A fundamental uncertainty in velocity measurement by Doppler scattering is caused by the finite residence time of the scattering particles in the observation volume; the arrival of scattering particles at arbitrary times gives rise to fluctuations in phase (and hence frequency) of the observed Doppler frequency. An estimate is obtained for the spectrum of these frequency fluctuations (called ambiguity noise). The frequency at which the spectral levels of a turbulent signal and the ambiguity noise are equal, provides a limit to the temporal resolution of an instantaneous velocity measurement; this limit is obtained, and shown to be quite restrictive. The influence of electronic noise is also analyzed and found to be negligible.
An experimental installation is described in which instantaneous fluctuating turbulent velocities may be measured by Doppler scattering using coherent radiation from a laser. Measurements are presented of the spectra of ambiguity noise end electronic noise. The agreement with theory is excellent
