>(2) How do you propose to calculate the coordinates
>of your observation point on the earth from latitude,
>longitude and elevation above sealevel?  I guess
>a database is required.  Or does one assume that
>"sealevel" is a perfect oblate spheroid? 
>
>Cheers
>
>Laurent S

Laurent S:

Sea level is actually an expression of the mass of the Earth. The 
overall mathematical generalized formula to describe the Earth is an 
oblate spheroid. However, this is a generalized formula and not the 
actual shape.

Think of the Earth like a squashed ball with irregularities about 
it's surface. There are areas in the ocean where sea level is 
actually "higher" and "lower" than the established "sea level". This 
is due to the pull of gravity which varies over the surface of (and 
within) the planet AND changes depending upon the influence of the 
moon, sun and even other planets (although much less).

What I find interesting about this satellite problem is that altitude 
of the satellite depends upon what the pull of gravity is at any 
specific location. Thus, the altitude of the satellite will change 
somewhat to accommodate variations in the Earth's surface -- with 
minor delays. So, this "footprint" thing may be a wash with respect 
to altitude.

tedd

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