Active adaptive arrays: The ASTRON approach to SKA

It has been argued, for a number of reasons, that the next generation radio telescope should be a multi-element interferometer with a collecting area of about 1 km(2). The remaining parameters of such an instrument - frequenc y range, angular resolution, instantaneous bandwidth, etc. - will be scienc e driven. The requirements for propagation studies are briefly discussed, a nd it is pointed out how variable-source confusion may differ from the norm al variety. Finally, the Dutch project to achieve a large collecting area u sing adaptive arrays of active antennas is described. A systematic approach has been adopted, with the construction of arrays of increasing complexity to test design features at each state. Recently, development of a low freq uency array (LOFAR) has become an additional option. It would facilitate te sts of some of the larger instrument's features, and provide real data on t he influence of the ionosphere and interfering sources.