Direction Dependent Self Calibration of Large Distributed Sensor Arrays

The LOFAR astronomical array currently under development in the Netherlands produce synthesis images of the most distant (and thus oldest) space objects by observing at unusually low frequencies (30-250 MHz) over a large aperture (100 km) using many antennas (on the order of 10,000). This presents some significant challenges for sensor calibration because at these frequencies Earth\´s ionosphere acts as a random refractive sheet which over the large aperture induces source direction dependent gain and phase errors that must be estimated and calibrated out. Existing array self calibration algorithms used at higher frequencies by other radio astronomy instruments such as the very large array (VLA) do not address direction dependence and does not work in the LOFAR environment. A new algorithm called "demixed peeling" is presented and assessed as a solution to the direction dependent calibration problem for large distributed sensor arrays