Concept design for a low frequency array

The LOw Frequency ARray (LOFAR) is the Digital Software Radio Telescope that has been under study by the NFRA and the NRL for the last two years. A scaleable configuration is presented for an aperture synthesis array operating in the frequency range from the ionospheric cut off at about 10 MHz to about 160 MHz where existing telescopes are confusion limited. Both the array and the antenna station have a fractal structure following an exponential scaling law, which by appropriate weighting can provide a field of view and a synthesised resolution pattern of almost constant angular size independent of frequency. The processing architecture is scaleable as well and allows optimum distribution of the total processing power over signal and data processing tasks, by trading processed bandwidth in beam-formers and correlators for advanced processing like interference rejection, multi-beaming, pulsar processing, decade-wide chirp processing. The phased array antenna station provides a set of beams, which extends about a steradian on the sky and can be configured such that all relevant science objects are covered simultaneously. The pipelined self-calibration processing provides a clean image every third of a day, improving the sensitivity by a factor eight after two month of repeated observing. Based on the fast expansion of high performance processing technology, it is only after 2003 that signal and data processing will no longer dominate the cost of LOFAR in producing a confusion limited sky survey at sub mJy level.