Sextant skills

[LadyInBed]

Would you buy a sextant these days?

Now let me think . . .
GPS on phone, tablet, dongle, 2 CP's and a hh GPS, so I would say the answer is NO!
But I do have an old plastic sextant onboard ?

 

[john_morris_uk]

GPS is also an excellent precise time source when taking sextant sights

Perhaps you can I understand why by I get a bit grumpy when conducting oral boards for wannabee YM Ocean candidates and they tell me they got the time off the GPS?

 

[st599]

It is very difficult to use a sextant accurately without an up-to-date Nautical Almanac. which costs a number of beer tokens a year.

Presumably there are apps etc that can calculate that data on a PC or smartphone but it rather misses the point of the sextant if you need your (GPS equipped) smartphone to use it.

But if I were planning to cross any oceans I would certainly pack the sextant and buy the Almanac "just in case".

You can get it in PDF format for free, or download the Python code and compile a Nautical Almanac for any year of your choosing.

I did a random check against our club copy of the Admiralty one - out of 100 random figures 99 were exact, one was out by a value of 1 at the 2nd decimal place.

The Nautical Almanac

 

[lustyd]

Realistically mos

it rather misses the point of the sextant if you need your (GPS equipped) smartphone to use it.

I don't think it does. If GPS is being messed about with by the military or some other bad actor then a sextant and computer are a perfectly suitable alternative which will give a more reliable position. That's one of the reasons military and weapons manufacturers don't always use GPS.

I also don't get the issue with using GPS derived time. These days almost all time is GPS derived one way or another, even if that's setting a Casio from it and using that time at a later date. If I use a cheap Casio on board I'd still want to cross reference against the GPS every day or so, so why not just use the GPS if it's more convenient? I can't think of a better way to get the time and all of my current wrist watches use GPS as their time source. Even if you were sat on the Greenwich meridian with a sextant you'd have no better option.

 

[Deleted member 36384]

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.

 

[bedouin]

Realistically mos

I don't think it does. If GPS is being messed about with by the military or some other bad actor then a sextant and computer are a perfectly suitable alternative which will give a more reliable position. That's one of the reasons military and weapons manufacturers don't always use GPS.

It depends what risks you are trying to hedge with your sextant - they all seem highly unlikely

By far the most likely cause of my GPS failing to work is electrical failure on the boat itself. Loss of the main batteries would take out the fixed GPS and any computing devices that rely on the boat electrics to charge. A battery powered HH (with spare batteries) would get round that - but they you wouldn't need the sextant.

Perhaps the second most likely risk is lightning as that could fry every piece of electronic equipment on the boat - but then the only accurate time I have comes from electronics too.

Whatever the risk is my smartphone is one of the most delicate pieces of electronic equipment I own (closely followed by my laptop) - I really wouldn't want to rely on either in a genuine emergency.

 

[laika]

I can't think of a better way to get the time and all of my current wrist watches use GPS as their time source. Even if you were sat on the Greenwich meridian with a sextant you'd have no better option.

If you were sitting on the Greenwich meridian at Greenwich around lunchtime you might have

 

[zoidberg]

I also don't get the issue with using GPS derived time. These days almost all time is GPS derived one way or another, even if that's setting a Casio from it and using that time at a later date.

I'd consider suggesting the shortwave 'time ticks' from Fort Collins/WWV ( and WWVH ) Radio Station WWV

Then there's the question of whether and how inaccurate IS the time derived by a receiver from a variable GPS signal set. The French name 'TMI' is the definitive source from which UTC derives 'time'.... which is offset from TMI by ( at the last count in 2017 ) an agreed 37 seconds..... This is subject to change.

Now, "GPS time is not corrected to match the ( changing ) rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds).

As of January 2017, GPS time is 18 seconds ahead of UTC because of the leap second added to UTC December 31, 2016. Receivers subtract this offset from GPS time to calculate UTC."

So, the question becomes "Is the time you take from your GPS receiver accurate for astro purposes, or is it out by 18 seconds, 19 seconds, or 37 seconds...... and in what direction."

There's more..... but that's quite enough geekery for one day.

 

[Buck Turgidson]

I'd consider suggesting the shortwave 'time ticks' from Fort Collins/WWV ( and WWVH ) Radio Station WWV

Then there's the question of whether and how inacccurate IS the time derived by a receiver from a variable GPS signal set. The French name 'TMI' is the definitive source from which UTC derives 'time'.... which is offset from TMI by ( at the last count in 2017 ) an agreed 37 seconds..... This is subject to change.

Now, "GPS time is not corrected to match the ( changing ) rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds).

As of January 2017, GPS time is 18 seconds ahead of UTC because of the leap second added to UTC December 31, 2016. Receivers subtract this offset from GPS time to calculate UTC."

So, the question becomes "Is the time you take from your GPS receiver accurate for astro purposes, or is it out by 18 seconds, 19 seconds, or 37 seconds...... and in what direction."

There's more..... but that's quite enough geekery for one day.

If it's labelled UTC is corrected. Few receivers (marine or aviation) show raw GPS time anywhere.

 

[laika]

So, the question becomes "Is the time you take from your GPS receiver accurate for astro purposes, or is it out by 18 seconds, 19 seconds, or 37 seconds...... and in what direction."

We want UT for navigation. UTC is always within 0.9 seconds of UT1 (which is probably what we want) so good enough. Information allowing UTC to be derived from GPS time is transmitted as part of the “almanac” data by gps satellites. So as long as your GPS receiver has been receiving for more than 13 minutes there should be no issues the above shouldn’t be a relevant question

 

[capnsensible]

I will echo others and say what's the point of doing it if you are gonna fanny about with any kind of gps, apps, whatever.

It's a physical Excercise in taking sights on a bouncy boat and a mental excise in doing a bit of adding up and taking away.

It's not compulsory. All this other sideline stuff is excuses. Either you want to do it properly...yay...or you don't. It's entirely a personal choice.

But for me and many others I know, having a sextant, a set of tables, a ships log and a watch is a great way of being more in touch with our planet and the heavenly bodies. Shed loadsa people won't take the time to take and plot position lines. Especially if they are cold, tired and Mebbe vomming after any time spent below. Fair one. Have two gps, three if you want and shed loads of batteries for your spare handhelds. Have done a few oceans on boats with that.

However, it's some personal satisfaction to run for days on your own calculations derived from objects billions of miles away....or from our local star, moon, planets and see Antigua pop up over the horizon almost where you thought and within a few hours or so too.

Quite understand it's not for everyone. Wouldn't it be boring if we were all the same.....?

 

[lustyd]

they all seem highly unlikely

On the contrary, military jamming is pretty common and bad actor attacks getting more and more common. Meanwhile electrical failure to the extent that every GPS device on board would be extremely rare in 2021, given how many and varied devices most modern sailors have with them.

I really wouldn't want to rely on either in a genuine emergency.

Who said emergency? Sextant and computer could quite easily be your primary position, with GPS as the backup to be checked every day or so. On a long passage it would fill a few minutes of your day. I doubt most people could do the simple maths without a calculator these days anyway.

If you were sitting on the Greenwich meridian at Greenwich around lunchtime you might have

Not really since solar noon drifts about quite a bit

So, the question becomes "Is the time you take from your GPS receiver accurate for astro purposes, or is it out by 18 seconds, 19 seconds, or 37 seconds...... and in what direction."

What difference does it make whether you take GPS time from the GPS or from a Casio which was set to GPS time? It's still GPS time. You could equally use a Harrison H4, but you'd be setting it based on GPS time, even if you received that time as radio pips.

 

[lustyd]

do it properly

You're confusing doing something properly with doing something the old way. It's entirely the proper way to use tools to make your life easier. Using a computer and sextant gets you a nice reliable fix, nothing improper about that whatsoever. The choice isn't GPS or 1800's, it's GPS vs celestial navigation, and celestial navigation has been evolving over time to now include calculators, computers and digital watches.

Sure, some people consider doing it the old fashioned way a hobby and good for them. That's quite distinct from using it for navigation.

 

[TiggerToo]

Quite understand it's not for everyone. Wouldn't it be boring if we were all the same.....?

I am with you all the way here.

But I really would like to know how to do the calcs. I tried several exercises, but "Got Lost" in the tables. I am sure it is a matter of learning... but I haven't got there yet.

 

[capnsensible]

You're confusing doing something properly with doing something the old way. It's entirely the proper way to use tools to make your life easier. Using a computer and sextant gets you a nice reliable fix, nothing improper about that whatsoever. The choice isn't GPS or 1800's, it's GPS vs celestial navigation, and celestial navigation has been evolving over time to now include calculators, computers and digital watches.

Sure, some people consider doing it the old fashioned way a hobby and good for them. That's quite distinct from using it for navigation.

Nope, my old electrical gadgeteer, not confused in the slightest. That's your job as usual. ?

The whole point is do Astro without a safety net. It's wot makes them wot can somewhat special. ?

As I keep saying, it's not compulsory......well except for those than have chosen a sea going career and seek promotion, Masters tickets, etc.

Now by all means, use those AIDS if you wish. Of all my past students, one and one only just couldn't do adding and subtraction in degrees and minutes. Absolutely first class sailor and looking for his ticket for a super yacht promotion. Spent lots of time including finding a suitable app to get him through. He pi&&led his oral afterwards, he understood it.

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. Nail that, then use every aid possible, in my view.

I think the ancient phrase is learn to walk before you try and run......?

 

[capnsensible]

I am with you all the way here.

But I really would like to know how to do the calcs. I tried several exercises, but "Got Lost" in the tables. I am sure it is a matter of learning... but I haven't got there yet.

I reckon it's mindset. If you want to do it, persevere. ?

What I've found from teaching the basics to people is to work through a typical sun proforma. Over and again. Then usually it starts to make sense.

There are some smart navigators on here who know masses more than I do beyond that basic level. But single step and all that. I don't know if there is an RYA book of questions anywhere like there is for the other courses, but that would be a way forward, perhaps.

 

[capnsensible]

Anyone got any real evidence that military jamming is 'common'? ?

 

[Minerva]

Anyone got any real evidence that military jamming is 'common'? ?

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.

 

[capnsensible]

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.

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.

 

[capnsensible]

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