Celestial Navigation - how good were you?

[SVHarmonie]

Originally Posted by skipmac

Sounds like navigating to Hawaii in the old days. Never did it myself but heard the plan was to leave California, dead reckon until you're a few days or week out of Hawaii then follow the contrails from the jets flying into Honolulu.

It is such a nice story...

https://fetchinketch.net/2015/11/07/...an-urban-myth/

[Lodesman]

Originally Posted by skipmac

That was a big deal for a while when GPS was starting take over especially with the initial GPS fixes degraded with selective availability. A lot of boaters, mainly for fishing and diving didn't want to give up Loran, for the accuracy and repeatability but also because they had a collection of Loran numbers for all their favorite spots.

The first time I dove the artificial reef site 25 miles off Clearwater with GPS I had to run a search pattern for about 30 minutes before I finally spotted the wrecks on my fish finder. If I recall my initial GPS fix put me about 1000 yards off the site.

Possibly the difference between NAD27 and NAD83.

[Lodesman]

Originally Posted by GordMay

A U.S. military document from the office of the Chairman of the Joint Chiefs of Staff, CJCSI 6130.01B,
"Master Positioning, Navigation, and Timing Plan"
has this to say:

e. Celestial Navigation. Celestial navigation, as traditionally
practiced, provides an average error in position of 2 nm...

➥ https://apps.dtic.mil/dtic/tr/fulltext/u2/a414737.pdf

I think you misquoted Gord. It says celestial "provides an error in position of rarely less than 2 nm,"

[robert sailor]

I sailed offshore for a couple of years before GPS made its debut. Always got where I was going. Then I bought a Celestialcomp computer and it allowed me to get more shots in and only seconds to do the reductions rather than my normal 15 minutes per shot, thought I'd died and gone to heaven. Along came GPS but it only worked for a few hours each day as just a few satellites were operational. I still used celestial navigation as I was so used to it but I did use the GPS to see how close my sights were. As long as the seas were not too big I easily averaged around 5 miles and often was in the 3 mile range, never better though.
One trick is you have to take your sight right on the top of the swell otherwise you'll be way off.
Fun in those days but I'd never go back. The one advantage was there were very few cruisers back then as learning navigation was just too much of a commitment for the average stiff.
Once GPS came in there was no more need to navigate and every dog and his master got into sailing.

[Adelie]

Originally Posted by Lodesman

Possibly the difference between NAD27 and NAD83.

Datum differences would account for a small part of the error. The biggest source I believe is that groundwave LORAN is subject to distortion by ground features and probably even the shape of the shoreline and maybe even sea state. It’s vey repeatable so once you’ve measure the TDS at a specific location you can return to that location within 50’ or so using those TDS. The analytic prediction of what the TDs are doesn’t account for the distortion and can be off by a lot more. Probably newer versions of LORAN could account for the distortion.

GPS is all line of site so the only distortion is right where the receiver is rather than along the whole signal path like LORAN. This is among the reasons for better accuracy with GPS.

[Moo]

Originally Posted by Adelie

Datum differences would account for a small part of the error. The biggest source I believe is that groundwave LORAN is subject to distortion by ground features and probably even the shape of the shoreline and maybe even sea state. It’s vey repeatable so once you’ve measure the TDS at a specific location you can return to that location within 50’ or so using those TDS. The analytic prediction of what the TDs are doesn’t account for the distortion and can be off by a lot more. Probably newer versions of LORAN could account for the distortion.

GPS is all line of site so the only distortion is right where the receiver is rather than along the whole signal path like LORAN. This is among the reasons for better accuracy with GPS.

Actually there are atmospheric issues which cause the errors in GPS position. That is what accounts for most of the error seen today.

[Lodesman]

Originally Posted by Adelie

Datum differences would account for a small part of the error. The biggest source I believe is that groundwave LORAN is subject to distortion by ground features and probably even the shape of the shoreline and maybe even sea state.

With Loran, the most accurate signal is the groundwave over water; local distortion was more pronounced inland. The biggest error was induced by atmospheric effects - this was all taken into account with the stated absolute accuracy of 1/4 mile. Assuming GPS lat/lon plotted on the Loran chart, it is unlikely that Loran inaccuracy alone would account for 1/2 mile displacement. Dithered and unaugmented GPS positions rarely exceeded 100m in error, but I suppose it was a possibility.
According to https://gisgeography.com/datum-transformations/

The average correction between NAD27 and NAD83 is an average of 0.349″ northward and 1.822″ eastward

Just a possibility.

[newlazydays]

Late summer in 1971 we were cruising along the south coast of Spain in my 44 foot ketch. The weather was hot, muggy and visibility was maybe 3 miles at best on a very hazy day. There was not a breath of wind. My engine was a two cylinder horizontally opposed Villiers, with a dry exhaust. The noise was terrible. An endless pap,pap,pap, that drove us nuts. After several hours of trying to make progress towards Ibitha at an exhilarating 4 knots I decided we needed a break. My DR put us about eight miles off shore. I waited till the sun was due west of us and took a sight that had my LOP running almost due north into Cartegena Harbour. Settling on the new course we steamed into the harbour a couple of hours later. What a relief!


I like many others learned celestial from Mary Blewitt using the Sight Reduction Tables. On a trip from Tenerife to Barbados we ran to within about 100 miles of the island and then used a small domestic radio receiver to hone in using the null of the aerial. At a much later date I learned to use a common calculator for sight reduction.