  Electromagnetic (EM) vector sensor arrays can track both the polarisation and
direction of arrival (DOA) of the impinging signals. For linear crossed-dipole
arrays, as shown by our analysis, due to inherent limitation of the structure,
it can only track one DOA parameter and two polarisation parameters. For full
four-dimensional (4-D, 2 DOA and 2 polarization parameters) estimation, we
could extend the linear crossed-dipole array to the planar case. In this paper,
instead of extending the array geometry, we replace the crossed-dipoles by
tripoles and construct a linear tripole array. It is proved that such a
structure can estimate the 2-D DOA and 2-D polarisation information effectively
in general and a dimension-reduction based MUSIC algorithm is developed so that
the 4-D estimation problem can be simplified into two separate 2-D estimation
problems, significantly reducing the computational complexity of the solution.
The Cramr-Rao Bound (CRB) is also derived as a reference for algorithm
performance. A brief comparison between the planar crossed-dipole array and the
linear tripole array is performed at last, showing that although the planar
structure has a better performance, it is achieved at the cost of increased
physical size.
