Why are headings still magnetic?

Bob Moore wrote:
Andrew Sarangan wrote
So, I still don't agree that navigation systems have
advanced to the point where we can abandon the
magnetic based instruments.

Hmmmm....I wonder how we used to navigate 'over-the-pole'
back before INS? Hint....Grid Navigation, an unslaved
DG referenced to true north.

I don't claim to know anything about navigation over the poles, but if
it is as simple as you say it is, why do we continue using the
magnetic compass for every-day navigation? My guess is that it must be
complex, difficult, expensive or unreliable.

Mxsmanic wrote:
"Grumman-581" writes:

And high enough that the sky was dark even when the sun was out -- at least
from the photos that I've seen published...

Yes, but the ANS could recognize stars even from the taxiway in broad
daylight. I still don't know how it managed that.

Jupiter, Saturn, and Venus are visible to the naked eye in broad
daylight at sea level, if you know exactly where to look. I've
seen them all, at times when I knew their approximate position
relative to the moon. You must look in exactly the right place --
an error of a half degree or so puts them out of the central field
of view of your eyes and renders them invisible against the
glare of daylight.

The brighter stars are just outside the ability of most people's
naked eyes to see in broad daylight, but a small pair of
binoculars will pick a few out nicely. Again, proper aim is
crucial. Only a handful of the brightest stars will typically be
visible at any given time, but you don't need very many for
navigation.

It sounds like the optics of the SR-71's nav system are at
least as good as a standard issue Mk 1 pair of eyeballs
coupled with a 7x35 pair of binoculars. It may be that, for
rapid startup, the system needs some hint of the location
and time of day to figure out where to begin its search for
stars, but I'm just guessing on that point.

wrote:
Mxsmanic wrote:
"Grumman-581" writes:

And high enough that the sky was dark even when the sun was out -- at least
from the photos that I've seen published...

Yes, but the ANS could recognize stars even from the taxiway in broad
daylight. I still don't know how it managed that.

Jupiter, Saturn, and Venus are visible to the naked eye in broad
daylight at sea level, if you know exactly where to look. I've
seen them all, at times when I knew their approximate position
relative to the moon. You must look in exactly the right place --
an error of a half degree or so puts them out of the central field
of view of your eyes and renders them invisible against the
glare of daylight.

I remember taking celestial shots of Venus and Jupiter in the daytime
but don't remember ever seeing Saturn. Pointing the sextant to the
right area of sky - not hard since I precomputed the shot - would put
them right in the field of view.

I would imagine the ANS could "see" just a bit better than I could.
:)

--Walt

writes:

Jupiter, Saturn, and Venus are visible to the naked eye in broad
daylight at sea level, if you know exactly where to look.

Are these consistently above the horizon at all locations?

From what I've read, the ANS looked specifically at stars, not
planets, but I may be wrong.

I suppose that technically stars are visible, too, it's just that the
difference isn't perceptible to the naked eye.

I've
seen them all, at times when I knew their approximate position
relative to the moon. You must look in exactly the right place --
an error of a half degree or so puts them out of the central field
of view of your eyes and renders them invisible against the
glare of daylight.

Hmm ... I'll have to try it sometime.

The brighter stars are just outside the ability of most people's
naked eyes to see in broad daylight, but a small pair of
binoculars will pick a few out nicely. Again, proper aim is
crucial. Only a handful of the brightest stars will typically be
visible at any given time, but you don't need very many for
navigation.

The ANS required three, supposedly. It also required a very accurate
clock. With the clock setting, it could find its way. I guess it
just looked around at the sky until it found a few stars, then checked
it against its own computed ephemeris based on the time of day.

It sounds like the optics of the SR-71's nav system are at
least as good as a standard issue Mk 1 pair of eyeballs
coupled with a 7x35 pair of binoculars. It may be that, for
rapid startup, the system needs some hint of the location
and time of day to figure out where to begin its search for
stars, but I'm just guessing on that point.

It definitely needs very accurate time of day, but I don't think it
needs a hint of its location--although may it does, or maybe a general
hint speeds up acquisition (which usually took a matter of minutes, I
think).

--
Transpose mxsmanic and gmail to reach me by e-mail.

Andrew Sarangan writes:

I don't claim to know anything about navigation over the poles, but if
it is as simple as you say it is, why do we continue using the
magnetic compass for every-day navigation? My guess is that it must be
complex, difficult, expensive or unreliable.

I think magnetic headings are still used because of the vast inertia
of the installed base of avionics and instruments, plus the inertia of
so many pilots who have been raised on that.

I certainly won't quarrel with using magnetic navigation as a back-up,
but I do question basing normal navigation on a compass, which is
relatively unreliable compared to more modern methods. If the SR-71
could navigate reliably by the stars half a century ago, I think most
aircraft could navigate reliably today using much more modern
satellite systems.

--
Transpose mxsmanic and gmail to reach me by e-mail.

"Mxsmanic" wrote in message
...
Yes, but the ANS could recognize stars even from the taxiway in broad
daylight. I still don't know how it managed that.

http://www.sr-71.org/blackbird/manual/4/4-3.php

"Mxsmanic" wrote in message
...
The leakage was trivial, and it rapidly warmed up enough to seal the
leaks.

I once asked Mary Shafer ( SR-71 Chief Engineer NASA Dryden Flight Research
Center, Edwards, CA) about whether it was true that the SR-71 leaked like a
sieve... She replied something like, "to say that it leaks like a sieve
would be to insult all sieves" or maybe it was, "a sieve that leaked that
much would be pretty useless"...

Here's some interesting readings if you are so inclined:
http://www.netwrx1.com/skunk-works/v05.n129

In certain situations, 1200 lbs would leak off prior to takeoff...

Mxsmanic wrote:
Andrew Sarangan writes:

So what instrument would you suggest we use for true headings?

A combination of INS and GPS would work. It would be at least as
accurate as a compass.

Accuracy isn't the problem. Would it be as reliable?
Would it be affordable and workable in airplanes that lack
electrical systems? And would it work when those
electrical systems failed?

If you're talking about high budget commercial aviation, any
nav system that has the processing power to decode the
GPS signals has the power to apply a local magnetic
deviation to its heading or course output. Updates to
the magnetic deviation can be distributed as needed
with the updates to the nav database. So for these
users, it's sort of arbitrary which reference is used,
as long as there is an agreed upon reference.

For the gliders, Piper Cubs, and other moderate budget
flyers, the price and reliability differences between a
compass and an INS weigh significantly in favor of the
compass.

There is the further issue of inertia. Every pilot flying today
learned to use magnetic headings. Every airplane cockpit
has an instrument (or several instruments) to prominently
display the magnetic heading to the pilot. To convert
wholesale would entail a period of minor confusion at
least.

Finally, what problem would it solve? The shifting of
the Earth's magnetic poles is slow, relatively
predictable, and something we have lived with for
awhile. While true headings may be more elegant
and stable over the very long term, that elegance
doesn't add much practical value.

--Rich

wrote in message
oups.com...
Finally, what problem would it solve? The shifting of
the Earth's magnetic poles is slow, relatively
predictable, and something we have lived with for
awhile. While true headings may be more elegant
and stable over the very long term, that elegance
doesn't add much practical value.

It's the typical case of a solution in search of a problem...

Mxsmanic wrote:
I certainly won't quarrel with using magnetic navigation as a back-up,
but I do question basing normal navigation on a compass, which is
relatively unreliable compared to more modern methods.

Unreliable? The magnetic compass is about as reliable is it gets. There's
one moving part, no power source, and the Earth's magnetic field is good
for another few thousand years. What's unreliable about that? Of the
cannonical "watch and compass" navigation kit, the watch is by far the less
reliable of the two.