  In this Note, a new approach to spacecraft positioning based on GGT inversion
is presented. The gravity gradient tensor is initially measured in the
gradiometer reference frame (GRF) and then transformed to the Earth-Centered
Earth-Fixed (ECEF) frame via attitude information as well as Earth rotation
parameters. Matrix Eigen-Decomposition is introduced to directly translate GGT
into position based on the fact that the eigenvalues and eigenvectors of GGT
are simplespecific functions of spherical coordinates of the observation
position. without the need of an initial position. Unlike the strategy of
inertial navigation aiding, no prediction or first guess of the spacecraft
position is needed. The method makes use of the J2 gravity model, and is
suitable for space navigation where higher frequency terrain contributions to
the GGT signals can be neglected.
