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Instruments for radio astronomical observations have come a long way. While the first telescopes were based on very large dishes and two-antenna interferometers, current Instruments consist of dozens of steerable dishes, whereas future instruments will be even larger distributed sensor arrays with a hierarchy of phased array elements. For such arrays to provide meaningful output (images), accurate calibration is of critical importance. Calibration must solve for the unknown antenna gains and phases as well as the unknown atmospheric and ionospheric disturbances. Future telescopes will have a large number of elements and a large field of view (FOV). In this case, the parameters are strongly direction-dependent, resulting in a large number of unknown parameters, even if appropriately constrained physical or phenomenological descriptions are used. This makes calibration a daunting parameter-estimation task.