GPS - A warning

[tome]

To everyone who believes GPS is infallible, this from the RIN website:

A significant GPS anomaly occurred on 1 January 2004, beginning at approximately 1833Z. The anomaly affected precise timing and navigation users over large portions of Europe, Africa, Asia, Australia, and the far northern reaches of North America.

The anomaly was due to a failed atomic frequency standard (AFS) on SVN/PRN23. The GPS system relies heavily on the accuracy and stability of its AFS. A failed AFS affects not only precise timing users, but can also significantly degrade navigation accuracy.

A lack of hard failure indications in satellite telemetry coupled with satellite visibility limitations in the Master Control Station's L-Band monitor station network made this anomaly difficult to characterize and resulted in the transmission of Hazardously Misleading Information between approximately 1833Z and 2118Z.

The position error from this satellite was around 300 km by the time it was set to 'unhealthy'

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[AndrewB]

And there was me blaming the weedy hull for the length of time it took between waypoints!

The only thing I noticed amiss on Jan 1st was a very brief "poor coverage" warning message, but that is not unusual. Do you have any indication of the effect of this problem?

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[tome]

You would have experienced a massive position drift - so big that it would have probably confused the GPS receiver and it would eventually fail to compute positions or speed at all until it received the unhealthy flag from the affected SV. Once flagged unhealthy at 2118, the SV would be ignored and the receiver would return to normal operation. The worst part would be the initial period as the SV clock started to drift where you would have seen your position moving steadily away at up to 15 knots (depending on how many SVs in view).

Scary that the ground segment didn't pick this up for almost 3 hours, and it could happpen again.

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[wishbone]

Glad I bought a s/h sextant for xmas, just got to sort out how to do reductions!
Maths aint me strong point, come to fink of it spelling aint upto much.

Wishbone
Rolling, rolling, rolling keep them doggies moving!
Where’s me chuck wagon gone?


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[BrendanChandler]

Sextant and Reductions using a calculator

I have a sextant which I use too infrequently to be able to do the reductions easily, quickly and correctly.

There was a program available for programmable calculators some years ago which took most of the hard work out of the reductions. That model calculator is now dead. Does anybody know of a calculator and program which is currently available?

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[extravert]

Re: Sextant and Reductions using a calculator

If you can come up with algorithms I could program it for a Symbian based phone.

<hr width=100% size=1>One day, I hope to own a real boat.

 

[LadyInBed]

Re: around 300 km

No sweat, well within my margin of error /forums/images/icons/blush.gif

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[bedouin]

Re: Sextant and Reductions using a calculator

It is pretty straightforward to do this in a spreadsheet - such as is available on most PDA devices.

However the best advice is to get a paper "pro forma" for doing sight reductions. That reduces the whole thing to a set of simple stages with very little room for error.

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[BrendanChandler]

Re: Sextant and Reductions using a calculator

It is pretty straightforward to do this in a spreadsheet - such as is available on most PDA devices.

My problem is that I do not do the sight reductions often enough to become familiar with the process. I take a sextant on most ocean trips and use it there a few times but each time I am rusty and have to reinvent the wheel. I become unfamiliar with the crip sheets so I was hoping to automate as much as possible.

Where is the algorithm available to use on a spread sheet or whatever?

I have a vague memory of seeing an article in PBO perhaps about an alternative method of sight reduction without tables, but midterm memory seems to be following short term memory.

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[AndrewB]

Re: Sextant and Reductions using a calculator

<blockquote><font size=1>In reply to:</font><hr>

<font size=1>I have a vague memory of seeing an article in PBO perhaps about an alternative method of sight reduction without tables, but midterm memory seems to be following short term memory. </font size=1>

<hr></blockquote>

Quite a while back! In place of tables it is possible to use the "analemma curve", a little graph which is often found pasted inside sextant boxes. Together with a sextant and chronometer this will provide passable accuracy for ocean voyaging, to ten miles or so. The method is given on several web sites, if you have PowerPoint <A target="_blank" HREF=http://instaar.colorado.edu/deltaforce/GEOL4060/Lectures/Lat_and_Lon.ppt>HERE</A>'s a good one.

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[Guest]

Calculators

Check out Micro-Mart or similar and often you will have Casio Basic / Sharp pocket computers available ..... fine for this work.

<hr width=100% size=1>Nigel ...
So WHAT does the EU really stand for ????/forums/images/icons/cool.gif

 

[Ovni]

Re: Sextant and Reductions using a calculator

If you own a Texas Instruments Ti 83, Ti 83 plus, Ti 83 Plus SE or the new Ti84 graphic calculator. (available soon in Europe) and you are looking for a freeware program on celestial and coastal navigation look here:

http://www.ticalc.org/archives/files/authors/74/7404.html

If you're willing to pay a lot of money for a Ti-89 or Ti-92 you can take the professional program Starpilot : (calculator + program about $360)

http://www.starpath.com

Starpilot can handle also the moon, planets and fixes with 3 Lop's, Navigate! '04.26 handles sun sights and star sights and fixes with 2 lops.

In both cases you can download the userguide in advance.

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[BrendanChandler]

Re: Sextant and Reductions using a calculator

Thanks Andrew, but the site that you pointed to is password protected.

Brendan

Quite a while back! In place of tables it is possible to use the "analemma curve", a little graph which is often found pasted inside sextant boxes. Together with a sextant and chronometer this will provide passable accuracy for ocean voyaging, to ten miles or so. The method is given on several web sites, if you have PowerPoint HERE's a good one.



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[steveh]

Re: Sextant and Reductions using a calculator

Does anyone know where it is possible to download a spreadsheet/program that will work out these reductions
for use on a PC?
Do you still need all the sight reduction tables?

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[Gunfleet]

Re: Sextant and Reductions using a calculator

Why use anything else? Within ten miles is as close as you'll get on the deck of a small yacht!

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[ubuysa]

Re: Sextant and Reductions using a calculator

Two possibilities:

1. Ftp the file celnav.zip from my website<A target="_blank" HREF=http://www.tcr.co.uk/Files> here.</A> It's a zip file containing an Excel spreadsheet template wot I wrote following my recent RYA ocean nav course. I've checked the calculations carefully but I make no claims to their accuracy. <font color=red>Use them at your own risk.</font color=red> You do need an almanac and the sight reduction tables to use these.

2. Visit <A target="_blank" HREF=http://www.tecepe.com.br/nav>www.tecepe.com.br/nav</A> where you can download a really neat shareware program ($25 to register) that does all the work for you (no almanacs or tables needed!). I can't vouch for it's accuracy since I've not tried it in anger, but it gave results within a few miles when I fed it the worked examples from the RYA ocean nav course.

Tony C.

<hr width=100% size=1>There are 10 kinds of people, those who understand binary and those who don't.<P ID="edit"><FONT SIZE=-1>Edited by ubuysa on 27/04/2004 16:50 (server time).</FONT></P>

 

[AndrewB]

Re: Sextant and Reductions using a calculator

Password protected? Well, my recommendation was some time ago. As an alternative, take a look at the <A target="_blank" HREF=http://www.davisnet.com/product_documents/marine/manuals/mark15.pdf>Davis Mark 15 Sextant Instruction Booklet</A>, which explains the method very clearly, though the analemma ("equation of time") is presented there in tabular form rather than as a graph.

All you ever wanted to know about the analemma and a whole heap more can be found <A target="_blank" HREF=http://www.analemma.com/Pages/framesPage.html>HERE</A>.

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