Doppler radars operating in the frequency band between 40 MHz and 55 MHz are used for the investigation of various dynamic processes in the middle and lower atmosphere. Their capability to detect coherent backscattered signals from altitudes starting around 1 km up to 100 km lead to the name MST radar, standing for mesosphere/stratosphere/troposphere. MST radars allow unattended and continues operation in the height range of about 1 km up to 20 km all year. 3-D wind vectors, radar reflectivity and turbulence can be derived from the received backscatter signals using the Doppler Beam Swinging (DBS) method as well as interferometry. However radar signatures can also be detected by MST radars in the mesospheric region during summer and winter months but under special conditions. The study of these so-called (polar) mesospheric echoes is one of the main applications of the MST radars of IAP, the OSWIN radar in Kühlungsborn (54.12°N; 11.77°E) and MAARSY on the North-Norwegian island Andøya (69.30°N; 16.04°E).