Real Navigation with SH4

[don1reed]

Hi, jmr.

Your AP is N48. You are N of the equator.

Dec = S26. The body is S of the equator.

then, Latitude is contrary to Declination.

Using your figures, I come up with Hc = 7* 51.9'

Intercept = Away 37.8' nm.

Z = 145*
Zn = 215*

You did good, but after a good night's sleep, you'll find it'll all work out

btw: my copy of HO 229 vol 4 (45* - 60*) p. 81, under 48* Lat & across from Dec. 26* shows:
Hc= 8* 08.5, d= -54.4, Z= 145.2*
North Lat then LHA < 180, then Zn= 360 -Z

[don1reed]

I'm not sure you're using the INTERPOLATION TABLES on the Front/Back covers of HO 229???

Since the Dec. Increments above are 18.3' moa. and "d" on page 81 is (-54.4), go to the inside front cover and use that Interpolation table. i.e.,

54.4 = 50 + 4.4

Under Dec. Inc. 18.3 (on the front cover), go across to 50' under tens = 15.1

Under Units 4'.4 = 1.4, then; 15.1 + 1.4 = 16.5

subtract 8* 08.5' -16.5 = 7* 52.0' <----Hc.

The SIGN (+ -) of "d" determines whether to add or subtract, and it looks as if you added the 16 rather than subtracted it.

Cheers,

[jmr]

Originally Posted by don1reed

I'm not sure you're using the INTERPOLATION TABLES on the Front/Back covers of HO 229???

I'm actually using what looks like a simplified interpolation table from a sextant tutorial linked in Mike's SH celnav tutorial.

This is it

The SIGN (+ -) of "d" determines whether to add or subtract, and it looks as if you added the 16 rather than subtracted it.

Cheers,

Yeah that's where the confusion stemmed from. I'm using the "advanced" navigation sheet that's in the tutorial and it shows +d and not +/- d and so I assumed that regardless of the sign for d in the table, I would always add that value. Now I know not to do that

Another thing I was unsure of was whether or not to use Zn = 360-Z for north latitudes or Zn = 180+Z for south latitudes. Since Antares is S I thought I had to use 180 + Z but it looks like I have to use 360 - Z for my latitude.

Thanks a bunch for your help, don1reed!

[don1reed]

Fair winds!

[jmr]

Still having a little trouble on the nav front. Here's a worksheet I completed and I'd be thrilled if you wouldn't mind verifying my results.




Using my numbers I get a funny plot where only the LOP of Capella and Arcturus intersect while Antares is off doing his own thing.

If I bypass all the paper work and grab my Hc from the USNO site I get a much more accurate plot of my position. So, is my math funny here?

BTW, the USNO Hc numbers:

Capella
Hc 7* 07.2
Ho 7* 28.5
----------------
-21.3 (pretty close to what I came up on the worksheet)

Antares
Hc 7* 32.1
Ho 7* 14.1
-------------
+18 (yikes, and I got +38.4)

Arcturus
Hc 25* 30.3
Ho 25* 33.5
----------------
-3.2 (I got +2.8)

[don1reed]

At first glance, jmr, I notice that your math is incorrect regarding Ho & Hc.

e.g.

Always Remember this nemonic memory jog: The Japanese god of navigation-

HO-MO-TO

that is, whenever Ho is MOre than Hc, then the Intercept is TOward the body. Toward the body is (+) positive and Away from the body is (-) negative.

In your Capella example, Ho is More than Hc, therefore the sign should be (+) positive as the intercept is toward the body. The sign would be negative if Hc were the larger of the two.

Likewise, in your Antares example, Hc is larger than Ho, therefore the sign should be (-) negative as the intercept is away from the body.


I calculated your position fix @ L 48* 22.8'N, Long 005* 02.7'W, a typical "cocked hat" of LOPs.
Same goes for your Arcturus example. The sign is reversed.

[jmr]

Hmmmm but the realnav tutorial says (-) is HO MOre TOward and (+) is away.

What I don't understand is why I'm getting such a large descrepancy between Antares between my worksheet results and my USNO results.

[don1reed]

Ah, I see what you're getting at, jmr.

Sorry for adding to the confusion.

It all depends on perspective...

as in...

I'm referring to ... Relative to your AP (+ or -) not actual mathematical quantities.

For instance when I say HOMOTO, I mean when your Ho is greater than the Hc, then from your assumed position (AP) measure off the intercept distance in nautical miles toward the body.

Conversely, if Hc is greater than Ho, then from your AP, measure off the intercept distance Away from the body. I hope that's understandable.


If, for example, our Hc > Ho and our resulting intercept was 23 nm Away and our Zn = 271*, this means we would construct a straight line from our AP, 23 nm in the direction of 091*, the reciprocal of 271*. Our heavenly body's GP (ground position) bears 271* from our present assumed position, and we are constructing a LOP 23 nm away from the body's GP relative to our AP.

Mathematically the realNav tutorial is correct. That is, from our AP Ho(if larger than Hc) is closer to the GP and therefore our distance to the GP is shorter (-). Away would be farther away from our AP and therefore (+).

BUT, what I'm trying to convey is the idea of construction of the actual intercepts.

If Ho > Hc, then the intercept will be drawn Toward the body.

If Hc > Ho, then the intercept will be drawn Away from the body.

Ales Klar?

[jmr]

Aye that makes sense.


I hate to badger you with my busy work, but I'm unsure of Antares from my worksheet attached above.

Antares using a nautical almanac and Ho229 tables I work out:

Hc 7* 52.5'
Ho 7* 14.1'
---------------
+ 38.4 (measure 38.4nm AWAY from Antares)

But using USNO numbers the Hc for Antares is 7* 32.1' Why does mine differ so greatly?

[don1reed]

Hc > Ho
Away

O<-------------------------o----------------------|
body's GP_____________AP_______________LOP


Ho > Hc
Toward

* <----|---------------------o
body__LOP_____________AP

[don1reed]

Your Antares example:
I get:

Hc: 7*32.1' - Ho: 7* 14.1' = Away 18.0 nm

Thats what I come up with using the Hc formula that was used to compile HO 229

Hc = arcsin [(sin L sin d) + (cos L cos d cos LHA)]

Z = [(sin d - sin L sin Hc) / (cos L cos Hc)]

Z = 145*, then Zn = 360* - Z = 215*

Legend:

L = Latitude
d = Declination
LHA = Local Hour Angle
Z = Azimuth Angle (measured E & W from 0-180*)
Zn = Azimuth (measured 0-360*)

[don1reed]

The answer to your question must be in the data you're using for GHA, SHA, and Aries.

Here's what I get:

Aries: for 03:17:01 = 287*04.1'
SHA: Antares = 113*31.7'
min/sec = 4* 15.9'
404*51.7' - 360*

GHA = 44* 51.7'
aLong = -5*(W)
LHA = 39* 51.7'

Dec = S26* 18.2'

[jmr]

Thanks for being patient with my, don1. :p I just realized the mistake I made when doing my calculations for Antares in my worksheet. It was another simple boneheaded arithmetic error when calculating d with my Hc.

[don1reed]

No problem, jmr.

This is worse than doing your own taxes...sometimes

[jmr]

Don, I'm curious, what's the longest you've gone without a sun/star shoot due to poor weather? In real life of course :}