Celestial Navigation

[Jason210]

Since getting the B377 I have become interested in pre-GPS/INS navigation methods and have installed the FS9 bubble se'xtant gauge in the B377 instead of the Moving Map.

When taking measurement of a star, I was wondering about the altitude of the aircraft (ASL) and whether this needs to be taken into account. I can imagine it's a consideration when using an ordinary sextant, because the horizon will appear lower at high altitude. But a bubble sextant uses an artificial horizon which is independant of the visual horzion, being dependant on gravity instead...a bit like the old Astrolabe.

Since stars are effectively inifinitely far away, the angle to the star ought to be the same at whatever altitude the plane is at. But I'm not sure, and have been unable to look this one up.

Any help appreciated!

[pjc747]

Yes your altitude does matter. However, celestial navigation will be quite tricky in FSX. Does it show true constellations? (not lockheed aircraft)

[Jason210]

Yes your altitude does matter. However, celestial navigation will be quite tricky in FSX. Does it show true constellations? (not lockheed aircraft)

Yes, FSX has accurate celestial projection that you can navigate with. There is even a bubble se'xtant gauge you can install on FSX that will compare your assumed position with the known position of a star, and I've been playing about with it. I've actually also just bought a Mark IX Bubble se'xtant off ebay that I'm waiting for and am going to experiment with when I get it.

Regarding altitude, I don't see how it can matter since the angular difference to the star (relative to the instrument) is not going to be significantly different if measured at sea level or at 30000 feet. Perhaps it matters the sun and solar system bodies?

Jason

[pjc747]

Well yes, but I think the globular shape of teh earth distorts their actual shapes in space, so it may matter, my celestial navigation knowledge, however is rudimentary, and from sailing, where you're always at sea level

[VulcanB2]

Hi,

It is a simple trig problem. It will have the result of making the celestial body higher above the horizon than it is, and thus will affect your positioning.

I'm a bit busy to run through some math on this right now, but I will come back and explain/show how altitude can adversely affect positioning when using the natural horizon.

If the se'xtant uses an artificial horizon then this problem is largely alleviated.

Best regards,
Robin.

[Killratio]

An old Bomber Navigator I saw interviewed said that he (top of his class in navigation) was able to fix his position to within 5 miles on the ground. he said he had not a lot of faith in anything he calculated at 18,000 in a heaving Lancaster.

Bomber Command averaged something like, I seem to recall, 5-10% of bombs dropped within 5 miles of target during night bombing before the Pathfinders, H2S etc improved it dramatically.


Darryl

[VulcanB2]

5-10% of bombs dropped within 5 miles of target

This is going to sound stupid but if I read this right, that means 95-90% were outside of 5 miles of the target? Wow.....

How many of the 5-10% hit the target?!

Best regards,
Robin.

[Killratio]

Robin,

The answer is pretty much ALL the 5-10% hit the target. Remeber also that with a City sized target, an error of 5 miles is not "huge" but......

The Butt Report found that, on average One Third of bombers that attacked got within 5 miles. That number does not include aborts, jettisoned, lost, etc etc. That was for June/Jul;y 1941 when experienced crews were available but "modern" technology was not.

The 5-10% figure is a modern, quoted estimate that looks at "overall" aircraft employed.

If you look at Dresden, Hamburg etc, obviously the large area made "effectiveness" accademic. Point targets were not generally a consideration for Night bombing (the obvious exceptions being Peenemunde and The Dams Raid) the US day bombers had a much smaller CAP and therefore tended to be used on point targets.

The Germans fared better but the Thames and short distances from coast to target for other places made their job easier. It has been argued that teh "Black Out" in London was pointless given the Thames and that it actually killed as many Londoners as bombing did. Personally I think that is a bit strong...

Regards


Darryl

[pjc747]

Its hard to beleive this, I think more than 5-10% would hit their target, if you see war footage allot seem to hit the place..... just sayin'

[pj_3]

, I think more than 5-10% would hit their target,

Entirely dependent upon many factors to generalize.. everything from the period you're referring to; the particular night; the particular target; the weather/cloud cover: whether there were a single, double or triple stream, with false targets being approached: Master Bombers: what technology was used and available at that particular time: the number of aircraft: what target marking was used on the night. If you want a good picture - get a copy of Len Deighton's "Bomber". It is the story of a single night's raid on Krefeld, in the Ruhr: the short prelude and the aftermath, told from both sides. "Deighton's cool, merciless, detailed reconstruction of the raid is overwhelming. The reader is denied the protection of distance or ignorance" (partial cover quote).
This book will give one the real sense of being in a war.

[VulcanB2]

It has been argued that teh "Black Out" in London was pointless given the Thames and that it actually killed as many Londoners as bombing did. Personally I think that is a bit strong...

Does seem a bit odd.

Thanks for the explanation.

Best regards,
Robin.