Space Forum / Space Flight / January 2006

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