Real Navigation with SH4

[don1reed]

@minimax:

You've made the same mistake as I have re sextant mod...

place the bottom of the sextant ( zero 0) on the bottom of the screen, not, I say again---NOT on the horizon. The sextant will not read low angle bodies & make sure the object you are observing is in the center of the screen, that is make sure you are view dead ahead, then press the (-) key to zero the screen. Do not elevate (up/down arrows), just rotate (left/right arrows) to the body of interest...get it centered in the screen. Bring the sextant from the side and maneuver it between the top/bottom of screen, (a small bit of yellow should be seen at top & bottom when properly adjusted. Registration marks). Now read altitude.

As I've requested and said elsewhere, the Observation scope would be a good candidate for substitute sextant, as it can be elevated to 90°; but, we need a way to read the elevation angle from horizon to body.

Minimax - how did you get that Sextant image to be draggable.....


And Don1 -- are you sure its not just course you started in the water west of Midway at campaign mission start... Witch would corrospond with Minimax's opinion about Base Time = time at start position in mission

[kv29]

I got confused

I calculated these 3 sunsets:

Pearl Harbor
Date: 07/10/44
157°59w 21°14N
sunset=18:24
SunAlmanac= shows a latitud of 10° at that hour. For 21°, sunset should be at 18:43
Starry night= visually shows sunset at 18:45

Brisbane
Date: 06/11/42
153°51E 27°9S
sunset=17:12
SunAlmanac=calculating time/degrees between 20° and 30° in SunAlmanac, you get exactly 17:12 :p
Starry night= visually shows sunset at 17:14

Perth
Date: 03/16/43
114°56e 32°11s
sunset=18:44
SunAlmanac=Shows a latitud of aprox 70° at that hour. For 32°, sunset should be at 18:16
Starry night= visually shows sunset at 18:18


wtf??

[Minimax]

@don1reed

place the bottom of the sextant ( zero 0) on the bottom of the screen,

Thanx for your explanation - my problem is:

Minimax - how did you get that Sextant image to be draggable.....

I don't get it draggable!


@kv29
Did you move your sub after starting the game?

I made some tests with the editor:
they're all like that:

******************
start position
40°00' S 140°00' W

sunset almanach:
19.32
sunset on-board-watch
19.32
******************

Same with in- and outside Pear Harbour: GMT sunset = local time sunset


I used that sunset almanac

http://www.vanjast.com/NavMod/RealNav_ReadMe2.zip

published by Vanjast here:

http://www.subsim.com/radioroom/show...eal+navigation

[don1reed]

Ooops, Sri, minimax, I misunderstood.
I believe you're correct, however, about the "base time".
I'm making some tests currently to simplify and/or clearify it, i.e, lets say we're based at Pearl Harbor and given a mission to patrol off Honshu and we'll refit at Midway Island both ways.

Pearl Harbor, Oahu, H.I. ZD +10 William.

Midway Islands ZD -11 Love.

Honshu Island ZD -9 Item.

Note 1."Bowditch" tells us to convert Zone Time (ZT) to GMT by adding positive ZT and subtracting negative ZT; conversely converting GMT to ZT, a positive ZD is subtracted, and a negative one added.

Note 2."Bowditch" further tells us re Date Line:

E Longitude----> 180°----> W Longitude = Loose 1 day
1 January 1942--->|---> 31 December 1941
5 July 1944<------|<-----4 July 1944

Even as virtual navigators, we still have to deal with this...so, the first thing we must do before casting off is to calculate GMT.

As we travel westward toward our objective the clock in the sim is still adjusted to "base time", this is the reason sun/rise/set and Local Apparent Noon (LAN) are happening at weird hours of the day and night. In order to compensate, see Note 1. above, we convert GMT to ZT by adding a negative ZD to give us local time at our present location, then we can correctly calculate sunrise/sunset LAN for navigation purposes.

What I've yet to discover is when we refit at Midway Island, does the base time revert to Midway's local time. Anyone know?

Nations have created Standard time/Daylight saving time rules and regs for commercial purposes; but, the sun's place in the heavens obeys no human command.

[kv29]

minimax, Im starting to feel reaaaally stupid...
Please do this: start a new career outside Pearl, 01/03/44 (157°59w 21°14n), tc till sunset and note what time is it.

Im getting 17:00 with my damn clock (or when the red ligh goes on, right?), and sunalmanac says 17:31 for 21°n.
Now, if your are getting another numbers, then....

don1reed, can you be a little more specific?? bad english is what I understand, can you post a step by step?? thanks

[don1reed]

Kv29

Lemme try what you're suggesting...and I get back.

Originally Posted by don1reed

Even as virtual navigators, we still have to deal with this...so, the first thing we must do before casting off is to calculate GMT.

As we travel westward toward our objective the clock in the sim is still adjusted to "base time", this is the reason sun/rise/set and Local Apparent Noon (LAN) are happening at weird hours of the day and night. In order to compensate, see Note 1. above, we convert GMT to ZT by adding a negative ZD to give us local time at our present location, then we can correctly calculate sunrise/sunset LAN for navigation purposes.

Same question here as Minimax.

Could we take an example to follow for all calculations please - Say Pearl Harbor departure on 22. Juli 1943 08:00 Base Time.

To me the above would give a GMT departure of:
21. July 1943 22:00

Is that correct so far?

[kv29]

Mav, when I try to start a career in 1943, the starting date is march 16th. where your date come from? I recommend to use defaults new career dates for testing purposes. Or are they randomized??

Nah i was just taking an example out of the blue.

[don1reed]

@Kv29: Closest I could get was a quick mission of 17 JAN 44 but from the same location: 21-14N 157-59W. My real life nav pgm for that date and year shows 18:02:40 for sunset, Azimuth 246° true.
The Sun tables of van shows 17:42
When the red light came on 17:08
34 minute difference is really not accurate enough to navigate by.

21° is 1/10 of the distance between the given angles in van's table.
There happens to be 21 minutes difference between the times for those angles. We can now interpolate:

1/10 x 21 = 2.1 min

We can subtract 2.1 min from 17:33 = 17:31 should be the time for sunset of our latitude IF the sun tables were accurate.

@ Mav:

Oahu is in Time Zone +10. This means we have to add 10 to obtain GMT.

Now, as we proceed westward, crossing the Date Line we gain a day (adding 24 hours to local time) GMT remains the same, but, to convert GMT to local time once we're west of the Date Line the sign is negative,


example: sailing west

ZT +10 local time 0100 date 3 JAN 44 = 1100 GMT
ZT +11 local time 0200 date 3 JAN 44 = 1300 GMT
ZT +12 local time 0300 date 3 JAN 44 = 1500 GMT
ZT -12 local time 0400 date 4 JAN 44 = 0400 GMT

The easiest way to do this if you use XCEL or some other spreadsheet is to build yourself a GMT conversion chart.

Left column top to bottom = local time 0000 to 2300.

Top row left to right Zone Times:
Y-X- ........etc to M
+12...................-12
then fill in the blanks with the spreadsheet's functions. Done. Now all you have to do for conversion is match column & row to determine GMT or visa versa.

One other thing, if you make your own time conversion chart, is to highlight the 00:00 in each column...this represents the Date Line. They should show up as diagonals on your chart. So, whenever your rows cross the diagonal it means you've crossed the Date Line...something to be aware of.

[Minimax]

mysterious

SH4 + 1.2 + date line bugfix (as in FTT Mod...)

Start new carreer 1943 / 1 in Pearl Harbour

Start date: 3. Jan 1943
Start outside harbour at 28°03,7'N 178°39,5'E
Sunset according to Van's almanac: 17:16 GMT









at this time the light changed to red

[kv29]

ok Don, lets follow your example then. why are we obtaining such differences with sunsets?

Im using starry night pro, set it to gmt-10.5 hours (as I think it should be for west time at that grid, correct me if Im wrong), and I get sunset at 17:42 247° true. (btw, does "true" sunset occur when the sun is 1° lower than horizon? because thats what this program displays).

I checked this at the US naval observatory and I get the same time & degree.

mav charts are wrong then? crap, nice headache I got...


Edited: I posted this before reading minimax post. Im not using fft mod, minimax are you talking about the career fix so you don´t get retired if your sub gets discharged? btw, Pearl is not at that grid, you should start at 157°59w 21°14n (outside the port, I mean)

[Minimax]

minimax are you talking about the career fix so you don´t get retired if your sub gets discharged?

No - I'm talking about the attack map fix, that lets you start at 179°E behind the date line in place of 157°W.

[don1reed]

Some of the reason our figures differ see this pic below:



The precise instant the "red interior light" comes on may occur when only half the sun is hidden by the horizon. If so, it is not the correct, real life, sunrise/sunset.

We're dealing with a simulation and as such we must attempt to find the right combo that allows us to have some simularity of navigation. I personally would prefer greater accuracy than 60nm (1°).

Another navigational fault with the sim is the chart, as once again in real life you cannot draw a straight line course over long distances between two points. In RL a long straight line course is only drawn on a gnomonic projection chart to compensate with the curvature of the earth. A gnomonic projection is transferred to a Mercatur projection as a series of short rhumblines that resemble a curved course overall. Be that as it may, a sim cannot be all things to all players...so what I'm saying is we have the choice of getting lost (frequently) or dragging the ingame course icon from point A to point B.