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Constraining ionospheric models with radio interferometric self-calibration data

Radio interferometers can measure the differential total electron content (TEC) between pairs of lines-of-sight through the ionosphere above the interferometric array very accurately. Such data potentially forms a nice complement to the absolute TEC measurements from GPS receiver networks that usually provide data with lower spatial sampling density and lower accuracy. This paper presents an overview of currently used ionospheric calibration schemes in radio astronomy. For each method, I describe how the data takes care of the 3-D structure of the ionosphere, quantify the accuracy of the differential TEC measurements and discuss the spatial sampling density.