The LOFAR correlator

Abstract

LOFAR is the first of a new generation of radio telescopes.Rather than using expensive dishes, it forms a distributed sensor network that combines the signals from many thousands of simple antennas. Its revolutionary design allows observations in a frequency range that has hardly been studied before. Another novel feature of LOFAR is the elaborate use of software to process data, where traditional telescopes use customized hardware. This dramatically increases flexibility and substantially reduces costs, but the high processing and bandwidth requirements compel the use of a supercomputer. The antenna signals are centrally combined, filtered, optionally beam-formed, and correlated by an IBM Blue Gene/P. This paper describes the implementation of the so-called correlator. To meet the real-time requirements, the application is highly optimized, and reaches exceptionally high computational and I/O efficiencies. Additionally, we study the scalability of the system, and show that it scales well beyond the requirements. The optimizations allows us to use only half the planned amount of resources, and process 50% more telescope data, significantly improving the effectiveness of the entire telescope.