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    AP terminology, WAS: 2-Body Fix -- take three
    From: Peter Hakel
    Date: 2009 Nov 11, 12:53 -0800
    Having started with CN less than a year ago, I am still a novice with fresh memories of learning the basics.  As a student I found the term "assumed position" confusing.  Having only reading materials to learn from and no human expert to ask questions, it took me a while to understand the details of the intercept method.  If I had John's book at that time it is likely that my confusion on this issue would have been averted.  In fact, it was this initial discomfort that led me to seek direct methods of solution such as the two-body and the one-body fix.  Even though these are academic problems, their study helped me put the practical St. Hilaire into perspective.   Then I became OK with it, including its less than ideal terminology.

    So what is AP for?

    Initially I did not know how to answer this question, even though I was already able to carry out the steps of the intercept method.  Perhaps it's because my first CN resource was mainly about "how it works" and less about "why it works."  Later my thinking went like: "We kind of know where we are (DR), this leads us to an AP.  The intercept method then corrects the error of that AP."  Even that I found strange because the HO249 rules (used by my first CN resource) led to different AP's for different celestial bodies (same integer latitude but different longitudes since LHAs must be integers).  So, "for the Sun LOP I assume to be here, but for the Moon LOP I assume to be over there, and then on top of that both are wrong and need correcting..."  Wouldn't AP=DR for both LOPs make more sense?  It took me more time and additional study resources to digest all that.

    I now think of the AP as the origin of a local coordinate system for a patch of Earth's surface that,
    a) contains the ship's location, and,
    b) is sufficiently flat and "zoomed in" in order to allow us to plot the relevant sections of the LOPs with acceptable accuracy.
    Then the intercept distance and azimuth are like the magnitude and phase of a complex number.  This local coordinate system is overlayed with the rectangular grid of parallels and meridians in the Mercator projection, which then can be used to extract the latitude and longitude of the fix.

    Peter Hakel

    From: "frankreed@HistoricalAtlas.com" <frankreed@HistoricalAtlas.com>
    To: NavList@fer3.com
    Sent: Tue, November 10, 2009 8:16:14 PM
    Subject: [NavList 10557] Re: 2-Body Fix -- take three

    Hi John. You wrote:
    "You may have not have found any student confused by the concept of assumed position.  But just try asking any CN navigator why we use an assumed position in the intercept method, and I'll bet you'll find that at least 98% of them respond with a misconception. "

    Really? Could you tell what answer those 98% would be likely to give and what the misconception is in that understanding? Maybe I didn't understand your point on this previously.


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