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    SA errors---averaged over a reasonable time?
    From: Mal Misuraca
    Date: 1997 Dec 15, 9:25 AM

      Do we know that SA errors are not randomly distributed over a reasonably
    short period of time?  It would be interesting to know if anyone can be
    certain of that.
    
      I have watched the progress of the SA watch program averaging, as it
    deposits one dot on the screen for each of the fixes it takes from the GPS and
    permits these accumulations to be measured against (1) the position you
    believe you occupy and (2) the position represented by the mean of the fixes.
    
      My impression---and I certainly intent to go back and look again---is that
    the GPS fixes are not random in the sense that they jump from one side of the
    putative position to another, but that they wander in a line back and forth
    across and around a point.  Thus, you could argue that you must let them
    wander quite a long time before being sure you are getting the mean well
    established.  On the other hand, after several hours, it seemed clear to me
    where the locus was, and I based that on seeing where all those fixes had been
    deposited.
    
      In maybe an hour, it seemed to me that the truth was beginning to
    emerge---there was a point that seemed to be the locus of all the fixes within
    that time.  After that, it was fine tuning, not a significant shift in the
    mean, no matter how many hours the GPS was left on.
    
      This suggests that in as little as an hour, maybe less if I had looked more
    carefully, there was in the plot the ability to say pretty much where it was
    all going to come out.  Maybe that's the "slowly" that the last two or three
    emails on the subject mean to suggest.
    
       This is for someone like me plenty fast enough to be useful---as in trying
    to correct some of the Baja charts, for example, in which it is not unusual to
    find that latitude is good, and longitude is off.  This arises, I believe,
    from the fact that the charts have not been resurveyed since Admiralty days.
    Thus, I have tried to tie down one or two important spots in each anchorage,
    figuring that when I went back, they'd be all I needed to get started.
    
      Mal Misuraca
      Passage East
      Sausalito
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