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Space exploration from a shirtsleeve environment



 
 
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  #21  
Old October 7th 05, 09:23 AM
Alex Terrell
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Henry Spencer wrote:
In article ,
Alan Anderson wrote:
That gives you attitude fixes, but it doesn't determine location on the
Moon unless you either have some closer reference (like Earth) as well,
or can measure the direction of the local vertical accurately.


Local vertical is easy: it's *down*.


It's easy only if your sensor is absolutely motionless during the
measurement period. This is not that easy to achieve aboard vehicles,
which are one of the major applications of such navigation devices.

Also, when you start wanting to get seriously accurate, you need a gravity
model of the Moon to determine the relationship between (so to speak)
local gravity vertical and local geometric vertical.


I think for lunar vehicles, a combination of inertial gyros, distance
measurement, and terrain contour mapping will be fine and cheap.

The Terrain Contour Mapping will be easy for a powerful enough computer
and some good optics.

An alternative is you shine a light to L1, and the L1 station tells you
where you are.

  #22  
Old October 7th 05, 06:10 PM
John Schilling
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In article , Alan Anderson
says...

(Henry Spencer) wrote:


John Halpenny wrote:
There is also a big black sky full of stars, so a set of automatic star
trackers could provide a good navigation system.


That gives you attitude fixes, but it doesn't determine location on the
Moon unless you either have some closer reference (like Earth) as well,
or can measure the direction of the local vertical accurately.


Local vertical is easy: it's *down*.


Well-calibrated accelerometers are one option, but even a reasonably
simple pendulum will tell you which way gravity is pulling. A pendulum
with a lit point on it can even use the star tracker to read its
orientation.



Measuring "down", to three significant figures, from a moving vehicle
(or even an only-temporarily-not-moving vehicle), is actually not that
easy to do. And if you can only get two significant figures, your
position fix on the lunar surface will likely be in error by farther
than you can walk before the oxygen runs out.


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