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We report some initial results of simulation and data analysis of a radio-frequency plasma impedance probe flown on sounding rockets. This instrument is used to measure the absolute electron density and electron-neutral collision frequency at ionospheric altitudes. A plasma-fluid finite-difference time-domain (PF-FDTD) simulation is used in conjunction with simpler analytical formulas by Balmain to match the measurements to theory. The effects of an inhomogeneous conical profile or a warm plasma on the impedance are then studied using an improved version of the PF-FDTD model. The effect of spacecraft plasma wake on impedance measurements is qualitatively analyzed using the conical approximation. In particular, a model of the electron wake structure is created for the Sudden Atom Layer (SAL) sounding rocket mission. The spin modulation observed in the electron density measurements during the SAL mission is analyzed. We use the fluid simulation results to suggest corrections to electron density and electron-neutral collision frequency estimates made during sounding rocket missions.