East is Least. West is Best.

[TheGreenGoblin]

Still harping on about the autonomic variation compass update analogue computer, I was looking at the modern charts associated with these magnetic variation lines on a global scale. Creating such a chart manually, given the issue of displaying the lines on a useful global projection must have also been pretty difficult. I am still intrigued to know how dental technology came into the fix as well in the gizmo PN described. I have got my proverbial teeth into that latter question.

This article describes the steps that are and would have been needed to use a chart of this kind, either manually, semi-automatically or fully automatically, even back in the 50's/60's. The fact that this process flow could even be partially automated using an analogue device is a tribute to the ingenuity of the boffins who designed the computer. Clever, clever people!

http://geokov.com/education/magnetic-de ... ation.aspx

Interestingly the NOAA refer to variation as declination. They may have Oceanic in the name but they are geographers and surveyors at heart.

[Pontius Navigator]

Thinks .. can anyone remember what interval the then current edition of the Air Almanac was re-published, not important but I woke up last night asking myself the question, and can't for the life of me remember !

Quarterly. UK version light blue, USN version yellow cover and spiral bound otherwise number for number.

I passed my Air Nav promotion exam first time and like to think it was my description of the basic astro position line:

Plot the sub Stella point and arc off the co-altitude.

I moved directly to the next question.

Remember the Admiral Marc St Hilaire (sic?).

DR ahead. Select a suitable assumed position. Use tge tables to find the calculuted altitude. Take your shot (stick with the Sun?). Calculate the difference. Plot the azimuth observed from the assumed position. Plot the position line remembering GOAT.

Will you mention coriolis? Or refraction?

Did you use the Air Ministry 3270s - red, yellow and green bands? Did you ever apply heading and speed change corrections?

Remember the 57 navigation stars in the red band?

In the Vulcan we had two sextant mounts and often carried two sextants. The offset mounts also made it impossible to take a 3 pl fix. We used to take sandwich fixes. The simplest was 3 sights, on the beam, across track and on the beam averaging the beam shots, no PL transfer.

In practise we would do a 5 or 7 sight fix. A BBB A , or ABABA or ABABABA. Inevitably we might lose a sight through turbulence. We then just dropped the corresponding opposite.

Anything over 5 miles was unusual. Heading changes of 0.2 deg and speed 1-2 kts was unusual. I tried astro in the Nimrod once. Heading changed 1-2 degrees and people moving about didn't help for speed. Never tried again: that was why the Nimrod got extra stabilisers on tail as yaw dampers for flight refuelling.

[Pontius Navigator]

TGG, re dentistry. The first cam we used was circa 1947-48 and probably made in the early 50s. The cam contours had to be rigid and operate over a large temperature range, impervious to moist and resistance to wear. Dental paste was presumably a better medium than plastics of the day.

As the computer was made by EMI they probably had a hand in creating the AVS unit.

[Undried Plum]

PN, were your sextants the two-minute averaging type?

What sort of chronometer did you use?

Did you ever do lunar distance measurements and calcs? Were they still publishing LD tables?

[Pontius Navigator]

Yes. We only used the one minute setting.

Standard nav aircrew wrist watch, we eventually got Lemania but that was for bombing rather than navigation. That bombing requirement only lasted 2 years but we retained the chronograph. A good watch would be in error no more than 2 seconds over a trip, say one second from time check to end of astro sights.

[boing]

Yes, during a Vulcan astro-fix even a sneeze from the pilots would cause a tirade of complaints from the navigator who happened to be taking the shot. Definitely no heading or speed changes.

Later, on the VC10 where we had co-pilot navigators, it was fairly normal for the pilots to make little heading and speed changes during in the shot. Nobody seemed to be particularly excited about the result.

When the 10's finally got inertial systems in the introductory period we were supposed to fly by the astro positions but only monitor the inertial for accuracy - in realty that period only lasted for a few weeks before, unofficially, the priorities were reversed.

The abilities of a good Vulcan navigator always amazed me. I am certain it must have included the sacrifice of black cats at the full moon. I do remember one navigator, who, by a very large amount of pre-flight work, would call for one heading after take-off and expect to arrive pretty darned close to Goose Bay with no further heading changes. I think they called it "pressure pattern navigation" or something like that.

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[Pontius Navigator]

Boing, rhumb line perhaps?

[boing]

PN

No idea, not my department. It seemed to be some sort of game between the the Nav. Plotter and Nav. Rad. Since we were flying 10,000 feet above the commercial traffic ATC were not concerned either.

There is actually some reference to Pressure Pattern Navigation here, All Chinese to me.

This category discusses pressure differential techniques and how they affect pressure pattern navigation. Topics, such as constant pressure surface, geostophic wind, pressure computations and plotting, D readings, effective true airspeed, K factor, pressure line of position, Bellamy drift, and the MB-4 solution of Bellamy drift are all discussed. Also explained are the limitations of pressure differential techniques and how they are affected my meteorological conditions.

https://www.flightliteracy.com/flight-n ... avigation/


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[ExSp33db1rd]

P.N. Thanks for the info. I thought Quarterly but then ???? maybe Monthly ? Thanks.

I'll mention Coriolis, which of course we had to build into our calculations, but intend to try to stick to practical in flight application, rather than too much theory - a lot of which I have forgotten, so I hope I don't generate too many questions ! I'll touch on Grid Navigation over the Pole, too, of which I have some experience.

[ExSp33db1rd]

I think they called it "pressure pattern navigation" or something like that.

Yes, learned the theory but never had the courage to put it into practice !

Undried Plum .... BOAC used Kelvin Hughes sextants on the Strat. and Brit. and early 707, but Kollsman on the later models of the 707. Both were rigged to average over 2 minutes. For a "fix" at 10.30 I would start to shoot the first star at 10.21, the second at 10.25 and the the third at 10.29. This would give position lines at 10.22, 10.26 10.30 and of course the first 2 would have to be moved along track for 8 and 4 minutes respectively at the estimated ground speed calculated at the previous fix,to give a new "cocked hat" fix at 10.30

Good fun, and I'd love to have another go ! A recent flying student asked me " what's a sextant? " One could weep.

[TheGreenGoblin]

I'll mention Coriolis, which of course we had to build into our calculations, but intend to try to stick to practical in flight application, rather than too much theory - a lot of which I have forgotten, so I hope I don't generate too many questions !

Talking about flight and different frames of reference I trust you will also spend a good half hour on the role of Schuler Tuning in the the design of the INS to take account and correct for the curvature of the earth.

Half an hour of such pain will definitely be character forming. You could just point them at this article and move on I suppose....

http://www.imar-navigation.de/downloads ... uction.pdf

[ExSp33db1rd]

G.G. Thanks, takes me back a bit ! I recall BOAC fitting their first, experimental, INS on a 707 freighter, it occupied an enormous section of the sidewall of the freight compartment. After start up we were cleared to taxy to the then 10 R and just before rolling I went back to see that everything was working, and was impressed to see that it had recorded a change of position of 1 mile West and a half mile South. Magic ! I still use a 17 year old GPS unit that fits into my pocket and is arguably more accurate ?

I doubt I will have time to do more than briefly describe the use we made of INS, let alone any detailed principle of it ? Our local airfield still operates, and publishes an NDB let down, tho' of course a GPS plate is also available.

I was on one of our early trips from New York to San Francisco, and the American hinterland was unkown to most of us, so the skipper had brought along a couple of road maps, and it being a clear day was able to point out cities of interest that were previously just names. Peering out of the window with his map, he was observed by a passenger at the open door, awaiting the nearby toilet. "Jeez," he said, "haven't you guys progressed beyond the Wright Brothers yet ?"

IFR I Follow Railways.

[Pontius Navigator]

I like the Kelvin prisextant with the graticule. Didn't like the Kolsman. The Kelvin just seemed more stable in its mount compared with the Kolsman which was technically more accurate but with the Nimrod 's yaw problem it was rubbish.

[ExSp33db1rd]

Boing, rhumb line perhaps?

Rhumb Line cuts all Meridians at the same angle, so could have been, but unless flying virtually in still air - maybe possible at those altitudes ? - he wouid have had to correct for differing drift angles as he crossed the Meridians. N'est ce pas ?

A Rhumb Line is not necessarily the shortest distance, that's the province of the Great Circle.

[Undried Plum]

Undried Plum .... BOAC used Kelvin Hughes sextants on the Strat. and Brit. and early 707
....
Good fun, and I'd love to have another go ! A recent flying student asked me " what's a sextant? " One could weep.

Ahh! Kelvin Hughes sextants. Swing that lamp and I'll tell ye a story.

When I was young sprog, newly qualified as a geodesist and hydrographer as a result of what turned out to be temporary loss of my Class 1 medical, I was hired by the FCO as a catman to work alongside battmen in Oman's Dhofar. The new Sultan wanted to win over the hearts and minds of the rebellious jebalis by bringing them goodies such as health, education, communications and good water. The bozos at FCO wanted a Hydrologist and didn't understand that I as a Hydrographer wasn't one, but they liked the cut of my jib and I was hired after an astonishingly short interview.

The cat team comprised myself and my survey technician. My job was to find water and to survey the local landscape to plan water channels and to find appropriate locations for village sanitation etc. The batt team comprised four soldiers of the British Army. One was a specialist medic; another a specialist signaller; a third was good at using explosives to make things change shape in useful ways; and a fourth spoke Arabic passably well.

Unusually for such a small troop, one of them was a rupert. Mike K, an excellent fellow whom I greatly miss after he perished in a stupid accident. He was a most unlikely looking soldier. Owlish in his round spectacles and rather bookish by nature, he was fascinated by the technicalities of navigation and had been studying how the WW2 guys who went behind German lines in North Africa managed to navigate.

I had taken my Kevin Hughes sextant with me. My most prized possession at the time. Mike K was fascinated by what hydrographers could do with a sextant. He already knew the astro stuff, but what we could do by techniques such as double angle sextant resection and vertical subtense blew his mind. I showed him how you can measure distances to an accuracy of less than the distance between your two eyes. I told him how the Great Survey of India was partly done with small sextants used on horseback at speed. Using my Mini Moke on the beach near town I let him practice using a sextant at speed when bouncing around.

He went on to draft a manual of Advanced Land Navigation for his unit, but he died before he could submit his final draft for official approval. His boss Mike R subsequently contacted me and asked me to to complete it and to set up a training course for their 'directing staff' instructors. He subsequently bought a dozen Kelvin Hughes sextants for the unit. I really should have submitted an invoice to Smiths Industries for sales commission!

I somehow forgot to return my Wild T3 theodolites to stores and it disappeared from the inventory. I still occasionally shoot some LoPs with it, just to keep my eye and hand in.

[Pontius Navigator]

Was that Rose?

[Pontius Navigator]

ExSp33, did you ever use an astro compass? I have the barest memory of its settings. It was mounted on a metal bar in the astrodome. You set LHA and declination, rotated it to the celestial body, usually the Sun,and read off your heading.

I guess in your no compass case you could set the desired heading and get Biggles to weave until the Sun Gun was aligned.

[TheGreenGoblin]

All this talk of Local Hour Angle etc. has set me off so, it being a down evening at Goblin Hall, I thought I might dust off my memory and conduct a thought experiment and see if could imagine how one might go about filling in a Sight Reduction Form in an aircraft, having never taken a sun-sighting from an aircraft in my life.

In my mind I am following the procedure I might follow in a yacht and have thus pulled out an imaginary Sight Reduction Form and am mentally filling it in . Having chosen my celestial body as being the sun and assuming the sextant has been calibrated, I am immediately beset by questions that may be relevant or different in an aircraft, as opposed to a yacht, when mentally filling in the Apparent Altitude Section.

First I come to the Dip Correction number. In an aircraft I assume that the aerial navigator of yore would have climbed up to some sort of periscope/astrodome from which he could take a sighting. I assume that the dip correction number would have been defined as the distance from sea level to the sighting port. If I am correct then this implies that one needs an accurate QNH to determine the altitude accurately, as arbitrarily choosing a standard ISA altimeter sea level of 1013.25 could be very inaccurate. The exact altitude number would then need to be corrected to allow for the height of the astrodome above the altitude datum port in the aircraft. Obviously I have to ensure that the pilot is flying straight and level, so I am leaning across for my rolled up newspaper with which to beat him about the head if he errs,...so that I can go to my almanac to look up the dip correction value based upon my altitude.

I have only just started and might already be all at sea. Can one of the real aerial navigators here let me know if I am on the right track in my thinking, or should I simply retire to the pub like the proverbial drunken sailor?

Something like this...

[Pontius Navigator]

Dip was not a factor. We used an artificial horizon. There were many iterations of airborne sextants culminating in the averaging bubble sextant Mk 9 as used in Lancasters etc in WW2 and also post war in aircraft with astrodomes and even the Canberra.

Inside was a liquid chamber that had a small bubble of air. Using a mirror this gave an horizon. However that bubble would change size depending on the cabin pressure. It could be adjusted by operating a diaphragm.

With pressurised aircraft a special hatch was fitted an the Mk 2 periscopic sextant which worked exactly like the Mk 9 with its bubble. Next came the Mk 2B which had a 'London Transport sign' suspended on a ligament. This had a fixed size.

When taking a shot you centred the body in the centre of the bubble or the IT sign. 30 seconds before the observation time you pressed start. After 10 seconds the light would go out to show it was running. You continued to track the body. At the half way time on the perisextant you flicked the azimuth switch to heading. At the end of the site you would read off the altitude and the heading.

That bubble or sign was subject to coriolis effect which deflected it to the right of track (IIRC) in the northern hemisphere. A typical value was 7 km at 480 kts. The assumed position was moved before the position line was plotted.

[Pontius Navigator]

Now thinking of the astro form and considering a single Sun shot.

Plan the time of the shot. Calculate the DR position for that time. Calculate the assumed latitude (nearest degree).
Enter the Air Almanac for the date and against the time get the GHA and Declination. Using Arc to Time calculate the LHA.
Enter the AP3270 Vol 2 (iirc) Yellow Band at the Assumed latitude page. Against LHA extract Altitude and Azimuth.
Check for refraction at low altitudes and altitude correction for depression shots.
And so on.