Calibratability by design for SKA´s Low Frequency Aperture Array

Early universe science with the Low Frequency Aperture Array (LFAA) Element of the Square Kilometre Array (SKA) requires the ability to produce thermal noise limited images after 1000 hours of integration. In this paper, we summarize the effects that determine the noise floor in such deep images and sketch the calibration strategy required to track variations in (ionospheric) propagation conditions and instrumental effects over an observation. Defining calibratability as the ability to produce deep thermal noise limited images, we show that calibratability imposes a number of constraints on the design of the array. We conclude that an array of small stations needs to meet a certain density of stations over the area covered by the array and that the antenna configuration within the individual stations should either have different randomizations or be rotated copies of the same randomized configuration.