are to be written in the positive offset passes. This ensures that nearly all adjacent tracks are in the same direction and minimizes the required guard bands.

The following tables give the head index position and the and the set of heads that are used for each pass for experiments using 1, 2, 4, and 8 passes per head position: Number: 1 2 3 4 5 6 7 8 9 10 11 12 13 14

             HEAD INDEX and HEAD SET for each PASS
---------------------------------------------------------------
 For 1 Pass Per Head Position:
   Passes 1 - 14
     Head Index    1  2  3  4  5  6  7  8  9 10 11 12 13 14
     Head Set      1  1  1  1  1  1  1  1  1  1  1  1  1  1
---------------------------------------------------------------
 For 2 Passes Per Head Position:
   Passes 1 - 14
     Head Index    1  2  1  2  3  4  3  4  5  6  5  6  7  8
     Head Set      1  1  2  2  1  1  2  2  1  1  2  2  1  1
   Passes 15 - 28
     Head Index    7  8  9 10  9 10 11 12 11 12 13 14 13 14
     Head Set      2  2  1  1  2  2  1  1  2  2  1  1  2  2
---------------------------------------------------------------
 For 4 Passes Per Head Position
   Passes 1 - 14
     Head Index    1  2  1  2  1  2  1  2  3  4  3  4  3  4
     Head Set      1  1  2  2  3  3  4  4  1  1  2  2  3  3
   Passes 15 - 28
     Head Index    3  4  5  6  5  6  5  6  5  6  7  8  7  8
     Head Set      4  4  1  1  2  2  3  3  4  4  1  1  2  2
   Passes 29 - 42
     Head Index    7  8  7  8  9 10  9 10  9 10  9 10 11 12
     Head Set      3  3  4  4  1  1  2  2  3  3  4  4  1  1
   Passes 43 - 56
     Head Index   11 12 11 12 11 12 13 14 13 14 13 14 13 14
     Head Set      2  2  3  3  4  4  1  1  2  2  3  3  4  4

---------------------------------------------------------------
 For 8 Passes Per Head Position
   Passes  1 - 14
     Head Index    1  2  1  2  1  2  1  2  1  2  1  2  1  2
     Head Set      1  1  2  2  3  3  4  4  5  5  6  6  7  7
   Passes 15 - 28
     Head Index    1  2  3  4  3  4  3  4  3  4  3  4  3  4
     Head Set      8  8  1  1  2  2  3  3  4  4  5  5  6  6
   Passes 29 - 42
     Head Index    3  4  3  4  5  6  5  6  5  6  5  6  5  6
     Head Set      7  7  8  8  1  1  2  2  3  3  4  4  5  5
   Passes 43 - 56
     Head Index    5  6  5  6  5  6  7  8  7  8  7  8  7  8
     Head Set      6  6  7  7  8  8  1  1  2  2  3  3  4  4
   Passes 57 - 70
     Head Index    7  8  7  8  7  8  7  8  9 10  9 10  9 10
     Head Set      5  5  6  6  7  7  8  8  1  1  2  2  3  3
   Passes 71 - 84
     Head Index    9 10  9 10  9 10  9 10  9 10 11 12 11 12
     Head Set      4  4  5  5  6  6  7  7  8  8  1  1  2  2
   Passes 85 - 98
     Head Index   11 12 11 12 11 12 11 12 11 12 11 12 13 14
     Head Set      3  3  4  4  5  5  6  6  7  7  8  8  1  1
   Passes 99 - 112
     Head Index   13 14 13 14 13 14 13 14 13 14 13 14 13 14
     Head Set      2  2  3  3  4  4  5  5  6  6  7  7  8  8
---------------------------------------------------------------

The following give the lists of the first track (head) for each channel for experiments using all heads in a pass. Channels using multiple tracks because of fan out and/or 2 bit mode use the listed track plus successive even or odd tracks. For the multiple pass per head position modes, the listed tracks are divided into groups and successive passes (at a head position) use the successive groups.

              FIRST HEAD per CHANNEL
---------------------------------------------------------------
    Tracks Per Channel:    8
    Modes:                 (VLBA1:4, 2 bit)
    First Heads:           2 18 3  19
---------------------------------------------------------------
    Tracks Per Channel:    4
    Modes:                 (VLBA1:4, 1 bit), (VLBA1:2, 2 bit)
    First Heads:           2 10 18 26 3 11 19 27
---------------------------------------------------------------
    Tracks Per Channel:    2
    Modes:                 (VLBA1:2, 1 bit), (VLBA1:1, 2 bit)
    First Heads:           2 6 10 14 18 22 26 30
                           3 7 11 15 19 23 27 31
---------------------------------------------------------------
    Tracks Per Channel:    1
    Modes:                 (VLBA1:1, 1 bit)
    First Heads:           2  4  6  8 10 12 14 16
                          18 20 22 24 26 28 30 32
                           3  5  7  9 11 13 15 17
                          19 21 23 25 27 29 31 33
---------------------------------------------------------------
    Mark~III Mode B with channels in ascending frequency order.
    Sidebands should alternate.
    First Heads:  18  4 20  6 22  8 24 10 26 12 28 14 30 16 32 2
                  19  5 21  7 23  9 25 11 27 13 29 15 31 17 33 3
---------------------------------------------------------------
    Mark~III mode E with channels in ascending frequency order.
    First Heads:  4   6   8  10  12  14  16   2
                 18  20  22  24  26  28  30  32
                  5   7   9  11  13  15  17   3
                 19  21  23  25  27  29  31  33
---------------------------------------------------------------

Mark III mode A is not normally used with VLBA systems because the gain of only 2 tracks over mode A is not worth the extra tape usage. When it is used, the assignments are special because a decision has to be made about what subset of tracks to use.

Perhaps an example will help clarify all of this. Consider an experiment that uses 2 passes per head position and 4 tracks per channel. There will be 4 channels and the following pattern is used for the first few passes.

Pass Head  Dir             Heads.
      pos        Chan 1       Chan 2        Chan 3       Chan 4
 1   -319  For | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
 2     31  Rev | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
 3   -319  For | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
 4     31  Rev | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
 5   -271  For | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
 6     79  Rev | 2 4 6 8 | 10 12 14 16 | 18 20 22 24 | 26 28 30 32 |
 7   -271  For | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
 8     79  Rev | 3 5 7 9 | 11 13 15 17 | 19 21 23 25 | 27 29 31 33 |
 etc.

Note that when there is both fan out and 2 bits per sample, the ``sign'' bits are fanned out over the first n tracks (eg 4 for VLBA1:4), and then the ``magnitude'' bits are fanned out over the next n tracks.


next up previous contents
Next: S2 Up: OBSOLETE SECTIONS Previous: Scheduling Mark II Observations   Contents
Craig Walker 2014-04-14