
Compasses don't work on Mars, so how do you navigate? - ingve
http://kottke.org/15/07/compasses-dont-work-on-mars-so-how-do-you-navigate
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gliese1337
I am surprised that no one has yet mentioned the Gyrocompass:
[https://en.wikipedia.org/wiki/Gyrocompass](https://en.wikipedia.org/wiki/Gyrocompass)

The post mentions manually-set directional gyroscopes, but gyrocompasses are a
step ahead of that; on any sufficiently-quickly-rotating planet, a gyrocompass
will point you towards the geographic poles by noting the axis of precession
of a gyroscope with arbitrary orientation. That's even better than a magnetic
compass on Earth, since the magnetic poles do not line up exactly with the
geographic poles, and the magnetic is non-uniform anyway (which is why
navigational charts include notations of the magnetic deviation in different
areas).

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creshal
How maintenance intensive are they, though? Moving parts in or near vacuum
tend to be a bit iffy (e.g. lubrication), especially for multi-year (or multi-
decade) robotic missions.

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davewasthere
There are non-moving part designs that utilise fibre-optics or laser rings.
I've only seen either the physical or laser ring gyros. Not sure about which
ones they'd use in space though.

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avian
Grandparent is talking about gyrocompasses, not ordinary gryoscopes though. I
haven't heard of a laser-ring gyrocompass. Not sure it if that is even
possible.

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darkmighty
The gyrocompass orientation should, in principle, be obtainable by properly
integrating gyroscope readings (and accelerometer? not sure if required).

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hamiltonkibbe
You still need a way of determining the initial position/pose

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darkmighty
I don't think so. The gyrocompass relies on gyroscopic precession. The
precession itself can be obtained by integration, which gives orientation. Of
course, the precision of this process may well be very low or unusable.

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hamiltonkibbe
you don't need an initial heading measurement but you still need to know where
you are

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darkmighty
It would suffice to know the radius of the planet and it's rate of rotation.

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hamiltonkibbe
Using only a compass and the knowledge that you were somewhere on the planet
earth, could you determine the shortest route from your locations to Chicago?

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darkmighty
Ah I thought you were referring to the gyrocompass functionality. Yes, for
general navigation you need an initial position to determine your position
from inertial measurement alone (as I said, you can determine orientation and
latitude, but not longitude).

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GregBuchholz
Celestial navigation plus accurate clocks. I'd think this would be especially
good on a planet without clouds (as long as your planet rotates fast enough,
so I guess Mercury is out). Anyone know if Mars has any polar stars?

[https://en.wikipedia.org/wiki/Celestial_navigation](https://en.wikipedia.org/wiki/Celestial_navigation)

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ars
The planet doesn't have to rotate to use celestial navigation. As long as you
can see the stars you are fine.

So the dark side of mercury would work fine. On the hot side as long as you
are not exactly on the equator the sun acts as a fixed direction and you can
use that.

> Anyone know if Mars has any polar stars?

The orbital tilt of Mars is 1.85 degrees (vs 0 for earth) and the Axial tilt
is 25.19 vs 23.44 for Earth. So Polaris would be pretty close to being a polar
star for Mars as well.

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vilmosi
That's still like a couple of degrees (4 full moons) away so not sure how
accurate it would be.

Still the thought of seeing literally the same stars on an alien world is
blowing my mind. Just imagine the same sky, but different planet... wow

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DanBC
I wonder if "giant head" enhanced depth perception works across planets?

[https://xkcd.com/941/](https://xkcd.com/941/)

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escherplex
Fun link. I was planning on similar setup using a couple of remote linked
Celestron C8s separated by 5 miles to observe thunderhead cloud formation in
SW Florida. Average natural IPD is around 64mm so it should be interesting to
see what the occipital lobe does when this is expanded to over 8km. (IPD =
InterPupillary Distance; eyeball separation)

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CapitalistCartr
IPD?

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Caer
Interpupillary distance, the distance between the centre of the pupil in one
eye to the other.

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stephengillie
Venus too - and this is also why neither have water - no strong magnetic field
to hold the heliopause at bay, so the solar wind blows water out of the
atmosphere.

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adekok
Mars doesn't have hydrogen because at temperatures where water is liquid, the
speed of hydrogen atoms due to temperature is greater than the escape velocity
from Mars.

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stephengillie
I thought it was just pressure - the more dense molecules push the Hydrogen
out of the way as gravity pulls at them.

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adekok
Pressure is the collective action of the molecules / atoms flying around and
smashing into things.

At "normal" temperature on Mars, hydrogen atoms have a thermal velocity [1].
That velocity is greater than the escape velocity from Mars.

A lone hydrogen atom will typically bounce around a lot in the lower
atmosphere. It eventually works it's way (via random scattering) to the upper
layers of the atmosphere. Once the atom reaches the upper atmosphere... it's
gone. It flies away, never to return.

Keep that up for a billion years, and Mars loses most of the hydrogen it
started off with.

Oxygen is heaver, so it's thermal velocity is smaller than the escape
velocity.

[1]
[https://en.wikipedia.org/wiki/Thermal_velocity](https://en.wikipedia.org/wiki/Thermal_velocity)

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JoeAltmaier
So the Mars atmosphere is a distillation column for gasses!

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lmm
Every atmosphere is a distillation column for gasses. Oxygen and Nitrogen are
similar enough that they don't separate (also Earth's atmosphere moves enough
to mix), but you definitely find that e.g. Radon will sink to the floor, and
lighter gasses like Helium will float up to high altitude.

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hacker234
Does having "no strong magnetic field" mean no Ionosphere and thus no
Shortwave Radio?

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batou
Interesting thought. Never considered that you have planet specific
propagation.

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kefka
Well, depending on frequency you would still have some traversable effects
regarding radio. You would have ground wave propagation at the minimum.
unfortunately, most radio would be line of sight because of no atmosphere.
Limited atmospherics would provide very restricted use on a multitude of
frequencies mainly lower than 6 meter. If you wanted to communicate with the
other side of the body, you would have to rely on a third entity like we do on
here with the moon to bounce and receive. Aside from that option you would
have little/no multi bounce

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batou
Very interesting. Thanks for the detailed reply.

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aaron695
KRET develops stellar navigation system for Russian strategic bombers

[http://www.janes.com/article/52661/kret-develops-stellar-
nav...](http://www.janes.com/article/52661/kret-develops-stellar-navigation-
system-for-russian-strategic-bombers)

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contingencies
I am seeing a cartoon forming in my head with a Russian strategic bomber
pilot, crashed in a kid's bedroom and looking up at a roof covered in those
glow in the dark stars and a big red flashing light warning about course
deviation.

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hybridwebtech
Already done? More than 40 years ago, but probably still workable:

[https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle#Control_a...](https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle#Control_and_navigation)

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stephentmcm
Did you read the article? That's exactly what the author was pointing out.
Also this article and wiki have ruined The Martian for me.

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cc438
Gyroscopes accumulate error with time, they make sense for an application with
a short range but they're useless over long periods of time without a
reference standard to crosscheck and recalibrate. That's one part of The
Martian the author got entirely correct despite the criticism. The mechanical
odometer in your car is only accurate to +/-3% of reading and the best
mechanical calibration standards are only 0.25% of reading, that puts the trip
described in The Martian off by ~90km.

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stephentmcm
Thanks, I was being mildly sarcastic in my post but having the math laid out
is great.

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cc438
NASA has the brains to design a better odometer but navigating by stars and
GPS makes more sense.

Also, remember the +/-3% next time you're comparing a used cars. Worrying
about 5-6,000 miles on a car with 100,000 is pointless since the odometer
itself lies by up to 3,000 miles. Then you have further error introduced by
new sets of tires which wear down 6-15mm depending on their tread depth and
can vary several mm in diameter despite having the same nominal size as the
OEM tires.

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adaml_623
stellar inertial navigation - It means that as well as an inertial system you
track the stars... which are actually visible even during daylight through an
automated telecope system... although not through cloud I guess.

Pricey but so is getting to Mars

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Florin_Andrei
[http://nmp.nasa.gov/st6/TECHNOLOGY/compass_tech.html](http://nmp.nasa.gov/st6/TECHNOLOGY/compass_tech.html)

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JoeAltmaier
GPS

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FreeFull
We haven't put GPS satellites around Mars quite yet. I'm guessing that won't
happen until it gets terraformed.

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grecy
That being said, how many satellites _are_ orbiting Mars right now that could
be used to help determine position?

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comrh
~7

[https://en.wikipedia.org/wiki/Category:Artificial_satellites...](https://en.wikipedia.org/wiki/Category:Artificial_satellites_orbiting_Mars)

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lmm
Looks like 5 of those are active rather than proposed.

