The low frequency array (LOFAR) is a large distributed radio telescope, consisting of phased array antenna stations that are combined in an aperture synthesis array. Antenna stations consist of many simple, omnidirectional antennas. Flexible station-based signal processing allows for trading bandwidth against instantaneous sky coverage. Central processing implements a software correlator, which can be reconfigured as a full tied array beamformer, and online calibration functions to handle the large data streams produced by the system. The key science programs for LOFAR challenge the technical specifications in several directions, which resulted in a highly reconfigurable architecture. This paper describes the LOFAR system design, the configuration, and the signal-processing chain. LOFAR has been developed by ASTRON and a consortium of universities and industrial partners. The instrument is currently being deployed in The Netherlands. Additional stations are being built in several other European countries. The telescope is considered an important pathfinder for the square kilometer array (SKA) in demonstrating the potential of (sparse) aperture arrays, in developing solutions to major calibration issues that are directly applicable to the SKA, and in paving the way for the mass-production and operations of such large distributed radio telescope systems.

The Low Frequency Array (LOFAR) opens a previously largely unexplored frequency domain for challenging radio-astronomical research. At the 10 to 240 MHz operating frequencies of this radio telescope, it is feasible to employ very large numbers of simple, all-sky antennas with wide-band early digitization. This means that almost the full signal processing chain can be realized in (embedded) software. This approach makes it possible to deal with Earth-based radio signals in effective and novel ways. The signal processing challenges in LOFAR are manifold, since the ultimate dynamic range in astronomical images depends on the quality of the full chain of operations that combines ten-thousands of antenna signals into a single multichannel image cube, while correcting for a large variety of instrumental and environmental effects.