  In this paper we study the approximation of an optimal control problem for
linear para\-bolic PDEs with model order reduction based on Proper Orthogonal
Decomposition (POD-MOR). POD-MOR is a Galerkin approach where the basis
functions are obtained upon information contained in time snapshots of the
parabolic PDE related to given input data. In the present work we show that for
POD-MOR in optimal control of parabolic equations it is important to have
knowledge about the controlled system at the right time instances. We propose
to determine the time instances (snapshot locations) by an a-posteriori error
control concept. This method is based on a reformulation of the optimality
system of the underlying optimal control problem as a second order in time and
fourth order in space elliptic system which is approximated by a space-time
finite element method. Finally, we present numerical tests to illustrate our
approach and to show the effectiveness of the method in comparison to existing
approaches.
