ABSTRACT

Clear-air radar refers to ground-based active remote-sensing instrumentation transmitting radio waves upward across the atmosphere at wavelengths allowing measurements of wind and atmospheric reflectivity at any time (day or night) and whatever the meteorological conditions. It is not to be confused with meteorological or weather radars devoted to precipitation and hydrometeor measurements and with airborne or satellite radars. The clear-air radar technique is also sometimes referred to as the MST (mesosphere, stratosphere, troposphere)

radar technique according to its range of measurements, which depends on configuration parameters such as the frequency, the transmitted power, and the antenna surface. The technique uses the weak backscattering from refractive-index fluctuations at radio wave frequencies due to atmospheric turbulence and temperature/humidity  sheets.  The  atmospheric  density,  and  thus  the refractive-index fluctuations, quasi-exponentially decrease with increasing altitude, so that the maximum altitude range for detecting atmospheric echoes is 20-25 km. Above about 60 km, in the ionosphere, backscattering arises due to the presence of ionized particles and electrons that enhance the scattering from turbulence.