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    Re: Digital Sextant
    From: David Pike
    Date: 2020 Sep 11, 03:40 -0700

    You would need a certain amount of navigation training to use this.  Otherwise, if you took everything you read in the manual as ‘the thing to do’ it could lead you into serious trouble.  Despite clever maths, in practice, you cannot get a fix from a single position line, but you can use one to refine a DR position.  I would want to see the algorithms used in the calculator before I trusted this device, even if I couldn’t fully follow them.  With no input for DR or assumed position, they must be using range and bearing from a sub-stellar point.  But where are you going to get the true bearing/azimuth from.  With GNSS lost and IN degrading, you would need to take a bearing on the body with the ship’s magnetic compass, then allow for deviation and variation, neither of which are likely to be known with sufficient certainty.  If Hs body is 60, the sub-stellar point is 1/12 Earth radius away, or 360x60/12=30x60=1800nm away.  Using the 1 in 60 rule, this relates to 30nm fix error per degree true azimuth measuring error.  The best I ever achieved air-swinging a Dominie at around 37,000’ using, Decca for position, a periscopic sextant mount with a Vernier scale attached to the azimuth ring, and ringing up the Admiralty Compass Laboratory after landing for an estimate of instantaneous magnetic variation at the time, place, and altitude was 0.1 degrees, but that was under trial conditions.  Using a lower Hs body might make measuring its azimuth easier, but that would mean the sub-stellar point was farther away, so gains and losses in accuracy would cancel out. 

    Cleverer people than me can produce algorithms to produce a fix from the intersection of two or three range circles, just as GNSS does it from the intersection of four spheres, but the instruction manual appears to use three Hs from the same body only minutes apart.  To misquote Clara Peller “Where’s the cut”.  Also, the units for Hs in the ‘RECORD’ appear to be for a different date, 14th Jan, to the earlier screenshots, 7th Nov. Moreover, the ‘CALCULATION’ screen shot for the 3PL fix lat & longs appears to be the same as the screenshot for the 2PL lat & longs.  I’m not even sure why they don’t just give a final position.  Clearly, the manual producer was more experienced graphic designer than navigator, and no one thereafter bothered to look closely at the screen shots, as a novice might.

    So, my feeling about this is this device is very nice if you know what you’re doing, but you definitely still need to know what you’re doing.  It would be no good as a rich person’s shortcut to becoming a Navigator. DaveP

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