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Posted by John Stoffel on January 27, 2006, 10:37 am
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I was wondering if anyone could explain how the new proposed NASA moon
probe will actually work when it's using it's laser (split into five
beams) to map the surface. Don't you need to have a known reference
point to get good measurements on the height?
Since the orbiter will be in a low orbit, won't the lumpy mascons in
the moon throw off the orbit, causing the measurements to vary?
Or will they just assign a zero datum to a point on the moon, and use
that as the basis of all future measurements? So it would go
something like this:
- orbit over point zero, measure height.
- as the orbit progresses, measure the height every 1 second.
- each measurement is of both the previous point, and a new point,
so that you have a known offset, irrespective of the actual
orbital height of the orbiter.
Then, once you've got enough data from enough orbits, you could start
integrating it all, using the zero datum as your reference point and
just crank through all the numbers for any arbitrary point?
Would you also setup sub-sidiary points where you could re-calibrate
and make sure you have some known points, and confirm that your orbit
is what you expect, etc?
Or would they be taking orbital measurements based on the doppler
measurements from a signal beamed from earth to act as a baseline
somehow?
I'm sorta just using the surveying technique I've read about where you
pick a zero datum, and just do all your measurements from there, with
cross-checking to make sure your errors don't add up too badly.
One thing I don't understand is how they get a reliable set of
measurements from an orbiter which can be affected by mascons.
Thanks,
John
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Posted by Mark Adler on January 27, 2006, 9:06 pm
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John Stoffel wrote:
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> Since the orbiter will be in a low orbit, won't the lumpy mascons in
> the moon throw off the orbit, causing the measurements to vary?
Yes. That's why that orbiter has to also use very frequent precision
radio tracking to both determine its position relative to the center of
the Moon and generate a very good gravity map to go along with those
altitude measurements. After all the processing, they will have both a
good map of the surface topography in distances from the center of
gravity of the moon, I'm guessing to the tune of meters accuracy, as
well as a good gravitational equipotential map that they will use to
define a standard geoid (lunoid?) from which to reference the radius
measurements and define altitudes relative to that reference.
mark
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Posted by Ian Stirling on January 28, 2006, 6:20 am
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>
> I was wondering if anyone could explain how the new proposed NASA moon
> probe will actually work when it's using it's laser (split into five
> beams) to map the surface. Don't you need to have a known reference
> point to get good measurements on the height?
show/hide quoted text
<snip>
> One thing I don't understand is how they get a reliable set of
> measurements from an orbiter which can be affected by mascons.
It's 'simple'.
You first take your best model of the masscons you have.
Now, map the surface while adding in doppler information and your best
guess of solar wind pressure, light pressure, ....
Now, watch it as it orbits at various altitudes and orientations.
An orbiter passing through exactly the same orbital path will of course
be affected the same, and will give no masscon mapping info.
However, if you go slightly lower, then a bit downrange of the masscon,
you're going to notice a divergance.
As a first cut, you might try adding divergance from expected models
1/2 orbit later for all orbits passing over each point on the moon.
Then, at big unknown masscons, you're going to get a peak.
If you've got a big enough dataset, then it reduces to merely a horribly
complex computing problem.
The mapping is not aiming to detect real-time orbital variations, which
would be very, very tricky, but to put together a map of the moon, so
that divergances from the expected orbit can be measured better.
Eventually, with increasing knowledge, you should be able to 'surf' the
masscons, rather than having to power your way through using massive
amounts of fuel.
Much like you might, with adequate knowledge, keep a ball sort-of on a
given track over lumpy terrain, instead of simply reacting when it gets
off track, you tap it slightly, so that the upcoming terrain keeps it on
track for you.
versus
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Posted by j.halpenny@rogers.com on January 28, 2006, 9:33 pm
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Ian Stirling wrote:
show/hide quoted text
> > I was wondering if anyone could explain how the new proposed NASA moon
> > probe will actually work when it's using it's laser (split into five
> > beams) to map the surface. Don't you need to have a known reference
> > point to get good measurements on the height?
> > One thing I don't understand is how they get a reliable set of
> > measurements from an orbiter which can be affected by mascons.
> It's 'simple'.
> You first take your best model of the masscons you have.
> Now, map the surface while adding in doppler information and your best
> guess of solar wind pressure, light pressure, ....
By tracking the satellite as carefully as possible as it passes low
over the moon, you can 'map' the gravity field. This allows you to
predict future orbits, but more importantly maps the density of the
underlying rocks. It appears that below the top layer of dust, the rock
density and type varies dramatically from plase to place.
John Halpenny
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> the moon throw off the orbit, causing the measurements to vary?