MST radar
In the troposphere and lower stratosphere (up to about 30 km) the radar signal is returned from refractive index fluctuations produced by turbulence in the neutral atmosphere (see clear-air echo). In the upper stratosphere and lower mesosphere (between about 60 and 100 km) refractive index variations are strengthened by the strong vertical gradient in electron density. Because the scale size of the refractive index fluctuations must be of the order of one-half the radar wavelength (Bragg scattering), and the minimum scale size of turbulence increases with height from a few centimeters in the lower troposphere to several meters in the upper stratosphere and lower mesosphere, most MST radars have operated in the VHF band (typically 30–60 MHz, 5–10-m wavelengths). Very sensitive UHF radars can detect echoes from incoherent scattering (thermal or Thomson scatter) by electrons above 60 km. MST radars are characterized by high-powered transmitters and large antennas. (VHF antennas range from 100 to 300 m across.) Similar wind profilers that lack the transmitter power and antenna area to detect returns from the upper stratosphere and the mesosphere have often been called ST (stratosphere–troposphere) radars.
