Frequency residuals retrieved from open-loop data from Ur and Sh compared to the residuals from closed-loop data from NNO, during occultation ingress in the session of 2012.04.27. Panel a: frequency residuals of the detected signal up to LOS. Panel b: zooms in the frequency residuals showing when the signal starts getting refracted by Venus’ ionosphere at around 19605 s. Panels c and d: corresponding altitude probed versus the frequency residuals of the data shown in panels a and b, respectively. Panel e: corresponding electron density profile, where the secondary V1 layer and main V2 layer of Venus’ ionosphere are identified.