NavList:

The radar technique uses no equations.  You simply run out a VRM (variable range marker) on the radar screen (a circle) to the point of interest (sharp headland is best) and then cross it with an EBL (electronic bearing line).  Then you take the bearing and distance from the radar screen and go to the chart.  Draw an arc equal to the range from the headland and then use a straight edge and protractor to draw the bearing line.  Where the two cross is your estimated position (I would not give this position the term fix because of the bearing ambiguity inherent with radar among other issues.).  I also would say that using a 3 cm radar is the way to go here as 10 cm doesn't give as distinct an image.  Does this work? Yes.  Do I use it, yes, every time I come into port.  Is it infallible?  Absolutely not.  It is much better to cross a radar range with a visual bearing line, or use two ranges.  As far as distance to be used. I would use this at the 12 mile scale at most.

 

As far as Celnav, you can't get an accurate enough bearing of a body to make it useful for fixes.  The best you can do is perhaps 1/4 or a degree or so at sea.  Run this bearing line over a thousand NMs to the GP, and there error range is impressive and probably not useful for navigation..  Still, when you try this technique in conjunction with a program such as Andres used and was copied on my last reply, the bearing, faulty as it is, will clearly indicate which of the two intersections of arc you should be using,.  I did not, and would not, say that this method can be used for a "single star fix"  It is far too inaccurate.  It is merely a technique to resolve the 2-star fix ambiguity if you have no DR on a world-wide scale.

 

Jeremy

 

It looks like that one-body fix problem again.  John Karl and I gave formal solutions to this a few months ago.  I don't know of any historical reference actually giving such a solution.  That is certainly because the method is rather impractical, so it is never used and described.  Your mention of radar, Jeremy, led me to look into the "Fix by Range and Bearing to One Object" in Bowditch.  The azimuth measurement accuracy is stated to be 5 degrees.  This makes me believe that this is really only useful across short distances (you mention pilotage waters).  Bowditch gives no equations how to do this; I assume it's because for such short ranges you can just plot directly without getting into the celestial triangle.

If you were to do such a one-star fix, Jeremy, how would you do it and where does your method come from?  I am still curious about this and appreciate any info on the subject.


Peter Hakel

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From: "Anabasis75@aol.com" <Anabasis75@aol.com>
To: navlist@fer3.com
Sent: Tue, November 17, 2009 9:04:51 AM
Subject: Re: [NavList 10725] Re: AP terminology, WAS: 2-Body Fix -- take three

 

In addition to twilight, another piece of data might be provided by the navigator if there was sufficient manpower to do this in the case where no DR can be provided.  In this particular case, only ONE star is needed to get an approximate position.  This of course is an altitude AND azimuth.  This is the same as using radar off of a single object for a position in pilotage waters and offers the same limitation.  That is to say that in both the case of Radar and Celestial, the observation of azimuth/bearing is not nearly as accurate as altitude/range.  however if no DR is available, you can get a rough position (dare I call it an EP?) with just this data.  If we have two stars and even if a single azimuth determination is made, we can easily determine which of the two fixes is the correct one to choose.

 

The trouble with all of this, and the reason it was not popular, is that you can't easily do this with the mechanical tools available at sea.  Sure a computer can do it all, as I said before, and Andres has demonstrated; but it's not very easy to do with plotting tools and the charts that we have.   I don't have a compass on my ship that can stretch that far on any chart so doing circles of equal altitude isn't an option except in very rare cases in the tropics (less than 2.5 degrees of Zenith distance).

 

[parts deleted by PH]

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