Are the days of satellite GPS navigation numbered??

[Jim Cate]

People forget how limited sextants were. First it couldn’t be used n a cloudy day, 2nd you could only take one sight a day at noon, and third it was rarely more accurate than 20 miles. The sextant was a way to find land so you could follow the coast by compass and eye to your destination.

The cloud issue is real. However the remaining thoughts are a bit off:

One can develop fixes many times each day... not just a noon sight. Running sun fixes are a standard method during the day and star rounds at each twilight are the navigators joy.
And even from the low height of eye and unstable platform of a small yacht one can usually achieve well better than twenty mile error. I've personally used celestial to make passages and expected (and realized) landfalls near to my destination.

I use GPS preferentially now that it is available, but your dismissal of celestial is a bit harsh.

Jim

[Pete7]

Originally Posted by requiem

It would be unusual to not have some idea of your general location. How often do you wander into a dense fog and find yourself in the opposite hemisphere Just do the same as with a traditional sight and use an assumed position.

My guess is that the greater issue is the ability of the camera to capture the stars, and the accuracy of the inclinometers. That last could be probably be addressed if the horizon is in view.

Surely this is similar to the early days of GPS when you moved a hand held GPS without it being turned on, so it has to start somewhere unknown. It knows the time and can pick up the satellites one at a time until it has enough. Same with stars surely.

Using a camera to identify stars was being looked at within military circles a decade ago. The RN continue to teach the use of sextant to Nav Officers.

[MicHughV]

What happened to:

"drive down this road until you see a red shed on the right side of the road, just past the red shed is a dirt road going to the left, take a left there and go about...hmmmm.....2miles or so, until you see a farmhouse with a green roof, just past that farmhouse is a road leading to your right, but it splits in two, take the left split and pretty soon you'll come to a cow pasture on your right, you can't miss this, as it's always full of cows, drive by this pasture and take the next turn to the right, that road will zig and zag a lot, but stay on it, and eventually you'll come to a river, follow that zig zag road for another mile...or maybe two until you see a bridge, relax, you are almost there, cross that bridge and about a mile...maybe two or three, you'll come to a store with a gas pump out front, the owner's name is Zeb, from there Zeb will tell you the rest of the way....."

[thinwater]

I've tested several iphone apps that emulate a sextant. The best one required you to calibrate the camera at a known location and was as accurate as any sextant. The math and tables are all in the app and the phone knows the time. It was a one-man project and was quite good. The only short coming was that it still displayed the info as a line of position, then a group of lines, but not as a "you are here" spot, which is what people want. He could get there.



I think that if GPS tanks a good iphone sextant app would appear in very short order, and all you would need to do is point it at the sun someplace with a reliable horizon. Soon to follow would be a star app. And before long, the need for the horizon would probably be eliminated somehow (a digital artificial horizon is doable). I would also be stunned if the military was now well down this path.


Traditional sextants will remain quaint dinosaurs unless a massive EMP comes along.Then just run a DR plot until you hit land.



Where there is a need ....

[ChrisJHC]

Surely part of the reason we sail is for the connection between the water and the wind and using the natural environment to get where we are trying to go.

To my way of thinking, an understanding of celestial navigation - let alone the ability to work out where you are using the sky - just extends that connection to a third dimension.

If you always want the answer right now wouldn’t you logically be on a power boat or an aeroplane?

And yes, I use GPS as my primary navigation tool and a magnetic compass as well!

[squarerigger]

Regarding the previous comment about the limitations of sextants:

Yes it's not possible to get an observation if the clouds prevent you from seeing a celestial body. Its also not possible, by the way, to make a decent observation if you can't see the horizon, which fog might also preclude. However, in many cases, even on a cloudy day, the sun peeks through sometimes and if you are alert you can get your shot.

Yes, you can get only one noon sight per day, at local apparent noon, obviously. But you can get an LOP from the sun any time you can see it and the horizon, though it is compromised once it gets close to the horizon. And , you can get the moon much of the time in daylight and stars and planets at either twilight, so you can do celestial for most of any day when conditions permit.

As for accuracy, I teach a course in the history of navigation where learning to do celestial is part of the syllabus. Even with a modest sextant, my students can expect to get lops within a few hundred yards in decent conditions by the end of it. That's not GPS accuracy of course, but is also not miles and miles off. Even in the 16th century, with an astrolabe, quadrant or cross staff, the professional standard for pilots were latitudes accurate to within 30 nautical miles. Accurate enough to establish seaborne empires and alter the course of history, for better or worse.

So, limitations yes. But for many people it's more satisfying, and makes one feel more connected to the history of the endeavour they love than simply pushing a button. Even pushing the button is more satisfying when you know you are not depending on it exclusively. But then, to each his own...

[SeanPatrick]

Originally Posted by coopec43

I wonder how many yachtsmen use sextant navigation these days? Not many I guess. (But I've got a plastic one and I am dying to try it).

If you need any help, I've got your back!

Originally Posted by coopec43

These days everyone is using satellite GPS however it has one huge drawback. What if an adversary destroys the system maybe by jamming the signal or "spoofing it" so that it positions you incorrectly?

With Frank Reed's GPS Anti Spoof app, you can cross-check the accuracy of your GNSS with your sextant instantly without any math. It displays the current sextant altitude of multiple celestial bodies based on your reported GNSS position. This means that you can directly compare your sextant reading with what the app displays. GNSS operates independently from cellular coverage and is normally available all the time, worldwide.

Originally Posted by coopec43

"quantum navigation"

Very interesting.

Originally Posted by CarlF

People forget how limited sextants were. First it couldn’t be used n a cloudy day, 2nd you could only take one sight a day at noon, and third it was rarely more accurate than 20 miles. The sextant was a way to find land so you could follow the coast by compass and eye to your destination.

As Jim Cate pointed out, there were (and still are) multiple types of sights that could be taken at almost any time of day (or in twilight or even sometimes at night, when the moon illuminates the horizon). In addition to what he mentioned, I would like to add time sights [longitude by chronometer] and lunar distances for longitude, and Polaris shots for latitude. All of which were known to mariners for at least a couple of hundred years.

Jim also correctly stated that the accuracy of celestial navigation can be well inside 20 nautical miles. But you are correct that celestial navigation was only used to get within sight of land, after which, pilotage was the preferred method.

Originally Posted by shrspeedblade

With the star gazing apps now it seems like it wouldn't be a huge challenge to design a program that uses the phone's compass combined with the date and what its camera sees on a clear night (polaris, southern cross, etc.) to give a reasonably accurate position similar to a sextant.

Long story short: the technology just isn't there yet. But I wouldn't be surprised if it materialized in my lifetime.

Originally Posted by requiem

It would be unusual to not have some idea of your general location. How often do you wander into a dense fog and find yourself in the opposite hemisphere Just do the same as with a traditional sight and use an assumed position.

Indeed.

[coopec43]

China and Russia are challenging the US dominance in satellite technology.


Norway of all countries has satellites orbiting to maintain communications!


https://www.ft.com/content/a4300b42-...c-bc9acae3b654

[coopec43]

The race to back up vulnerable GPS

" Other PNT backups determine position using celestial navigation or terrain referenced navigation, which uses a digital map of the surrounding terrain to match a vehicle’s movement and update its position."

https://spacenews.com/the-race-to-back-up-vulnerable-gps/

[requiem]

Originally Posted by MicHughV

"until you see a farmhouse with a green roof, just past that farmhouse is a road leading to your right"

I think "country directions" need to include at least one local knowledge reference, such as "about a quarter mile past where the old Murphy barn used to be".

[coopec43]

"
Xona in 2025 plans to start building a constellation of between 250 and 300 small satellites in low Earth orbit. The company worked with the Air Force under SBIR contracts, and counts Lockheed Martin Ventures as one of its investors.
Manning said new space-based PNT services in low orbits provide higher accuracy. The signals only have to travel a few hundred miles, compared to GPS which broadcasts from satellites about 12,000 miles above Earth.

Xona does not plan to make user devices, and instead is partnering with existing GPS equipment companies to integrate the company’s software in their receivers."


Well I won't chuck out my old GPS equipment just yet

[Lepke]

I'm an old navy trained navigator and still use a sextant for LOPs and bearings.

Just like my dad use to hand add numbers to double check the adding machine.

[sundrifter]

Originally Posted by CarlF

People forget how limited sextants were. First it couldn’t be used n a cloudy day, 2nd you could only take one sight a day at noon, and third it was rarely more accurate than 20 miles. The sextant was a way to find land so you could follow the coast by compass and eye to your destination.

Hi Carl, I disagree with your above statement and feel this may dissuade some cruisers from carrying a sextant onboard and would feel remiss in not pointing this out.



It appears you have not used a sextant at sea. Why would you limit yourself to a noon shot? I use the stars and get two accurate fixes a day at morning and evening twilight. I have been doing this before GPS was available. Also it is not always necessary to get a fix everyday. I delivered a lot of boats from Cape Town to Caribbean and after a few days out of Cape Town there is nothing to bump into before St. Helena so for at least a week would steer 330M and just enjoy the ride. I would get one fix, adjust my course and then dead reckon till we saw the loom of lights in St James Bay. So 1 fix in 1700 miles. Easy Peasy



Evening was my favourite time for a star fix, I would put rice on to boil, go on deck and take in the fishing lines, trim the sails, shoot 3 stars and be back below with the rice ready.



When I ran my sailing school, which I abandoned after a year to do deliveries again, my students were regularly getting to within 1 mile of a verifiable position.



I seldom use my old Freiberger-Zeiss sextant anymore but it travels with me. I encourage all to get familiar with their 'sexy', something very rewarding when taking a sight and then doing a sanity check by asking a pssing ship for a position.

[Benz]

Originally Posted by coopec43

Good thinking. Thanks!

As far as a binnacle compass is concerned I haven't got one as yet (I have been buying an EPIRB, VHF Radio, Flares etc)

The compass I have been looking at is a Plastimo Olympic 135 which at $531 is a lot of money. Are they really necessary these days or can I get by with something less expensive?

https://www.bluebottlemarine.com/pro...8&gad_source=1

I've had good success with Ritchie compasses. I don't think they cost nearly as much as the Plastimo.
You might find a used one at a consignment shop--there's tons around here.

[coopec43]

Originally Posted by Benz

I've had good success with Ritchie compasses. I don't think they cost nearly as much as the Plastimo.
You might find a used one at a consignment shop--there's tons around here.

You must be reading my mind! (I've just been doing that - looking at S/H and other makes) Thanks.