GPS. Or is it GNSS?

One of the Coastal Skipper candidates muttered something about GPS satellites being in geosynchronous  orbits.   Another muttered something in reply about them being confined to equatorial orbits.

This arose from the realization that the variation of a magnetic compass would increase dramatically as one approached the Poles.   Unless,  of course,  one approached along a meridian which passed through the magnetic poles.   Even so,  the dip of a magnetic compass close to the poles would make it very inaccurate.

This led to a discussion that the use of Global Navigation Satellite Systems would be a better way to navigate close to the Poles (or indeed,  anywhere else!).

The LG (in this context,  a Lazy Sailor) was reminded of the time when he attended a five-day Astronavigation class which showed him that for the price of a decent sextant he could buy between 5 and 10 GPS instruments and that for the annual cost of tables he could buy all the batteries those instruments would ever need.

GNSS satellites are,  of course,  in geosynchronous orbits:  they pass over the same spot on the surface once every day or once every half day.   To do this they orbit at a precisely predetermined height at a predetermined speed.   Each has a different orbital angle to the Equator so that a constellation of satellites covers most of the Earth's surface most of the time.

Because the satellites are moving with respect to the receiving instrument the latter must constantly search for other,  more appropriately positioned,  satellites.

By contrast,  communications satellites are placed in geostationary orbits:  they 'hover' above the same point on the surface all the time so that directional antennae (TV dishes!) can remain fixed pointing in one direction.   These satellites orbit at the same speed that the Earth rotates,  and they do so above the Equator.

The LS has always been an advocate for paper charts.

Yes,  they get damp in the well of a small boat,  especially a dinghy,  but they can be enclosed in a waterproof case.

On a recent visit to Imray's offices Amy showed him their new-ish electronic charts on an iPad,  and they were a revelation.   They were,  in appearance,  scanned copies of the paper charts.   No more disappearing detail that is so annoying with vector charts.   Straightforward zoom by 'pinching' and tapping in the classic iPad way.  And when you need the detail charts,  simply zoom in to the detail charts.

The LS has always sailed away from teaching electronic navigation.   He's used the old mantra "better to practice with the machine on the boat",  which doesn't convince even he.

But now he has something sensible to teach.

As soon as he can persuade his iPad to communicate with his monitor and projector he'll use Imray Navigator.

He might even use it on the boat.

The Bris Sextant

Everyone knows about Sven Yrvind,  the brilliant,  crazy Swede who wants to be the first to sail around the world in a 10 foot (3 metre) boat of his own design and construction.

Fewer people know that he designed a small instrument for measuring the time at which the sun reached a pre-calibrated altitude.   This is useful for plotting a position,  with an accuracy of a few miles,  at sea.   He called this instrument a 'Bris Sextant'.

Sven Yrvind with his Bris sextant

It's not a sextant,  strictly speaking (possibly a fixed solar goniograph),  although it does a similar job.

'Bris' is the Swedish word for 'breeze':  at least one of Sven's boats was called Breeze.

Almost no-one,  it seems,  has built or used a Bris Sextant (or,  if they have,  they haven't written about it),  although there are some very learned discussions of its reflections and angles on the internet.

So here goes . . .

Astronavigation

The art,  science or craft of finding one's position using a sextant and several books of tables.

A sextant is an optical device for measuring the vertical angle of the sun,  moon,  planets and stars above the horizon (the sea horizon,  not the hills).

It's a difficult and dangerous instrument to use.

Difficult because it requires a degree of knowledge and skill,  first to find the correct heavenly body,  and second accurately to bring the reflected image of that body down to the horizon.

Dangerous because if you get the light filters wrong you'll burn out your retina.   Seriously.   Literally.

In passing,  you might note that a cheap sextant costs twice as much as a decent hand-held GPS instrument.   A good sextant costs 10 to 15 times as much.   It's also 10 times bigger and weighs a lot more.   Astonishingly,  it's more easily damaged by salt water than most GPS sets.