NavList:

   

Reply

I've been trying to follow the dialogue under this threadname between Dave
Walden and Joe Schultz. This started with a posting on Jan 11 by Dave
Walden. Within the text he referred to 3 attachments, but only one
attachment arrived, headed jkarl.jpg. What am I missing?

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: 
To: 
Sent: Monday, January 11, 2010 5:30 PM
Subject: [NavList] Running Fix vs Estimated Position -- Monte Carlo
assessment


Running Fix vs Estimated Position -- Monte Carlo assessment

The question of the relative merits of a traditional running fix vs an
estimated postion based on minimizing the distance from the DR position to a
point on a second LOP has been discussed recently.  Three attachents are
given which summarize a Monte Carlo simulation that was carried out in an
attempt to provide some quantitative results to further the discussion.

A quote from the renowned naturalist Louis Agassiz: "Go to nature; take the
facts into your own hands; look, and see for yourself."

First, a plot of average distance from true postion to Rfix or EP in
nautical miles.  This shows a number of things.  When the crossing angle
between the two LOP's is small, the traditional running fix can produce
large errors.  Not surprinsing.  In fact, well known and expected.  (And
reassuring to find in the result!)  When the crossing angle between the two
LOP's is small, the EP does well.  Not surprising after a little thought.
The EP is "anchored" by the DR and doesn't get too far away.  At 90 degrees,
the two methods give the same result.  Again, expected and reassuring to
see.  Interestingly, for crossing angles between about 25 degrees and 90
degrees, the traditional running fix method gives better results.

Next, is a scatter plot showing error in nautical miles as a function of
crossing angle between LOP's.  As expected from the above, when the angle is
near 90 (or equivalently, 270), the error is lower.  At 0 (or 180) the
errors in Rfix are large.  (In fact there were Rfix values off scale at very
small crossing angles, but the scale was chosen so as to provide a
'reasonable' view over the range of anagles.)

Last, a view of the spread sheet that was used to produce the data above.
The Rfix and EP are both plotted.  A visual basic macro was written to copy
the inputs and results of this sheet to a table on another sheet.  The excel
rand() function was used to provide variation and 1000 runs were made. The
input values for the above results were:

Latitude:  2 degrees North  (but not expected to affect the conclusions)

Longitude: 2 degrees East   (but not expected to affect the conclusions)

ships course: 45 degrees  (but not expected to affect the conclusions)

error in estimated ship course:  +/- 5 degrees, uniformly distributed
randomly

Ships speed: 6 knots

error in estimated ship speed:  +/- 2 knot, uniformly distributed randomly

time interval: 1 hour

Azimuth of LOP 1: 0-360 degrees, uniformly distributed randomly

Azimuth of LOP 2: 0-360 degrees, uniformly distributed randomly (independant
of LOP1)

Error in initial DR postion: +/- 3 nautical miles along LOP1, uniformly
distributed randomly
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------

File: 



File: 

   

Reply