Why are headings still magnetic?

In article ,
Stubby wrote:

Roy Smith wrote:
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.

I met a sea captain that piloted an old ship full of refugees from
Latvia to Nova Scotia in 1939 with only a sextant and magnetic compass.
And he said it was overcast most of the time.

Probably had a watch, too. And a taffrail log. In 1939, that would have
been a pretty standard navigational kit for an ocean-going ship.

On Sat, 02 Sep 2006 19:56:46 +0200, Mxsmanic wrote:

And the poles occasionally
reverse, which would also be somewhat of a disaster for
magnetically-based aviation.

It's expected the poles will flip sometime over the next couple of
centuries. For those of you interested in this phenomenon.
http://www.pbs.org/wgbh/nova/magnetic

Grumman-581,

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


You got that right.

--
Thomas Borchert (EDDH)

Mxsmanic,

And while a compass shows magnetic north, that's all it shows. You
have no idea how far north or south you are, or which direction to fly
to your destination.


And having true heading (and only that) changes this problem how?

--
Thomas Borchert (EDDH)

Roy Smith wrote:
In article ,
Stubby wrote:

Roy Smith wrote:
Mxsmanic wrote:
From what I've read, the ANS looked specifically at stars, not
planets, but I may be wrong.
Planet, star, it's all the same. It's a point of light in the sky. The
ephemeris calculations are a little more complicated for a planet, but
that's only something you'd notice if you were working it out with pencil
and paper.
But aren't the stars stuck to the celestial sphere so that their motion
is fairly simple and easy to predict. Planets are zipping around the
sun, as is the Earth, and the Earth is turning on its own axis. Much
more complicated.

Depends on your definition of "Much more complicated".

If you're doing it the traditional way, working from the Air (or Nautical)
Almanac with paper and pencil, reducing a planet sight is a couple more
table lookups and a couple more additions or subtractions. Some hulking
mainframe did all the really messy math for you a year or two earlier, in
plenty of time for the tables to be typeset, printed, and bound.

If you're doing it all from scratch with a computer, all the formulas you
need can be found in Jean Meeus's "Astronomical Formulae For Calculators"
(http://www.willbell.com/math/mc3.htm). The book was published in 1979,
and gave formulas usable on the popular hand calculators of the day to
achieve accuracies exceeding any practical navigational need.

I would worry about running out of fuel while I'm trying to figure that
book out and fat-fingering the calculator.

I believe all U-2s have been retired. The satellite folks are winning
the high altitude intel game. Too bad the U-2 was a nice plane (glider?
rocket???).



Mxsmanic wrote:
"Chuck Peterson" charles.petersonxxx@comcast(removethis and xxx).net
writes:

Did (or does) the U-2 employ a comparable ANS

The U-2 predates the ANS, I believe, and today I'd expect it to be
using GPS instead, which is much more accurate.

Greg Copeland wrote:
On Sat, 02 Sep 2006 19:56:46 +0200, Mxsmanic wrote:

And the poles occasionally
reverse, which would also be somewhat of a disaster for
magnetically-based aviation.

It's expected the poles will flip sometime over the next couple of
centuries. For those of you interested in this phenomenon.
http://www.pbs.org/wgbh/nova/magnetic


Interesting, alarmist speculation.

"Walt" wrote in message
ups.com...

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.

Bob Moore

Way back when I was a navigator on a KC-135 using Grid Navigation we
referenced the DG to Grid North, not True North. Big difference between
the two, although I think I know what you're hinting at.

And, I remember taking a celestial shot every 15-30 minutes or so to
check for gyro precession. That would be hard to do in the Warrior I'm
flying nowadays. :)

--Walt Weaver
Bozeman, Montana

Yeah, Walt. During the Cuban crisis I flew B-52Hs out of Minot AFB, ND.
We flew the "North Country" route. From Minot fly East to the "Black Goat"
refueling area in the Atlantic just off the U.S. East coast.. North to the
Artic..SW to "Cold Coffee" refueling area in Alaska..out the Aleutian chain
to the periphery of the Soviet Union...back to Seatttle; Spokane, Minot and
land 24 hours after takeoff.

The Navigator had to convert Magnetic/True headings to/from Grid while also
observing celestial references for sextant shots. Not the time to have a
weak Navigator. Story was that one Navigator got it all screwed up and
actually penetrated Soviet airspace. Shots were fired in front of the
bomber by Soviet interceptors. The B-52 immediately reversed course and
nothing further happened. As with all B-52s in Airborne Alert it carried
nuclear weapons and could have been a disaster. Due to the sensitivity of
the crisis nothing was ever published about that situation that I know of.

Darrell R. Schmidt
B-58 Hustler Web Site URL (below)
http://members.cox.net/dschmidt1/

"Mxsmanic" wrote in message
...

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

When I was a tech in the B-2 program about 10 years ago I gained some
experience with its astro inertial navigation system. It was made by
Northrop, and word of mouth was that it was descended from the SR-71
system, but I don't know for sure.

The B-2 AINS had a catalog of 61 stars. No planets. The rapidity and
complexity of planetary motion would necessitate separate algorithms
for stars and planets.

Precise time came from an ATTU (airborne time transfer unit?) which we
synchronized to UTC on a lab time standard and installed in the
aircraft just before the crew arrived, to minimize clock drift.

The star tracker used a telescope of about 3 inch diameter on an
alt-azimuth mount. To find a star, it first aimed at the expected
point in the sky (based on the current nav solution), then did an
expanding square spiral search. After aquisition, it tracked for only
a short time before moving to the next star. If the search failed
(perhaps due to cloud), the tracker kept trying different stars. It
could shoot through holes in the clouds. And it worked just fine in
broad daylight.

I was able to verify that during a long ground test outdoors. The test
had nothing to do with the AINS, but there was enough slack time that
I could play around with it. First I aligned it to the GPS position,
then changed the AINS mode to pure inertial. Slowly its coordinates
drifted away from the GPS. Then I switched to stellar inertial mode,
and watched that star tracker drive the AINS position right back on
top of the GPS.

In those days the AINS accuracy was classified, and I'm not sure if
that's still the case, so I won't say exactly how well it did. But it
was impressive.

There was a period when we had trouble with star tracking during the
day, due to contamination on the inner surface of the window. (It
appears as a dark opening about the size of a dinner plate, a few feet
to the left of the cockpit in B-2 photos shot from above.) There was a
metallic grid that made the inside hard to clean. You couldn't just
wipe it off. I remember seeing some poor guy individually cleaning
several hundred tiny squares of glass with Q-Tips and solvent!


Getting back to the magnetic topic, the only magnetic compass on the
B-2 is the standby compass. It's made by Airpath and looks just like
one you'd see in a light plane. By default the glass cockpit heading
readouts show magnetic (you can select true), but that's synthesized
from gyro-derived true heading and a variation table in the aircraft
software.


Maybe someone else has already mentioned that VORs and TACANs are
aligned so their radials are close to the magnetic direction. Changing
them to true would be pretty expensive and disruptive.

--
Paul Hirose
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Stubby writes:

But aren't the stars stuck to the celestial sphere so that their motion
is fairly simple and easy to predict.

Yes, relatively speaking.

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