From: john@stoffel.org   
      
   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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