A device tested in the late 1950s and 1960s that used acoustic waves to create clear-air targets for a continuous-wave radar. The acoustic source produces sound waves that perturb the atmospheric refractive index. The perturbations propagate at the local speed of sound and serve as targets for the radar. When the acoustic wavelength is matched to half the radar wavelength, a resonance condition develops that leads to the production of a detectable radar echo (see Bragg scattering). The Doppler shift of this radar echo is a function of the virtual temperature and the wind component along the direction of the radar beam. Radio acoustic sounding systems (RASS) also use this principle, but with a vertically pointed beam so that the Doppler shift depends on virtual temperature and the vertical wind component. The vertical wind component in RASS is removed either by time averaging or by making independent measurements of the wind. The EMAC probe was designed to make wind measurements with an operating range of 400 m. The virtual temperature (and corresponding speed of sound) was assumed, based on an independent measurement at the radar site.