[TEC] Real navigation Tutorials

I've been calculating my LAT for the past 3hrs but I just can't figure why my math is incorrectly. Could someone please help me out?

I'm using Tutorial TDC as my mission(TWOS), starting location is 46.41N , 15.10W

My math below:
1/1/1941, sun's highest point is at 9am local or 10am GMT.
Sextant Altitude=56.31, DEC is S23.04
ZD=89.60-56.31=33.29
Latitude=33.29-23.04=N10.25

What have I done wrong here? Why is my LAT 10.25N instead of 46.41

Any help would be appreciated!

[Sean C]

Well, the good news is that your math is correct. The bad news is that it seems your noon altitude is wrong.


The equation for figuring out the predicted altitude of a celestial body is:


asin(sin(Dec)·sin(Lat)+cos(Dec)·cos(LHA)·cos(Lat))


"Dec" is the declination of the body, "Lat" is your latitude and "LHA" is the local hour angle of the body. By definition, at local noon, the LHA is 0°. So, plugging in the other figures from your example gives us:


asin(sin(-23°04')·sin(46°41')+cos(-23°04')·cos(0°)·cos(46°41') = 20°15'


In other words, the altitude you should have measured given these parameters is 20°15'. Subtracting this from 90° (or 89°60') gives 69°45'. Subtracting the declination, 23°04', from 69°45' gives 46°41' ... your DR latitude.


Now, why your altitude is so wrong is another question altogether.

[vdr1981]

Originally Posted by Lymark1

I've been calculating my LAT for the past 3hrs but I just can't figure why my math is incorrectly. Could someone please help me out?

I'm using Tutorial TDC as my mission(TWOS), starting location is 46.41N , 15.10W

My math below:
1/1/1941, sun's highest point is at 9am local or 10am GMT.
Sextant Altitude=56.31, DEC is S23.04
ZD=89.60-56.31=33.29
Latitude=33.29-23.04=N10.25

What have I done wrong here? Why is my LAT 10.25N instead of 46.41

Any help would be appreciated!

Try it in the campaign. I don't think that time and date are correct in single missions...

[sckallst]

So I've jumped back in to sub-simming recently. I've always played Real Nav/no map contacts, but this time I've decided to go all-in on celestial navigation with no or minimal help from the in-game navigator and scripts.

I understand cel nav well enough to get by, but I'm having a tough time getting accurate-enough sights with even small wave motion. I'd really like to be able to mess around with three star fixes and such, but I'm just bouncing around way too much.

I see others in this thread have had this issue as well. Any new advice or fixes out there?

Thanks to all who've posted the various tutorials and other resources here and in the SH3/SH4 forums. Really looking forward to doing navigation to add something to the game.

[vdr1981]

Originally Posted by sckallst

So I've jumped back in to sub-simming recently. I've always played Real Nav/no map contacts, but this time I've decided to go all-in on celestial navigation with no or minimal help from the in-game navigator and scripts.

I understand cel nav well enough to get by, but I'm having a tough time getting accurate-enough sights with even small wave motion. I'd really like to be able to mess around with three star fixes and such, but I'm just bouncing around way too much.

I see others in this thread have had this issue as well. Any new advice or fixes out there?

Thanks to all who've posted the various tutorials and other resources here and in the SH3/SH4 forums. Really looking forward to doing navigation to add something to the game.

Small tip: If you select "deck awash" depth, your sub will be much less bouncy and you can still use built in sextant...
You can also try to pause the game with "backspace" while measuring the elevation of celestial bodies...

[sckallst]

Thanks for the tips. Will try them tonight.

I've been doing pretty well with just noon sights for latitude, dead reckoning, using the radio beacons, using an almanac and timepiece, but those only get you so far.

I get that DIY navigation isn't for everybody, but it sure adds to the immersion factor if you have the time and patience to do it. Also makes the whole intercept process on radio contacts a lot more interesting when you have to count on yourself to know where to even start from in your calculations.

[ouPhrontis]

I show sunrise and sunset times as being off, I seem to recall this was an issue with SHIII, is that the case with 5?

https://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html

^this shows sunrise for 56°47'N 1°3'E on 08/11/1939 as 'apparent' 0719, whereas the board in-game has 0705 (118).

Will this interfere with Cel-Nav?

Currently have bad weather, so pitching/rolling deck is messing with my results.

[Lanzfeld]

Yes I have noticed that the in game board sunset and sunrise times I’ve been off for a long time.

[vdr1981]

Originally Posted by ouPhrontis

I show sunrise and sunset times as being off, I seem to recall this was an issue with SHIII, is that the case with 5?

https://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html

^this shows sunrise for 56°47'N 1°3'E on 08/11/1939 as 'apparent' 0719, whereas the board in-game has 0705 (118).

Will this interfere with Cel-Nav?

Currently have bad weather, so pitching/rolling deck is messing with my results.

I don't do full celestial navigation my self so forgive me if my answer is pointless, but can this built in function be of any help for you?

 

[palmic]

Originally Posted by vdr1981

I don't do full celestial navigation my self so forgive me if my answer is pointless, but can this built in function be of any help for you?

 

What is he asking about is if the sun is accurate enough to use with manual celestial navigation (i don't know)
You don't have to use it since navigator can do it accurate enough...

[Vlaxie]

I can't figure out the procedure to get the correct longitude tbh, all I know is that the sun will pass over Greenwich when GMT is at 12:00pm but I don't know what the other steps are for getting my correct longitude

[Sean C]

The best way to determine longitude using only the Sun is by doing a "time sight". You measure the altitude of the Sun several hours before or after local noon and then use tables (or a calculator) to find your longitude.


If you want to use tables, a good set is available for free here.


If you want to use a calculator (which is quicker and, IMO, easier), you can use the following formula:


cos(LHA) = (sin(Ho) - sin(Dec.) · sin(Lat.)) / (cos(Dec.) · cos(Lat.))


... where:

• "LHA" is the local hour angle of the Sun
• "Ho" is the measured altitude of the Sun, corrected for index error, height of eye, refraction and semi-diameter
• "Dec." is the declination of the Sun at the time of the sight
• "Lat." is the latitude of the boat at the time of the sight (usually determined from dead reckoning or "run up" from a noon sight or sight of Polaris

Once you find the local hour angle of the Sun, you can compare it to the Greenwich hour angle (GHA) and determine your longitude.



*Note: the tables linked above do this in a roundabout way by using the "apparent time at ship" and comparing that to Greenwich Mean Time (GMT). But this is really the same concept, because the difference in time is directly linked to the difference in longitude.

[Vlaxie]

Originally Posted by Nathaniel B.

The best way to determine longitude using only the Sun is by doing a "time sight". You measure the altitude of the Sun several hours before or after local noon and then use tables (or a calculator) to find your longitude.


If you want to use tables, a good set is available for free here.


If you want to use a calculator (which is quicker and, IMO, easier), you can use the following formula:


cos(LHA) = (sin(Ho) - sin(Dec.) · sin(Lat.)) / (cos(Dec.) · cos(Lat.))


... where:

• "LHA" is the local hour angle of the Sun
• "Ho" is the measured altitude of the Sun, corrected for index error, height of eye, refraction and semi-diameter
• "Dec." is the declination of the Sun at the time of the sight
• "Lat." is the latitude of the boat at the time of the sight (usually determined from dead reckoning or "run up" from a noon sight or sight of Polaris

Once you find the local hour angle of the Sun, you can compare it to the Greenwich hour angle (GHA) and determine your longitude.



*Note: the tables linked above do this in a roundabout way by using the "apparent time at ship" and comparing that to Greenwich Mean Time (GMT). But this is really the same concept, because the difference in time is directly linked to the difference in longitude.

Hello, thank you for the quick response

I took a sun shot a few hours after the local noon and I applied the formula
(sin(Ho) - sin(Dec.) · sin(Lat.)) / (cos(Dec.) · cos(Lat.)) and I'm pretty sure I've input all the correct information, the latitude is correct, the declination taken from the almanac for the exact day and hour when I took the shot and the altitude angle as well but I'm not sure what to do with the resulting number (I did make sure to set the calculator to display the result in degrees). For reference, the number that I got at the end was 0.82, and the sun's GHA at the time of measurement is 59° 58.0'.
I know that my actual longitude is around 19° 4'E but how do I get to that number from what I calculated?

[Aktungbby]

Vlaxie! after a 3 year silent run!

[Sean C]

Originally Posted by Vlaxie

I took a sun shot a few hours after the local noon and I applied the formula
(sin(Ho) - sin(Dec.) · sin(Lat.)) / (cos(Dec.) · cos(Lat.)) and I'm pretty sure I've input all the correct information, the latitude is correct, the declination taken from the almanac for the exact day and hour when I took the shot and the altitude angle as well but I'm not sure what to do with the resulting number (I did make sure to set the calculator to display the result in degrees). For reference, the number that I got at the end was 0.82, and the sun's GHA at the time of measurement is 59° 58.0'.
I know that my actual longitude is around 19° 4'E but how do I get to that number from what I calculated?

Could you provide the latitude, declination and Ho (altitude) of the Sun? That would help me double check the calculation.


You left out the "cos(LHA)" part of the equation above, so I'm not sure if you took that into account. It means that you must find the arccosine of the result from what you did quote. Another way to write the equation (which might make more sense) is:


acos((sin(Ho) - sin(Dec.) · sin(Lat.)) / (cos(Dec.) · cos(Lat.))) = LHA


Also, one thing I forgot to mention is that, technically, the result you get is not necessarily the LHA proper. It is actually what is sometimes referred to as "t": the "meridian angle" of the Sun (or how many degrees away from the local meridian the Sun is, either east or west). If you took your sight after local noon, t = LHA. If you took your sight before local noon, 360° - t = LHA.


The difference between the LHA and GHA of the Sun is your longitude. If your actual longitude is indeed 19°04' E, then the Sun will be 19°04' farther west at your location. If we add your longitude to the GHA of the Sun, we get: 59°58' + 19°04' = 79°02'. That is what the LHA of the Sun will be at your location. (Hint: when comparing GHA to LHA, add longitude to GHA if east of Greenwich, subtract if west.)



If we take the arccosine of 0.82, we get t = 34.9152... or 34°54.9'. Since you state that you took the sight after local noon, t = LHA. This is clearly way off from the expected value of 79°02'. On the other hand, assuming you did take the arccosine of the result, 0.82° = 0°49.2' ... also way off.


But, again, I would need all of the actual data to begin to track down exactly what went wrong. It could very well be that the sim does not have the Sun at the correct altitude for your location. Multiple people on this forum have come to the conclusion that SH does not correctly model the positions of the heavenly bodies. I do not play SH, so I cannot personally verify this.