Celestial Navigation Plugin Redux

[NAV]

I now see that the refraction is calculated according to the Bennett formula, as stated in Meeus' Astronomical Algorithms, where the abbreviation Ho is used for the apparent altitude. Confusing indeed.

[rgleason]

I'm going to be trying to use this plugin so I do have an interest in getting it easy and understandable with respect to the way the books treat these measurements.

1. ObservedAltitude = Measurement - IndexCorrection - EyeHeightCorrection - RefractionCorrection - LimbCorrection
2. CorrectedAltitude = ObservedAltitude - ParallaxCorrection

... what is called CorrectedAltitude in the plugin, is called Observed Altitude by Bowditch and Admiralty

Plugin.............Bowditch...............Admiralt y..............Dutch
Measurement..sextant alt (Hs).......sextant alt (Hs).......instrumentshoogte (Hi)
-.......................-..............................-..............................gemeten hoogte (Hg) [=sextant alt corrected for index corr only]
-.......................apparent alt (Ha)....apparent alt (H).......'apparent alt' (Ha)
ObservedAlt.....-..............................-...............................-
CorrectedAlt.....observed alt (Ho)...observed alt (Ho).....ware hoogte (Hw)

NAV what would you like to see?
1. Change the Plugin name for "2. Corrected Altitude" to "2. Observed Altittude"? (Hs)
2. What does "1. ObservedAltitude = Measurement - IndexCorrection ." become? Apparent Altitude? (Ho)?
3. Where does Eye height correction go then?

I hope changing this stuff around does not screw up the programming, because sean probably has routines and variables named a certain way...etc.

I think the entry and interface should be similar to the standard hanc calcs, but if there is any obvious shortcut or improvement to the process because of the computer tables, I would not be s slave to the format!
---Just who am I to be talking here! Don't program and am just learning.

[rgleason]

I now see that the refraction is calculated according to the Bennett formula, as stated in Meeus' Astronomical Algorithms, where the abbreviation Ho is used for the apparent altitude. Confusing indeed.

Sorry NAV you just lost me here. Can you explain a little what the problem is and what Bennett formulat is .

Maybe if you put out the process for calculating that Bowditch uses, and compared it to the entries in Cel_Nav, we could see what is needed, reduced to a few simple changes? Hopefully...

[rgleason]

It would be great if the suggestions were a list of changes that could be added to Tracker, under the Celestial_Navagation_pi project. I understand you are going through the plugin step by step and explaining which is also great for all of us.
Thank you

[NAV]

Originally Posted by rgleason

Can you explain a little what the problem is and what Bennett formulat is .

No problem at all with this refraction formula from Bennett. However, the abbreviation Ho as used in this formula (in Astronomical Algorithms) for apparent altitude is confusing because the standard abbreviation for 'apparent altitude' is Ha (or H by The Admiralty). Ho is normally used for 'observed altitude'.

This formula is suitable for all altitudes between 0 and 90 degrees. The formulas used in Bowditch and by The Admiralty are suitable for altitudes between 15 and 90 degrees (that's why in nautical astro navigation you shouldn't use heavenly bodies with altitudes < 15 degrees.

In the plugin also a second formula for refraction is used. I haven't looked into that one already.

[NAV]

Originally Posted by rgleason

NAV what would you like to see?
1. Change the Plugin name for "2. Corrected Altitude" to "2. Observed Altittude"? (Hs)
2. What does "1. ObservedAltitude = Measurement - IndexCorrection ." become? Apparent Altitude? (Ho)?
3. Where does Eye height correction go then?

The standard is:
Apparent altitude Ha = sextant altitude corrected for index correction and dip
Observed altitude Ho =Apparent altitude corrected for refraction, parallax and semi-diameter

The plugin uses (as shown in the Calculations window):
ObservedAltitude = Measurement - IndexCorrection - EyeHeightCorrection - RefractionCorrection - LimbCorrection
CorrectedAltitude = ObservedAltitude - ParallaxCorrection

(Note: EyeHeigtcorrection = dip, Measurement = sextant altitude)

The possible problem I see (but perhaps I'm overlooking something and Sean can explain) is that the refraction calculation should be a function of the 'apparent altitude', in the plugin refraction seems to be a function of the sextant altitude in these lines of code: double Ha = m_Measurement - m_EyeHeightCorrection; and double x = tan(M_PI/180 * m_Measurement + 4.848e-2*(M_PI/180) / (tan(M_PI/180 * m_Measurement) + .028)); (to keep in mind: I'm not a programmer, too ...).

Secondly, generally I would stick to the naming conventions and calculation procedures as used in Bowditch.

[NAV]

Sean updated some of the code. I'd like to continue testing. Can somebody compile a new Windows version for me?

[rgleason]

NAV
I can recompile later tonight or in the morning. Thanks for alerting me. Did not realize Sean was back with a computer.

[neworleansrich]

Maybe its just too much rum lately, but I could not get a working version of the OpenCPN Celestial Navigation Plugin installed. Could someone post an idiot proof set of instructions on where to download the most current version and how to get it installed. Thanks

[rgleason]

NAV, the current git did not compile and Sean will be making some changes to fix internal file reference problems.

Neworleansrich
Look at Opencpn Beta File thingie, further up in this thread. There you can download the current working celestial nav. Remember that celestial nav requires a DATA directory. Read the Readme file about Celestial Nav in File thingie.

[seandepagnier]

gah, my email server is down

[rgleason]

http://www.cruisersforum.com/forums/...ml#post1468165

[NAV]

Three more observations:

1. In the plugin, eye height (h) can only be entered in whole meters (m). For more accuracy and easier checking against example data in The Nautical Almanac and The Admiralty Manual of Navigation, I suggest to offer the possibility to enter eye height in meters with one decimal (m.m).

2. Dip in the plugin is calculated by the formula 1'.753*SQRT(h). Both the Admiralty Manual of Navigation and the Nautical Almanac use 1'.758*SQRT (h). I suggest to use this last formula.

3. The Nautical Almanac uses a standard atmosphere for the calculation of refraction (Temperature=10 degrees Celsius and Pressure=1010 hPa). I suggest to set the default values in the plugin accordingly.

[rgleason]

Thanks very much NAV for your suggestions. Otherwise how are things looking to you?

[NAV]

Originally Posted by rgleason

Otherwise how are things looking to you?

Well, things look fine, I guess. But checking goes slow when example data cannot (yet?) be entered with same accuracy and when - partly - slightly different formulas are used. A difference of 1' in altitude is relatively large in astronomical positioning, so while checking I try to establish the cause of differences > 0.1' I see (that's 0.00167 degrees).

This brings me to a fourth suggestion:
Although calculations are probably done to full precision, for the presentation of data 3, 4 or 5 decimal figures are used. I suggest to make that 4 decimal figures for all values in degrees. For presentation of minutes (angular) 1 decimal should be sufficient.

Question for Sean: will the POT-file be updated too?