Advances in orbital theory have also kept up with
these increasing accuracy demands, due to the
progress made in manipulating complex algebraic
problems by computers. A recent example is the
development of an orbital theory accurate to the
third power of the earth's flattening [30]. Suitable
satellites, which reduce the non-gravitational per
turbations and which enable the measurements to
be reduced to a well defined mass center will also
be necessary in the future. Such a satellite is now
being planned by the Smithsonian Astrophysical
Observatory [31] for observing accurately the
motions of the earth about its center of mass.
1 hope that 1 have succeeded in some of the ob
jectives I set out in the introduction and that the
choice of paths at the cross-roads is more clear for
the specific problem discussed here. We have to
develop precise laser tracking stations, well distri
buted around the world, and we have to study some
of the proposed new complementary techniques.
On the theoretical side we have to investigate the
optimum methods for using this new data; we have
to improve our orbital theories and the represen
tation of the geopotential. Finally we have to con
tinue work on the interpretation of the results ob
tained and to investigate their relationship to other
geophysical quantities.
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