Precisely simultaneous radar and satellite measurements at the altitude of reflection of a strong HF heating wave above the Arecibo Observatory were made on June 7, 1977. Parametric instabilities produce strong enhancements in the plasma line and ion line incoherent scatter radar echoes. These echoes also exhibit periodic deep fading that is attributed to a self-focusing instability. This explanation was confirmed by the in situ observation of electron density fluctuations with peak-to-peak amplitudes reaching at least 3% and a spatial dependence that corresponded closely to the radar fading pattern, at least for irregularity wavelengths ranging from a few hundred meters to a few kilometers. The correspondence implies that the radar fading is associated with the convection of the density irregularities through the radar beam. The radar and satellite observations also provided values for the important parameters of the ambient ionosphere, making possible a quantitative comparison of the data with existing theories of the self-focusing instability. In particular, the agreement with the theory of Cragin et al. (1977) is fairly good.