The application of the phase Doppler technique for characterizing coaxial rocket injector sprays is a challenging problem. In order to obtain reliable measurements in the high density, high pressure, and high velocity environments that are typical of liquid rocket engines, many issues need to be satisfactorily addressed. The high particle number density environment gives rise to the problems of trajectory dependent scattering and coherent scattering by multiple particles. Also, the dense spray gives rise to multiple scattering effects which cause the Doppler signals to be very noisy. This, coupled with the fact that the particles are moving very fast, imposes exceptional performance requirements from the signal processor which should be able to accurately and reliably measure the frequency and phase of Doppler signals having low signal-to-noise ratios (SNR), high frequencies, and short transit times. These issues have been addressed in the present research. Also, the spray characteristics of a scaled-down version of a coaxial rocket injector has been studied at elevated chamber pressures using a two-component phase Doppler particle analyzer (PDPA) along with a frequency domain signal processor, namely the Doppler Signal Analyzer (DSA). The validity of the number density measurements obtained with the phase Doppler instrument in a coaxial injector spray has been verified by using the classical light extinction technique in conjunction with the Abel transformation technique. By varying the chamber pressure and the liquid-to-air mass flow ratio, it has been possible to decouple and quantify the individual effects of chamber gas density and the shear air velocity on the liquid atomization process.