NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Range by height beyond the horizon
From: Greg Rudzinski
Date: 2009 Nov 16, 16:03 -0800
From: Greg Rudzinski
Date: 2009 Nov 16, 16:03 -0800
Frank, Sounds very familiar ;-) A good explanation of ranging beyond the horizon is described by US Navy Captian H.H. Shufeldt in his book SLIDE RULE FOR THE MARINER on page 29: "In clear weather , an object such as a mountain, can be seen even when it is well beyond the horizon. If its height is known, its approximate distance can be determined with a sextant. The first step is to correct the sextant altitude for any index error, for the dip of the horizon, and for refraction. For this purpose refraction is found by dividing the estimated distance in nautical miles by 13.75 : it is obtained in minutes and tenths of arc, and subtracted from the sextant altitude. Next, correction must be made for the curvature of the Earth. This is done by multiplying the square of the estimated distance in nautical miles by .907 ; the product, which is in feet, has to be subtracted from the height of the object. The corrected height is then divided by the corrected sextant altitude, stated in minutes of arc, and the quotient is multiplied by the factor .566 to obtain the distance in nautical miles. When the distance thus found varies considerably from the estimated distance used to obtain the refraction and in correcting for the curvature of the Earth, the problem should be reworked, using the distance found as the estimated distance" Greg On Nov 16, 2:29�pm,wrote: > Here's a story from USS Growler, the diesel sub formerly armed with nuclear cruise missiles, now a museum ship in New York City: > "Growler's first deterrent patrol began on 12 March 1960. [...] Lieutenant John J. "Joe" Ekelund, Executive Officer and Navigator, developed an innovative method to determine the submarine's position in the assigned operating area. The technique was quite simple and similar to that used by submarines to determine the range of a target ship. Using navigation charts, Ekelund identified mountain peaks and their height as listed. He then observed the mountain through the periscope and, utilizing the built-in periscope stadimeter, he could superimpose the image of the base of the mountain on its peak. This double image and known peak height provided a good approximate range to the mountain that was read on the stadimeter dial. Using the range so determined, one can could calculate the amount of height which was not seen (was below the horizon) and correct the charted height to the observed height. Using the observable height a second, more accurate range could then be measured. Three iterations of this sequence would yield a navigationally useful range. Using more than one peak, he could accurately determine his position. " > > That should sound familiar! Greg? > > -FER -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com