LOFAR is among the first radio telescopes in which RFI mitigation forms an integral part of the system design. This is needed, as LOFAR will operate in a hostile RFI environment. Unlike the majority of present-day radio telescopes, the signals at the input of the LOFAR ADCs can no longer be characterized predominantly as white noise. RFI mitigation in radio astronomy is a relative new field, and many RFI mitigation methods are not yet "fully proven." However, many techniques have been extensively demonstrated on prototype systems.

The overall LOFAR approach towards RFI mitigation is to rely in the first place on such well-proven techniques, but to provide both fall-back techniques and options to study new and promising approaches. It is therefore important to ensure that the LOFAR system is designed in such a way that future, more complex RFI mitigation algorithms can be applied without redesigning parts of LOFAR. LOFAR also offers a unique opportunity to develop and demonstrate new techniques for application in future instruments, in particular the Square Kilometer Array.

Calibration
The development of a robust calibration system for LOFAR is among the most challenging and high-risk tasks within the project. Without such a system, the instrument cannot hope to perform much better than, for example, the VLA 74 MHz system. In fact the 74 MHz calibration strategy employed at the VLA (using a Zernicke polynomial representation of ionospheric refraction) assumes a stable antenna primary beam pattern, and since this assumption is grossly violated by stations consisting of fixed dipole arrays, such a strategy cannot be used for LOFAR without modification. The fact that the pioneering VLA system has successfully enabled the use of baselines longer than a few km is by no means a proof of concept for the calibratability of LOFAR, even when considering only the shorter baselines corresponding to VLA scales.

Instead, what is required is a much more rigorous approach to calibration, in order to correct for severe position-dependence of complex station-dependent gain, and to achieve dynamic ranges and residual sidelobe confusion levels commensurate with the high sensitivities enabled by the rest of the LOFAR design.

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