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An image processing technique was designed to be utilized in combination with a Fabry-Perot interferometer for time-resolved velocity measurements in gas flows. The method was employed to obtain the Doppler shift in Mie or Rayleigh scattered light from the measurement volume. The technique minimizes the unwanted effects due to the drifting of laser frequency and interferometer alignment. The scattered light is mixed with unshifted light from the interrogation laser for reference before it is passed through the interferometer.

An experimental study revealed the velocities of atomic hydrogen and helium in an arcjet thruster to be similar, thereby implying velocity measurements near the exit plane to be representative of the average mass velocity. This suggests that species specific velocity-measurement strategies are versatile in quantifying velocity measures of the related species near the exit plane.

Oxygen laser-induced fluorescence was studied experimentally to quantify significant error and uncertainty sources and to evaluate the technique feasibility for gas temperature measurements in the 1-6-atm, 300-500-K regime. The dominant uncertainty source is quantum noise. For low-speed flows, the dominant error source is nonlinear process errors originating from oxygen ion fluorescence and saturation. For flows where the pressure is unknown or variable, collisional quenching may introduce an additional error up to 3 K/atm. For high-speed flows above 300 K, the dominant error source is the slow vibrational relaxation time of oxygen molecules. The optimum experimental conditions are determined by budgeting the allowable temperature inaccuracy to various uncertainty and error sources. In general, the nonlinear process errors limit the laser fluence that may be used, and the quantum noise determines the minimum number of laser shots per measurement.