Note: An Advanced GG Phase A Study has been funded by ASI in 2001-2002 in order to assess the feasibility of the experiment with a spacecraft in sun-synchronous high incliantion orbit, for which the high latitude Russian launcher DNEPR can be used at a favourable cost. The GG error budget in sun-synchronous orbit has been proved to be compatible with the GG target of testing the Equivalence Principle to 1 part in 1017 (see paper published on Physics Letters A)