  The azimuthally symmetric WASA detector and the polarised proton beam of
COSY, enable investigations of energy and angular dependence of the beam
analysing power in the pp --> ppeta reaction. The aim of the studies is the
determination of the partial wave contributions via interference terms which
are inaccessible from spin averaged observables. The partial wave decomposition
is mandatory for the understanding of the reaction dynamics and for the
determination of the eta-proton interaction, independently of the theoretical
paradigm used.
