Quote:
Originally Posted by CaliEs
I dont see "excellent results". What i see are posts of ambiguity and questions.
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Thank you for joining us. You certainly came to the right place for ambiguity and questions.
Seeing "excellent results" comes from understanding the long challenging history of implementing in-game real navigation within SH3.
Usually I just see stars though...
Quote:
Originally Posted by CaliEs
I look at your first calculation with the stars Alioth, Antares and Sirius
My result with your given data is this: 3StarFIX = 000-41.1'S 030-28.3'W, means 50.7nm (= 101.4km) from your real position 000N 031W.
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It looks like you took my observations and dumped them into a program that spits out a position on earth. Without having the program and understanding what assumptions it is making, I can't tell you why it gives a different result. But I can clearly explain how I am getting my results.
Quote:
Originally Posted by CaliEs
Show us the concret calculation steps from the sighted position to the calculated Hc.
Also give us evidence of the taken shots of the stars. For example Sirius at 12°20 at 0203 GMT.
Why do you put in an index-error of 0.6' to the calculation?
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Most of what you ask for has been covered in this thread.
Hs is the measured altitude of the star using a linear sextant with certain parameters set in the cameras.dat file of the game. Screenshots are available in earlier posts.
Index is a correction based on observing the error in 23 navigation stars from a known position with the sextant. The results are available in an earlier post. I notice that my errors largely are positive, so I subtract their average in an attempt to improve accuracy. For the flak station sextant, that is 6'.
Ha is what I am calling the apparent altitude after the index correction.
Alt is my main correction to the measured altitude. It was based on a camera calibration curve I developed for the external view at the station where the sextant is used. It depends strongly on the screen position, and therefor sextant position. Covered in earlier posts above.
And that brings us to my Ho. Let me dig out and post that Sirius screenshot. I will put the Alt correction chart on it also. Here we go.
SH3's Sirius displayed in my sextant together with the corresponding Alt correction based on in-game measurements of the display error of the horizon. Take a look at the horizon. That is a feature of how information is displayed in the game, and my Alt correction puts it in the right place after you measure Hs and optionally apply an Index correction. I squint and measure the star to the nearest 10'. Go ahead and hit "CTRL" + "+" a few times and stick your nose on the screen. How did I do?
Also notice how the SH3 clocks are handled. When you start a mission, SH3 remembers your starting nautical time zone and applies that as an offset to the "local time" and the "screen time". I determined how to handle that by making observations of sunrise/sunset data. Also covered in detail in this thread. I have a few more hours of testing ahead of me for that though. That will be the next post.
Now we have everything we need to plot the Sirius line of position on the nautical chart. Here is my result.
Celestial navigation works by comparing the altitude or height above horizon of a body from your unknown position to a value calculated from an assumed position nearby. Here I have created a test mission that puts me at 0°N 30°W. I shot the star. I consult an almanac for an assumed position AP of 1°N 31°W.
Here is what I know.
1. If my boat was at the assumed position AP, the star Sirius would be observed to be 12°19' above the horizon at a bearing of 253°.
2. From my real position, I observe the star Sirius to be 11°40' above the horizon which is lower by 39'.
3. Things further away look smaller and lower in the sky.
4. My position must be further away from Sirius than the AP at 1°N 31°W is from Sirius.
5. Planet SH3 has 1° = 120 km so that 39' difference in altitude would be 78 km is distance.
6. There are an infinite number of positions on planet SH3 that are 78 km further away from Sirius than is the AP. But the ones near my boat run on an almost straight line at right angle to the Sirius bearing.
7. I must be on that line.
Using the ingame mapping tools, this is what I do.
I plot the bearing from AP to the star. I advance or retreat with the ruler the distance toward or away from the star. I draw a right angle and call it the Line of Position for the star.
Repeat two more times and I get the error triangle that should contain my position if everything was done right and everything was right.
Read the whole thread. I went into this thing with my eyes open. I am attempting to quantify the errors. I am stating my assumptions and I am showing my results, including the errors.
My mods are available for download, link in my signature.