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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » mathematically advancing an LOP?

Author Topic: mathematically advancing an LOP?

 - posted April 04, 2020 05:33 PM      Profile for bruce           Edit/Delete Post 
As I work through this stuff my old brain-cells are reconnecting and I'm remembering some of the "tricks" I used to use Back In The Day.

One of the tricks was in advancing an LOP. The textbook way to advance an LOP is to pick a spot on the LOP, advance it the right distance along the DR line of travel, and draw a parallel line.

If I recall correctly, I used to "shortcut" that by effectively calculating a separate DR line, using the altitude-intercept as the initial point.

In other words, if I know that the point where my LOP crosses Zn is at 48*00'00"N, 120*00'00"W, and I've gone 28 miles at 220T since that sight, I can calculate the projection and plot a point at 47*38'33"N, 120*26'48"W, draw a line through that point at right angles to the Zn value, and... have my advanced-LOP

is that a valid approach, or does that introduce error (or mask nuances) that are handled by the normal approach?


From: Everett, WA
David Burch

 - posted April 04, 2020 08:25 PM      Profile for David Burch           Edit/Delete Post 
Yes, you can compute a modified a value to account for the advancement. It is discussed in the back of the Hawaii by Sextant book... maybe also in our textbook. I think called "automatic advancement" or something like that. Just a simple sin or cos correction as i recall
From: Starpath, Seattle, WA

 - posted April 05, 2020 07:00 PM      Profile for bruce           Edit/Delete Post 
Thanks! I checked out the "automatic advancement" section in the back of HBS.

Back in the day, I used to use an HP-41 with the "nav pack" (I think we talked about that when I was in your shop a couple of months back. I mounted an expedition into the scary corners of my garage where my old sailing gear is, and found it... and it still works! The "DR" function is fairly crude but it does the job - enter a position, course and distance, press the button and it gives the lat/lon of the resulting projection. Pretty handy.

As an aside there are a couple of really good HP-41 "apps" available for iPhone (probably Android as well), and I've started using one which allows the import of code modules. I've imported the "nav pack" code and it works great. Wonder if that would work with the StarPath code you stored on magnetic cards?


From: Everett, WA
David Burch

 - posted April 05, 2020 07:57 PM      Profile for David Burch           Edit/Delete Post 
That is very interesting about an iOS version of HP41cx, but the timing is bad for us.

We just two days ago gave away our entire HP41cx package including nav pac, card reader, and tons of other parts. I do not think we have a copy of any of that any longer.

From: Starpath, Seattle, WA

 - posted April 06, 2020 11:56 AM      Profile for bruce           Edit/Delete Post 
Gahhhh! I wish I'd known. I'd love to have grabbed it. Oh well... next time

If you still have a set of the cards, I could check and see if my card-reader still works. if it does, you could at least have a copy of the routines in case you want to re-use them at some point.

From: Everett, WA
David Burch

 - posted April 06, 2020 01:56 PM      Profile for David Burch           Edit/Delete Post 
Frankly, we have long since then developed our own StarPilot calculator for TI 89 which has IOS and PC versions, and this tool has far more than anything that was in the HP41c series. www.starpilotLLC.com

I have fond memories of those days, but we have much better navigation tools now.

From: Starpath, Seattle, WA

 - posted April 06, 2020 07:52 PM      Profile for bruce           Edit/Delete Post 
Yes, I purchased the StarPilot app for my phone, and it looks awesome, but.... I'm ignoring it for now in favor of actually (re)gaining depth in the concepts and techniques before I start using the tools that make them easier...

Thanks for everything. These resources you provide are really wonderful

From: Everett, WA

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