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Antoine,

There is a temperature and pressure correction applied from table A4 of the Nautical Almanac for this low altitude example. I had listed the early morning temperature at 57* F when it should be 47* F and the pressure as 30.00 inches. This is right on the table line between column F & G. Column G has a zero correction and column F has a -0.3' correction. I went with -0.3' to have a value for demonstration purposes. This explains most of our difference. The rest of the difference can be attributed to Pub 229 interpolation and rounding.

As you say:

" the differences between both end results can be appreciable at such low altitudes, should the DIP correction be performed first, and then the Refraction correction, or the other way around ? "

The index correction and dip are applied first to get Ha from Hs then altitude refraction is applied and then parallax to get Ho.

Greg Rudzinski

Re: USCG Student Example for Low Altitude
From: Antoine Couëtte
Date: 2013 Apr 25, 23:26 -0700
RE : http://fer3.com/arc/m2.aspx/USCG-Student-Example-for-Low-Altitude-Rudzinski-apr-2013-g23682

Hello Greg,

Thanks for this example.

From your chart, I can guess that for your published Azimuth of 077.5° you obtained an intercept (between GPS position itself and LOP) close to 0.8 NM (A).

I find i = - 1.4' (A) / AZ = 077°7

Our intercept difference (close to 0.6') comes from a difference in the Value of Refraction (the SUN is at quite a low height). As an example, the refraction differences between US N.A. and French N.A. Tables reach 1 full arc-minute by 0° observed height.

Best Regards

Kermit

AN INTERESTING QUESTION (Any reply anybody ?)

As the differences between both end results can be appreciable at such low altitudes, should the DIP correction be performed first, and then the Refraction correction, or the other way around ?

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