Sextant skills

[Stemar]

Imagine the chaos if gps was jammed in the Solent on a bank holiday weekend. ??

... a cloudy bank holiday weekend, so you lot can't use your sextants either

 

[Buck Turgidson]

Understand. Joint Warrior I think is the Excercise. But, there is always a but, that's a small area for a limited period. Personally I've never had a gps jammed anywhere across a number of seas and oceans.

Common place in the eastern med. We have GPS outage flying into Egypt all the time.

 

[neil_s]

I used Andrew Evans' Step by Step Sextant User's Guide to get started. Got my own spreadsheet, now, but still like to use the almanac and sight reduction tables every now and then. Astro Nav is a hobby all to itself - you don't have to use it in anger. Just try doing some stars!

 

[bedouin]

Get warnings frequently every year around the Minch during military exercises that there could be issues with GPS jamming - the boys in the grey ships are clearly playing GPS jamming tech regularly.

If we are talking Astro we are talking about crossing Oceans and taking sights in the middle of the Atlantic.

Hardly likely to be subject to GPS jamming there.

For those who don't know - Astro is not appropriate for coastal or pilotage situations. In those circumstances the fall back is a set of paper charts and the Mk 1 eyeball.

 

[capnsensible]

If we are talking Astro we are talking about crossing Oceans and taking sights in the middle of the Atlantic.

Hardly likely to be subject to GPS jamming there.

For those who don't know - Astro is not appropriate for coastal or pilotage situations. In those circumstances the fall back is a set of paper charts and the Mk 1 eyeball.

Agree. Plus echo sounder, hand bearing compass, tide tables. A brief run up of a tide curve is very useful. And computation of rates for tide streams is generally straightforward to interpolate with ones grey matter. Just takes practice!

 

[capnsensible]

Common place in the eastern med. We have GPS outage flying into Egypt all the time.

Wonder why? What do all the ships in and out of the Suez Canal do?

 

[lustyd]

Understand. Joint Warrior I think is the Excercise. But, there is always a but, that's a small area for a limited period. Personally I've never had a gps jammed anywhere across a number of seas and oceans.

That you were aware of. The position can be changed as easily as jammed.

 

[capnsensible]

That you were aware of. The position can be changed as easily as jammed.

Mebbe easy.....but where and when? I'm not seeing the worlds huge merchant fleets in disarray. Or myriads of small craft in dire circumstance. ?

 

[lustyd]

Apparently there's a lot you don't see, I wouldn't worry too much about it, you seem happy enough.

 

[Buck Turgidson]

Wonder why? What do all the ships in and out of the Suez Canal do?

https://www.eurocontrol.int/sites/default/files/2021-03/eurocontrol-sf-gnss-iata.pdf

 

[zoidberg]

'All-aids navigation' is for trained pros and those few amateurs who have studied the stuff to pro standards.

For the rest, it's Garmin.

 

[Laminar Flow]

As an aid to position fixing for vertical sextant angles on objects, I have used a sextant often in the past. It’s a very quick and accurate way of position fixing when an object’s height is known and the distance off is marked on a single position line. In my case a low cost plastic sextant was adequate.

Indeed, and it even works as a handy pelorus for taking horizontal bearings.

 

[AntarcticPilot]

Understand. Joint Warrior I think is the Excercise. But, there is always a but, that's a small area for a limited period. Personally I've never had a gps jammed anywhere across a number of seas and oceans.

A really bad solar flare (see Carrington Event) would kill all forms of GNSS stone dead. It would probably disable a fair amount of the satellite constellation as well as the direct effects.

 

[TNLI]

An app called "astro Calculator" on android - does the sums for you, shows you the working and plots your position. Really quite good

I used to use an Astrofix calculator to do a running fix. That calculator was very easy to use and did cover all the main stars and planets. I presume there is a download of the program and files available these days.

 

[zoidberg]

Indeed, and it even works as a handy pelorus for taking horizontal bearings.

I do have a 'pelorus' made/modified for my 'Ocean' viva from a reclaimed 'Grey Funnel Liner' dumb compass, specifically for the Suns Amplitude stuff. My examiner, a notoriously grumpy ex-Lifeboats cox'n, was so intrigued he pulled out his own 'Blue Peter' model he'd made in perspex years before, and we spent the rest of the examination comparing each others'.....

I s'pose our Lammie F. was thinking about the venerable Station Pointer device, which is admittedly rather tricky to utilise on an iPad screen or a Raymarine Integrated Display.

One can use the 10" Portland Plotter for much the same purpose, but at much lower cost. So much so that I printed the thing onto photocopier transparency, with instructions, and gave a renamed copy to my YM students 'back in the day'.

 

[laika]

Not really since solar noon drifts about quite a bit

I'll take a punt that you're overlooking something that happens in greenwich at 1pm...

 

[zoidberg]

Bolleaux!

 

[laika]

So yeah, use calculators. But reductions using apps aren't gonna make people learn it properly, yeah that word again, until they've used tables and obtained a position after a couple of days using basic techniques.

I'm going to respectfully disagree with your criticism of lustyd's point here. If we rewound 40 years before most recreational yachts had GPS and sextant was the primary means of ocean navigation, As I Understand It (older folks who were there please chime in...) use of calculators for sight reductions was normal.

Now I do use tables because I learned the "RYA method" but surely if we're "cheating" with tables, we can't criticise those who "cheat" with calculators.

 

[Buck Turgidson]

I'm going to respectfully disagree with your criticism of lustyd's point here. If we rewound 40 years before most recreational yachts had GPS and sextant was the primary means of ocean navigation, As I Understand It (older folks who were there please chime in...) use of calculators for sight reductions was normal.

Now I do use tables because I learned the "RYA method" but surely if we're "cheating" with tables, we can't criticise those who "cheat" with calculators.

This is the crux of the matter.
The sole reason I learned astro was to navigate offshore if GPS was not available for any number of reasons. I do it to get a fix. Not to practice an ancient art.

 

[zoidberg]

This is the crux of the matter.
The sole reason I learned astro was to navigate offshore if GPS was not available for any number of reasons. I do it to get a fix. Not to practice an ancient art.

I learned astro 'cos my employers - HM Queen and her RAF senior officers - deemed I needed to be able to do astro reliably to navigate their bombers around in time of war, when every other navaid would be switched off 'just like that'.

And astro by itself is not the essential core skill. That is actually quality Deduced Reckoning, or DR.... which was run continously. The astro LOP and/or fix - and anything else that became available, such as a 'running fix' on a prominent mountain or headland - was used to update the DR plot sequentially.

In the Vulcan and similar aircraft, the fixing cycle was typically 'one complete fix every 20 minutes' rising to 'one every 10-12 minutes' when required, the info derived being fed into an analogue DR Computer. There were various techniques used, such as '3-star fix', '4-star fix', '2-star sandwich ( five or seven shot ) fix' and continuous 'Along/Across Track Fix Monitored Azimuth'..... I don't think people bothered with the nautical 'round of morning star sights', for stars were always available up around 40,000 feet.

At low-level, one saw plenty of daylight stars, bouncing one's head continuously off the insides of the crew comparment.