Insight into the plasma properties of high-speed ionized flows is key for the validation of predictive aerothermodynamic tools for atmospheric entry applications, namely plasma radiation which will be highly dependent on the flow electron densities, which accordingly need to be accurately estimated. We performed an extensive review on past re-entry microwave diagnostics, including both in-flight and ground-test experiments. This work assesses the peculiar characteristics of entry plasmas, and derives functional requirements for microwave diagnostics tailored for such applications. A preliminary analysis, including a simulation of electromagnetic environment for the RAM-C II flight experiment, obtained through a CFD code coupled to an electromagnetic propagation code, is presented. The obtained results outline the adequacy of such diagnostics for improving verification and validation for high-speed entry plasma flows.