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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.





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