Astronavigation - what do I need to take besides a sextant?

[BelleSerene]

There's a lot of stuff to remember and a fair bit of 'fetching and carrying' with numbers and rules (i.e is east best or least and what does that exactly mean! And if latitude is LESS than declination but of the same name..do you add or subtract zenith distance??

Buy, download or even better make up your own step by step pro forma so you dont end up leaving something out or adding when you're supposed to subtract. Easy enough to do at home, never mind down below when you're attempting to work up a sight for the first time!

I agree, and I did the same, actually inspired by the Cunliffe book.

I always find it preferable to understand what's going on rather than slavishly follow a formula or mnemonic: if you can picture it it's obvious whether for example to add or subtract ZD. Just as if you picture True North with Mag North and your own heading on an imaginary compass disc, you don't need to bother remembering what CADET is supposed to stand for or recall the relevance of True Virgins Making Dull Companions: you just use common sense and add or subtract an angle.

Personally, I found sufficient clarity in the short Cunliffe book to enable this, and then as you suggest I made up my own step-by-step proforma with diagrams to hand-hold me through the process when too many months have passed since my last practice!

 

[prv]

You will need an accurate source of the time. A watch is fine.

The closest I've come to astro was being walked through a check of the gyrocompass against the sun. You have to work out the predicted bearing of the sun from tables in a way that I imagine is quite similar to working a sextant sight, and have the same need for an accurate time.

We used the GPS for time

Pete

 

[mcframe]

I own a sextant. Only used in the garden.

If every piece of electronic equipment was fck'd, what are the chances that my paper tables would survive?

Counter argument: Worsley's navigation on the sail to South Georgia.

http://www.amazon.co.uk/The-Barefoot-Navigator-Jack-Lagan/dp/1574092324

Latititude by stick; Longditude by stick (and a cheap quartz watch).

Also has some handy EoT, sun declination at noon tables/graphs to laminate and stick in your grab-bag.

Why have I just thought of an opportunity to sell those tables/graphs laser-engraved on small Ti or Al tablets? ;-)

 

[Porthandbuoy]

Have a wee look at my signature link.

 

[alant]

LPP

I have a few long distance deliveries coming up, the perfect time to practice astronav. The aim is to get to RYA YM Ocean standard.

I haven't time to fit in the theory course, so was thinking of reading a few books and trying it out while on passage.

I have a selection of books, i will get a sextant and ebay it after the exam. What I want to know is what else I need to take on the boat and where to get it from. Can I download the tables and almanac or do I need to buy them.

Presumably your aware, that in the absence of a Theory ticket, the examiner will set an exam, which you will have to do in front of him.
This, is in addition to the normal grilling he will give you.

 

[lpdsn]

The closest I've come to astro was being walked through a check of the gyrocompass against the sun. You have to work out the predicted bearing of the sun from tables in a way that I imagine is quite similar to working a sextant sight, and have the same need for an accurate time.

We used the GPS for time

Pete

GPS time differs from UTC by, IIRC, 12 seconds. The difference is the accumulated leap seconds since GPS was set up. So to use the time accurately, you need to understand exactly what time the GPS set is giving you and whether it has corrected accurately for the difference.

The last place I worked installed NTP servers that worked off GPS, but they corrected for the differences and there is the facility for them to be automatically updated when new leap seconds are introduced.

 

[Robin]

Been a very long time but I too had a plastic EBBCO sextant. I loaded a 50 quid Casio programmable calculator with the astro programs written in 'basic' that were published years back in PBO that could compute all the almanac data. I also made a bunch of crib sheets that said ' do this write result here' then 'do this and write it here, add this to that and write result here' that kind of thing, the real idiot's guide but personalised for my lame brain to follow. I had the 'Air tables' book for average northern Hemisphere latitudes Like others I liked Mary Blewitt's book on celestial Nav methods. For time I cheated and used the time taken from my GPS which I know is not precisely correct with UTC but it beat any watch that I had available or could afford. Oh and later I downloaded a freebie program ( from a KIWI site IIRC) for doing sight reductions to my on board laptop, that too went tits up ( so much for dire warnings of what to do when all your on board GPSs pack up)



I no longer have a sextant, the Casio went tits up and I sold the books before we move to the USA. I think I could still do it in extremis.

 

[SteveSarabande]

Thanks for all the advice.

 

[GHA]

Have a wee look at my signature link.

I was just about to do a google for that site - everything you need is on there. Is it your site?

Good cheap kindle book here as well...

http://www.amazon.co.uk/Astro-Navig...?s=digital-text&ie=UTF8&qid=1427950063&sr=1-2

 

[Skylark]

There's much more to the RYA course than simply craming a lesson in plotting a sun Position Line. Well worth doing inmho and will then exempt you from having to do the course assessment while sat in front of an examiner. The examiner will also spend an hour or so talking to you, looking through your log, to ensure that you took a full and active part in the planning and execution of your submitted passage. Followed by a general chat about ocean sailing (this is where the course scores highly, too).

I also strongly agree with the comments about using your own derived sight reduction sheet. It's not always easy to follow someone else's logic. Create your own to your own logic.

Unless you have access to a radio time signal, it may be wise to carry two, cheap digital watches.

Personally, I wouldn't buy Plotting Sheets. Very simple trigonometry relationship between lat and long to create your own. Either take some plain A3 paper or use the back of a chart.

Using a sextant is a lot of fun and very rewarding. Makes you reflect that there were some very clever people about long before Bill Gates and Steve Job.

 

[snowleopard]

If every piece of electronic equipment was fck'd, what are the chances that my paper tables would survive?

Some years ago, pre-gps, I shipped as navigator on my boss's boat for the Cowes - St Malo race. as we came out of Lymington he switched on the instruments..... nothing. We found out at the end of the trip that it was corrosion in the fuse box. I got us as far as the Channel Light Vessel on DR but vis closed in and we found ourselves somewhere to the W of Guernsey with a rock-strewn approach to our destination. The crew was amazed when I trotted on deck with my sextant, got a position line from a hazy sun and took us into St Malo without further incident.

I've been sailing with a gps for the last 15 years but I still find it comforting to know I can make a landfall without electronic aids.

 

[snowleopard]

On a normal passage you need no more than an ordinary watch. If you make a note in the log every day of the difference between the watch and gps time you are then in a position to calculate the accurate time if the gps then goes down.

Personally I would always want to know how to use the tables and carry copies of them rather than rely exclusively on electronics of any kind. Imagine the frustration if you had the sextant but your calculator was fried by a lightning strike!

 

[laika]

GPS time differs from UTC by, IIRC, 12 seconds. The difference is the accumulated leap seconds since GPS was set up. So to use the time accurately, you need to understand exactly what time the GPS set is giving you and whether it has corrected accurately for the difference.

The last place I worked installed NTP servers that worked off GPS, but they corrected for the differences and there is the facility for them to be automatically updated when new leap seconds are introduced.

The idea that the time normally displayed by GPS receivers is not UTC may be a common misconception. The offset between "GPS time" and UTC is currently 16 seconds and will be 17 seconds after a new leap second is added this summer. However, that offset is sent as part of the GPS navigation message from the satellites. The average punter really doesn't need to know this: The info carried in NMEA messages is UTC and this is what your plotter will display unless you correct it for summer time which on my rubbish last generation Raymarine you have to do manually but I suspect that newer linux-based ones will do that automatically.

GPS clocks can be very accurate and are used by financial institutions in their time keeping, including for high frequency trading where super-accurate timing is needed. Note that the NMEA sentences you receive from a GPS unit are NOT very accurate in the scheme of things: the receiver I've been playing with for the past few days kicks out RMC sentences which if I believed them the time would be about half a second off with a jitter of about 20ms. The trick is taking advantage of the "Pulse Per Second" (PPS) signal from GPS which is super-accurate:
http://en.wikipedia.org/wiki/Pulse_per_second
For DIY-ers, a PPS signal isn't available from low-end GPS receivers but the popular £35 adafruit GPS breakout board I've been playing with this week has it and this week I've rigged up a raspberry pi as a GPS time server using it. The clock should be accurate to a few microseconds and ntp is reporting a jitter of 2 microseconds from the PPS signal.*

So....I don't think the average punter *does* need to worry about the GPS time offset, although if their battery on their GPS unit is dead it will explain why their clock is 16 seconds off until the receiver picks up the UTC offset. Your plotter's time should be pretty accurate. *How* accurate? Good question. If the GPS receiver is external to the plotter I wouldn't count on more than a second. I really don't know if any of the new generation of plotters with internal GPS receivers make use of PPS. Anyone know? Suspect not. Still, probably good enough for navigation. Contrary opinions welcome though

*Of course keen observers will note that the raspberry pi is essentially rubbish hardware with all its networking on a rubbish usb, so there'll be a whole lot of added jitter in what would be received by a network time client from the pi.

 

[BelleSerene]

There's much more to the RYA course than simply craming a lesson in plotting a sun Position Line. Well worth doing inmho and will then exempt you from having to do the course assessment while sat in front of an examiner.

David - interested in your experience here. I have not done (and am far too rusty currently to pass) the Ocean YM. But if I took it it'd be as a personal challenge for it either to endorse my competence to myself, or to point out where I need to improve my understanding or skill or experience. So I'd rather 'face the music' in front of a live examiner, take my chances and benefit from the feedback, than likely improve my chances with the attendance certificate from a course which will have been more forgiving of my lapses. No?

 

[KellysEye]

We carried sight reduction table but used Astrocalc that comes in 32 and 64 bit: http://www.astrocalc.com/blog/astrocalc-trial/

 

[[70521]]

Have a wee look at my signature link.

Thanks for posting that.

 

[fergie_mac66]

Anyone got a link to mercator plotting sheets i can print out

seem to recollect that Eric Hiscock in his atlantic crossing book detailed that he used to plot on a sheet a4

 

[laika]

This is the fancy plotting paper:
http://www.bookharbour.com/astro-navigation/plotting-sheets-and-sight-forms-for-yachtsmen-25-sheets-per-pad/

Nice to plot on if you've (a) got a saloon table to work on not some crummy laptop shelf which passes for a chart table on some modern boats and (b) have the space to take them with you (if flying to/from the the boat) without them getting bent. As fergie_mac56 and others mentioned though, not really necessary.

 

[Skylark]

David - interested in your experience here. I have not done (and am far too rusty currently to pass) the Ocean YM. But if I took it it'd be as a personal challenge for it either to endorse my competence to myself, or to point out where I need to improve my understanding or skill or experience. So I'd rather 'face the music' in front of a live examiner, take my chances and benefit from the feedback, than likely improve my chances with the attendance certificate from a course which will have been more forgiving of my lapses. No?

I did the Ocean shore based one-to-one with a retired Master Mariner. We shot off at all sorts of tangents during our enlightening and informative discussions. Off syllabus, he introduced me to the delights of Nories Tables. Very enjoyable learning experience.

I also did a lot of self study to reinforce the sight reduction process. I found the Mary Blewit book too superficial. I usually like anything from the pen of Sir Tom but I don't think his Ocean book is his finest work. I really liked the Adlard Coles Ocean book. As a special treat for myself, I bought a copy of Bowditch, American Practical Navigator.

My RYA examiner was a lady. We sat down together very informally, drinking tea as she scrutinised my qualifying log. She also willingly shared lots of ocean sailing, anecdotal good advice. I learned a lot from her. Looking back, we covered a lot of ground in about 90 minutes. Victualling, weather and routings were fairly predictable as topics. We also talked about managing sickness, the wisdom of a curry and several pints before embarking upon an ocean crossing (don't ask), sail and boat maintenance during an ocean passage. It was a thorough assessment of my knowledge and experience. She found a lot of gaps but evidently thought this office desk bound wannabe sailor from Manchester knew something about the subject.

As you rightly say, it's nice to have your competence endorsed by a 3rd party.

 

[alant]

I did the Ocean shore based one-to-one with a retired Master Mariner. We shot off at all sorts of tangents during our enlightening and informative discussions. Off syllabus, he introduced me to the delights of Nories Tables. Very enjoyable learning experience.

I also did a lot of self study to reinforce the sight reduction process. I found the Mary Blewit book too superficial. I usually like anything from the pen of Sir Tom but I don't think his Ocean book is his finest work. I really liked the Adlard Coles Ocean book. As a special treat for myself, I bought a copy of Bowditch, American Practical Navigator.

My RYA examiner was a lady. We sat down together very informally, drinking tea as she scrutinised my qualifying log. She also willingly shared lots of ocean sailing, anecdotal good advice. I learned a lot from her. Looking back, we covered a lot of ground in about 90 minutes. Victualling, weather and routings were fairly predictable as topics. We also talked about managing sickness, the wisdom of a curry and several pints before embarking upon an ocean crossing (don't ask), sail and boat maintenance during an ocean passage. It was a thorough assessment of my knowledge and experience. She found a lot of gaps but evidently thought this office desk bound wannabe sailor from Manchester knew something about the subject.

As you rightly say, it's nice to have your competence endorsed by a 3rd party.

Will ask, can you expand on the curry/pints pls.