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    Re: Zenith Fixes
    From: David Pike
    Date: 2019 Mar 30, 15:33 -0700

    Andres, Jeremy

    Thank you for your replies.  Sorry not to have responded sooner, but I’ve been away for a few days.  The one thing that keeps coming up is how the emphasis in navigational techniques has changed over the years and between different roles.  A late 18th Century marine navigator might have been running three plots.  One a longish standing DR plot plus updated DR plots with resets to rated chronometer alone, and to lunar corrected chronometer time.  A 20th century marine navigator was still travelling sufficiently slowly to use celestial values based upon accurate chronometer time transferred forwards for several hours.  With the arrival of mechanical DR plots in the mid 20th Century, but before the arrival of GNSS, especially with aircraft, your DR position wasn’t something to be ignored once the next fix was plotted.  You DR position was being continuously calculated by an automatic mechanical system, and it was something to be nurtured.  Celestial position lines and fixes were used instead to monitor ‘the system’, and depending upon the crews confidence in both the fix and DR position, were used to decide by how much to ‘tweak’ the automatic DR position towards the fix.  The arrival of inertial navigation needed roughly the same sort of monitoring, but the arrival of GNSS, digital computing, and digital charting has changed the required techniques yet again.  DaveP

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