Ionospheric drift measurements with ionosondes

In the last forty years many attempts have been made to measure ionospheric plasma motions with HF radio sounders, generally specialized ionosondes. Starting in the 1950's, the so-called similar fading and the correlation methods were applied to estimate the velocities in the ionosphere. More recently, the interferometric Doppler technique was introduced which successfully measured ionospheric drifts that were verified by incoherent scatter radar measurements. The latter technique is discussed in some detail in this paper