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AstroNav
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Celestial Nav - Sextant Ports

Thu Jun 18, 2020 1:40 pm

Hello,


I'm studying for my RYA YachtMaster Ocean certification.

I'm curious to know from pilots/ navigators - military or civil - how it's possible to take shots of stars, planets or the Sun when flying at 5 or 600mph to gain an accurate (>3 point) astronomical position fix.

I know that before Inertial NS, older aircraft had sextant 'ports' and I wondered how they worked if we assume that a typical aircraft may travel 1 mile very 7 seconds - in the cruise.


Questions:

1. Do they teach this subject - practically - at flight school these days?

2. The FAA Nav booklet has it but how do you actually do it while flying at those speeds?

3. Is it academic rather than practical?

4. Does anyone (private, civil, military) practice that skill and use it?

--

The USN withdrew their celestial nav course in 2007 but then reinstated it.

Personally, I would like to do a spherical trig course at night school for a few months - does anyone have any ideas for this sort of thing in the UK?


Thank you
 
extender
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 4:15 pm

That was the original means of navigation for the SR-71. That was moving at 6 miles per second.
 
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flyingturtle
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 5:05 pm

Somebody on a.nut said that some crews attached the hose of a vacuum cleaner to the sextant port...

AstroNav wrote:
2. The FAA Nav booklet has it but how do you actually do it while flying at those speeds?


That's not so much of a problem. You have watches... and celestial navigation gives you a better precision than the early inertial navigation systems.

AstroNav wrote:
4. Does anyone (private, civil, military) practice that skill and use it?


Because celestial navigation cannot be jammed and GPS satellites can't be shot down with missiles, it's still practised. You rather have a computer that does the number crunching, instead of looking up values in the almanach.
Reading accident reports is what calms me down
 
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Francoflier
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 5:10 pm

extender wrote:
That was the original means of navigation for the SR-71. That was moving at 6 miles per second.


Hmm... The SR-71 was flying nowhere near 6 miles per second. That's almost Earth's escape velocity.


As for sextant, they are not taught or used in modern commercial aviation. I doubt even the best cockpit windows would provide enough viewing angle for an accurate reading.

I didn't know the USN reinstated celestial navigation courses. That said, when you see how easily jammed GPS can be, especially in a conflict area, it might still be a valuable skill for military pilots.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
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flyingturtle
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 6:05 pm

Francoflier wrote:
I didn't know the USN reinstated celestial navigation courses. That said, when you see how easily jammed GPS can be, especially in a conflict area, it might still be a valuable skill for military pilots.


https://www.npr.org/2016/02/22/46721049 ... r-officers
Reading accident reports is what calms me down
 
Woodreau
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 6:12 pm

extender wrote:
That was the original means of navigation for the SR-71. That was moving at 6 miles per second.


Yeah Mach 3 is somewhere around 27 miles per minute, or roughly 1 mile every 2.5 seconds, not 6 miles per second.

From a commercial aviation standpoint, celestial navigation is not taught and there are no means to use that method in commercial airliners.

There is still a flight navigator certificate that can be obtained in the US from the FAA, but the issue is actually finding anyone who can actually give you the check ride for the flight navigator certificate.

I believe the US Navy stopped teaching celestial navigation at the US Naval Academy, but celestial navigation is still done aboard ships, so they never stopped teaching it in the fleet. It has always been taught in the Quartermaster rating schools and for navigator training for officers assigned to a ship as a navigator.

I have no idea how celestial is done aboard aircraft, but aboard ship celestial is just another method to obtain a line of position to plot on a dead reckoning plot that is always run even when not using celestial techniques. These days (or 20 years ago) you would use a computer to select the best stars to do a sighting. Then when you did your star shots, you took your results back inside and plugged them into a computer (Which did the sight reduction) and plot the resulting data on the chart where you ran your running fix and dead reckoning plot. There were still workbooks where you could do the sight reduction by hand if you wished.

There is still practical uses of celestial at sea. It’s used to determine gyro error and to correct the magnetic compass and to determine local noon.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
mmo
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 7:19 pm

This information is somewhat dated so bear with me. When I was on active duty, one of my several assignments was as a B-52 Aircraft Commander. Every training sortie required the Nav to have a celestial leg logged and that leg was later graded for procedures and accuracy. The aircraft had a sextant port built into the upper deck on the topside of the fuselage back by the EW's station on a D model and by the EW and Gunner's stations on a G model. The EW generally took the star or sun shot for the nav and then read off the data for the Nav to plot. The aircraft was stable enough to get a fairly good star shot or sun shot. I think the biggest problem was if you were IMC.
I can't say if that is still being done, I would assume there is a requirement perhaps not every mission. The aircraft also had an astrotracker and the D when I flew had a digital nav system. It was reliable but there were problems with the data on position fixes which the DBNS system did not like.

Hope that helps.
If we weren't all crazy we'd all go insane!
 
mikesbucky
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 7:49 pm

Celestial navigation on aircraft is done using either a periscopic sextant or an astro tracker depending on the aircraft. The periscopic sextant would attach to a mount on the ceiling of the fuselage. You would attach the sextant to the mount and move a lever which would open a hole in the ceiling and basically suck the sextant up so the top of the sextant was outside the aircraft skin.

The navigator would use the air almanac and sight reduction books to make the necessary computations. You would make corrections for motion of the celestial body and motion of the observer (aircraft) in your calculations. Depending on how you made your celestial observations, you could cancel the motions out by bracketing the observations around your fix time. Observations were usually a two minute average, but could be done instantaneously if time was short. Day celestial normally was accomplished using only the sun to give a single line of position. That line of position would optimally be either parallel or perpendicular to your course so you could get an accurate speed or course line. Sometimes the navigator would also use a moon observation to get a two body fix. If the observations were near sunrise or sunset, the navigator may have been able to use three bodies the get a three line fix by using either Mercury or Venus in addition to the sun and moon. Night Celestial would use stars and planets to get a three body fix.
 
889091
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 11:36 pm

Isn't this the reason for the cockpit eyebrow windows on the B737?
 
GalaxyFlyer
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Re: Celestial Nav - Sextant Ports

Thu Jun 18, 2020 11:56 pm

No, there’s a port to mount the sextant so it views the sky without looking thru the windows. The eyebrows are for circling to land, cross-cockpit.
 
fsnuffer
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Re: Celestial Nav - Sextant Ports

Fri Jun 19, 2020 1:24 am

mikesbucky wrote:
You would make corrections for motion of the celestial body and motion of the observer (aircraft) in your calculations.


One of the main differences between taking a celestial fix on a ship and in an aricraft is you have to account for the "Coriolis effect" which caused by the rotation of the earth while taking your fix in an aircraft. At 20 knots it is not a big deal at 420 knots it needs to be accounted for. Here is what I googled.

"The Coriolis effect is caused by the rotation of the Earth. An object, such as an aircraft, travels in a straight line through space. Because of the rotation of the Earth, the object will appear to veer to the right in the northern hemisphere or back to the left in the southern hemisphere.

Coriolis acts on the air as well as an object flying through it. Air drawn towards an area of low pressure will actually move along the Isobars, the gradient force created by the pressure difference being balanced by the Coriolis effect. Thus wind travels clockwise around a area of high pressure in the northern hemisphere and anticlockwise around an area of low pressure. The Coriolis effect varies with ground speed (or wind speed) and is greatest at the Poles and zero at the Equator."
 
GalaxyFlyer
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Re: Celestial Nav - Sextant Ports

Fri Jun 19, 2020 2:17 am

Always liked the ATC clearance given to out tanker after a refueling and they were off to a cel NAV leg, “Tanker xxx, cleared to wander aimlessly about North America between BTV and”.
 
mmo
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Re: Celestial Nav - Sextant Ports

Fri Jun 19, 2020 5:43 am

GalaxyFlyer wrote:
Always liked the ATC clearance given to out tanker after a refueling and they were off to a cel NAV leg, “Tanker xxx, cleared to wander aimlessly about North America between BTV and”.


Reminds me of a typhoon evac from Andersen to Kadena. The aircraft we had was in somewhat less than optimal condition. We were a flight of 3 going to Kadena so getting up there was not a problem. The return was somewhat different. Due to ramp congestion and other issues, each aircraft returned as a single ship. Just trying to get going was a task. Multiple problems delayed our takeoff by a couple of hours. We finally got going and things seemed to settle down. About 2+30 out of Kadena the Nav comes on the interphone and asks me to tune in IWO Jima TACAN (I had done it already) and he asked me the range and bearing. My Nav was a great person and is now a physician, however, his navigation skills left room for a little improvement. Always ready for a little chain pulling, we proceeded to have a dialogue on the intercom about why he needed a fix. Finally, in utter frustration, he responded, "I'm the navigator and I have the right to know where we are!". Priceless! His DR and Cel position was a circle just about as big as the pacific and he didn't have a clue as to where he was. Glad he's a better Doc than Nav. The best thing is, we still friends and talk and see each other.
If we weren't all crazy we'd all go insane!
 
AstroNav
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Re: Celestial Nav - Sextant Ports

Sat Jun 20, 2020 4:43 am

Lot's of interesting commentary including anecdotes. Thank you.

For anyone interested in a couple of good intro's to this topic, the reinstated USN CN course is online (plus some atmospheric/ weather/ Hydrography and GNSS content) as well as one from Harvard. Both free and accessible, here:

USN

https://www.meted.ucar.edu/training_module.php?id=1150

https://www.meted.ucar.edu/training_module.php?id=883

Harvard

https://courses.edx.org/courses/course- ... 17/course/
 
strfyr51
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Re: Celestial Nav - Sextant Ports

Sat Jun 20, 2020 4:59 am

when I flew as a flight engineer in the navy we sometimes took star shots while on station to update the Plot of where we laid sonobouys for the front door to our pattern if we were handing off our prosecution of a sub to an oncoming crew or squadron. other than that? We marked the Lat-Long and climbed out for home and started our Loitered engine or engines. for the transit.
 
SAAFNAV
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Re: Celestial Nav - Sextant Ports

Sun Jun 21, 2020 8:12 am

As an ex-navigator, I missed out on celestial nav as it was retired long before I started training.
It always fascinated me, but I must say, if I still had to do that in the nav course it might have gotten pretty hard.

Our C-130's still carries the sextant. It isn't serviced/calibrated anymore, but the idea is that you could still use it to check for physical damage. The sextant port is also useful for the smokers to stand underneath. Also an interesting thing I've picked up from lore - on top of the fuselage there is a little stud that you use to get your 'zero-angle' between the aircraft and the sextant.

As I don't have practical experience, I can can't cite it as fact, but I reckon taking a sun-shot through any kind of window would really mess up the angles due to the refraction involved.
In a sextant there is just the mirror, and if a lens is present, it would be calibrated around it.
L-382 Loadmaster; ex C-130B Navigator
 
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SheikhDjibouti
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Re: Celestial Nav - Sextant Ports

Sun Jun 21, 2020 11:46 am

AstroNav wrote:
Hello,

I'm studying for my RYA YachtMaster Ocean certification....

If it hasn't been mentioned already, another (RAF) term is "astrodome" (not to be confused with the Houston Astrodome!)

Presumably you are aware of Eric Tabarly, winner of the 1964 single-handed transatlantic yacht race?
Coincidentally a former French Aéronavale (Fleet air arm) pilot, Tabarly fitted Pen Duick II with an astrodome scavenged from a decommissioned Short Sunderland flying boat.
From a flying boat, to a boat that almost flew.... :scratchchin:

Here is a Short Sunderland; note the large perspex bubble half-way along the upper fuselage, ideal for the typical navigators position.
I believe astrodomes were also useful for co-ordinating the various gun positions when under attack from say... eight Luftwaffe Ju-88s.
https://en.wikipedia.org/wiki/Short_Sun ... ith_Ju_88s



Both the RAF Shackleton at Keflavik in 1980, and the C-130 at Southend in 2005, feature a crew member looking out of the astrodome, possibly to assist the pilot whilst manoeuvring on the ground. (click on "view large" if you don't believe me!)



Pen Duick II, with ex Sunderland astrodome clearly visible.
Image

Thx as usual to wikipedia
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