# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Problems with AstronavPC**

**From:**George Huxtable

**Date:**2004 Feb 15, 15:15 +0000

Problems with AstronavPC. Do any listmembers possess the book (with CD), "AstronavPC and compact data 2001-5" or an earlier edition? Or perhaps another version of the same thing, published in the UK as "NavPac and compact data 2001-2005". What follows will be of interest only to those that do. Even better, have any listmembers attempted to implement the procedures given, in the chapter "Compact Data-Explanation"; in sections 7.4, "Position from intercept and azimuth by calculation", and/or 7.5, "Estimated position error"? Better still, does anyone claims to understand those procedures? I'm not asking whether the built-in program gives the right answer when you ask it to "display : plot of position lines". I've no reason to think it doesn't, though unable to try it out because it doesn't work on my Mac. All I an questioning is the written explanation. 1. Go to section 7.5, "Estimated position error". This shows how a "confidence ellipse" can be calculated, with major axis a, minor axis b, and orientation of the major axis "theta". Near the end of the section is stated "The ideal situation is to produce a circular distribution of errors, with A = B and C = 0, so that the errors are the same in all directions." This seems to me to be wrong: to produce a circular distribution of errors, I suggest that what's needed is instead for A = C and B = 0. Only then would the ellipse become a circle; (a = b, and theta becomes immaterial). Does anyone agree (or, more interesting, disagree)? 2. A few lines down from the start of 7.5 is stated "The standard deviations sigma(L) and sigma(B) in longitude and latitude are given by-" and so on. I don't question the value for latitude, but I suggest that for longitude, the value given is for standard deviation of the East-West displacement, in nautical miles (that is, the "departure"), rather than in longitude (which is always expressed in terms of angle). 3. Toward the end of 7.4 is stated- "If d exceeds about 20 nautical miles set L(F) = L(I), B(F) = B(I) and repeat the calculation until d, the distance between the position of the previous estimate and the improved estimate, is less than about 20 nautical miles". In the sentence above, "repeat the calculation" must first require, for each body sighted, the following- [Using the newly updated values for B(F) and L(F) as Lat and Long, rework the calculated altitudes and azimuths, in 7.2.2, 7.2.3, and 7.2.4, and obtain a new intercept p from (observed alt. - calculated alt.). For this to be possible, the original values for dec., GHA, and observed altitude of the body must have been retained.] It's a pity that there's no specific mention of that necessary part of the iteration procedure, given in square brackets Then the new values Z and p are collected together for all these bodies in the expressions for A to G in 7.4. This allows new values for dB and dL to be calculated for a further iteration. In 7.4 is the statement- "Additional observations may be included in the solution by simply adding the extra terms to the summations A,B,C,D,E, and F and calculating dL and dB again." To me, that seems to be rather futile, in that it would be impossible to work any further iterations using those added terms unless their original values for dec, GHA, and observed altitude had been preserved. =============== I'm tempted to submit these as suggestions for the next revision of this publication by HM Nautical Almanac Office (which must be due soon), but before making a fool of myself in front of them by getting things wrong, I would rather risk making a fool of myself in front of Nav-L readers. Any comments would be most welcome. George. ================================================================ contact George Huxtable by email at george---.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================