Radio astronomical observations with connected element interferometers and very long baseline interferometers are affected by atmospheric (i.e. tropospheric and ionospheric) refraction. If both horizontal and vertical gradients in the ionospheric electron density distribution are combined, it is possible to correct for ionospheric refraction and its variations. Interferometer phase appears to be more sensitive to ionospheric irregularities than, for example, Faraday rotation. Observations made with the Westerbork synthesis radio telescope have been analyzed to improve knowledge of the climatology of ionospheric irregularities. These observations have been combined with differential Doppler measurements using NNSS satellite signals to determine spatial parameters of medium-scale traveling ionospheric disturbances.