A continuous effort has been undertaken at ONERA over more than 30 years to develop non intrusive methods to characterize various kinds of flows: subsonic to supersonic aerodynamic flows, combustion and reactive flows, hypersonic flows. The paper is giving an overview of the methods dedicated to measure the velocity; they are mainly particle scattering based for aerodynamic flows (such as LDV, PIV or DGV), but for reacting and hypersonic flows the molecular scattering based methods avoid the particle lag problem (such as CARS velocimetry, diode laser absorption spectroscopy or electron beam fluorescence).
Alain Boutier, Alain Boutier,
"Overview of laser velocimetry techniques developed at ONERA", Proc. SPIE 4448, Optical Diagnostics for Fluids, Solids, and Combustion, (26 November 2001); doi: 10.1117/12.449401; https://doi.org/10.1117/12.449401