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From: Joel Jacobs (no email)
Date: Mon Jul 26 2004 - 07:49:59 EDT
YOUR OPINIONS ARE SOLICITED
GO: http://www.seamanship.co.uk/deck/navigator/ASNAv/ASNAv%20Site/
Joel Jacobs
ASNAv is an astronomic navigation program written by a Merchant Navy
officer.
ASNAv calculates the user position by means of stars observations. A
nautical almanac is not required.
The user enters only his estimated position and sextant altitudes of
heavenly bodies (stars, planets, sun, moon). The program computes and plots
(see example) the most probable position and an ellipse of probability
around it, together with the lines of position (LOPs).
ASNAv is something new in the world of celestial navigation programs.
Because ASNAv is designed by a seaman for seamen.
The program aims to be a practical option as backup of the today omnipresent
GPS. This means that ASNAv is trying to decrease the additional workload to
a minimum. There is nearly no time consumed outside the time required for
the observations themselves.ASNAv is written by a Merchant Navy chief
officer and is user-friendly from a seaman point of view.
Because ASNAv is definitely smarter that the existing astronavigation
programs.
Indeed, a statistical analysis is performed:
a.. each observation can include some small errors due to a bad horizon,
clouds or the ship's rolling during the measure. It is difficult to estimate
oneself the quality of a given observation because the small errors can
counterbalance each other. That's why ASNAv gives a certain weight to each
observation according to its reliability in the normal law model.
b.. very often an observer is repeating a same constant error for each
observation (uncorrected sextant index error and personal error). This
possible systematic error of the observer can be computed and eliminated;
c.. the program can also correct the assumed course and speed if enough
observations are provided (exactly the same way the GPS is able to give the
course and speed of the vessel if enough satellites are visible).
This last feature can greatly improve the accuracy of the results compared
with a traditional method if the exact course and speed are not known
(under- or overestimated drift). However, you need at least 8 observations
during 3 hours in order to make course and speed correction possible. So,
it's a good idea to combine a star fix and a number of sun observations.
See an example of the ASNAv skills.
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