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» Online Classroom   »   » Public Discussion of Cel Nav   » Calculator Celestial Fix

   
Author Topic: Calculator Celestial Fix
Dirk


 - posted June 19, 2007 02:22 PM      Profile for Dirk           Edit/Delete Post 
I went through my calculator manual and now I am learning to use it.
I do this by redoing table based exercises from my RYA Yacht Master Ocean course.
For the sight reductions I come to the same results (with roundings off course).
But for the celestial fix, e.g. based on 2 sight reductions at different moments, the calculated fix sometimes differs 10 miles.

I tried to simplify the problem by setting up an imaginary test situation, just using internal calculator data.
Assume Position A: 49°N 9°E at 8:00 UTC, Position B: 50°N 10°E at 18:00 UTC: 10 hours travelling. Both SR at 19 June 2007, Sun centre, no index error, no dip.
Star Pilot precompute (set to high) indicates for A:
Hs 42° 33.4'
Zn 103.5°
GHA 299° 41.5'
Dec 023° 25.2'
SD 000° 15.7'

For B
Hs 11° 51.5'
Zn 292.4°
GHA 089° 40.2'
Dec 23° 25.6'
SD 000° 15.7'

Calculator Route Sailing Rhumbline indicates
Distance travelled: 71.6 nMiles
Route: 033.1°

Speed: 71.6 nMiles / 10 hours = 7.16 kt

Then I entered 2 sight reductions with the given information.
Then I tried to do a celestial fix at WT 18
I assumed that the result should be exactly the coordinates of position B, but the celestial fix resulted in
Lat 49° 29.1'N
Lon 009° 47.1'E
R 0.55 nMiles
Brg 197.2°

More, if I try to calculate a fix at WT 10, this results in a position SW of position A???????

What am I doing wrong?

Thanks for your help.

Best regards,

Dirk

From: Belgium
David Burch


 - posted June 19, 2007 02:57 PM      Profile for David Burch           Edit/Delete Post 
If you are doing a sight reduction with the sight reduction function using data input from the precompute function then you are still missing the refraction corrections even with HE=0 and IC=0. and since you are taking sights that are nearly the same LOP ie 189° apart in Zn is just 9° intersection angle, so small errors are enhanced in a sense. You might try using the usno aa data at http://aa.usno.navy.mil/data/docs/celnavtable.html

which will show all the altitude correctons so you can fine tune the input. Please try that first and if still not getting the right answer post the new results and we will look farther...

when all the input is right you should still get a very close answer since you are computing the fix rather than plotting it. Plotting a fix at 9 deg intersection would be more questionable, but i do not know if 0.5 is large or small till all the input is right... recall the rule we use in emergency nav course.. the refraction at 5° is 10' and the refraction at 10° is 5' so you are dealing with a refraction correction of 4' in the low sight, which is not being handled properly... ie you are taking it out on the sight reduction, but not putting it in on the sight data (precompute function is pure Hc).

From: Starpath, Seattle, WA
Dirk


 - posted June 20, 2007 07:10 AM      Profile for Dirk           Edit/Delete Post 
You are right indeed. The impact of the correction is huge.
If I deduct from Hc A 14.8', Hs becomes 42°18.6'.
From Hc B I deduct 11.2', so Hs becomes 11°40.43'. I am using SP89 internal Hc. USNO data differ .1' (because SP89 based on Sun center and USNO based on LL?), which has little effect in the final result. I also changed from Sun center to lower limb, which has a big impact.

Rerunning the Celestial Fix with calculated rhumbline data (71.6 miles, course 033.1°, v=7.16 kt), results in position 50°03.1'N, 010°02.0'E, 3.35 miles (from point B) at 22.1°.

Results become better by using the precalculated great circle data: 71.54M, c=032.6°, v=7.154kt, the calculated position becomes 49°59.4'N, 009°59.6'E, .65M from point B at 204.3°

I can come to an "exact" position by increasing the speed to 7.185kt. Apparently one always needs speed for Celestial Fix: log data are only accepted for DR updating, or am I wrong?

Knowing that steering within 5° is a good job, the .5° difference between GC and RL is neglectable, as is the difference in distance of .06 Miles. Nevertheless, is there a way (other than hourly fixes) to avoid that this small difference results in a 3.35 miles position difference, certainly because in this case I made no sextant reading error?

From: Belgium


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