  The joint NASA-ESA mission LISA relies crucially on the stability of the
three spacecraft constellation. Each of the spacecraft is in heliocentric
orbits forming a stable triangle. The principles of such a formation flight
have been formulated long ago and analysis performed, but seldom presented if
ever, even to LISA scientists. We nevertheless need these details in order to
carry out theoretical studies on the optical links, simulators etc. In this
article, we present in brief, a model of the LISA constellation, which we
believe will be useful for the LISA community.

General Relativity and Quantum Cosmology;Astrophysics