Complementary Code and Digital Filtering for Detection of Weak VHF Radar Signals from the Mesosphere

Measurements of the structure and the dynamics of the middle atmosphere with a fine height resolution have been carried out using the SOUSY-VHF-Radar (SOUSY = SOUnding SYstem), operated by the Max-Planck-Institute for Aeronomy in the Harz mountains in Germany. Since the echoes from the middle atmosphere are coherent within a time scale of the order of a second, the received signals are over-sampled and added coherently for that time period. This integration, which is equivalent to a digital combfiltering, improves the signal-to-noise ratio. It is carried out by a hardware adder. In order to detect echoes from the mesosphere, an increase in the effective radiated power is necessary. By applying a 32-element complementary code to long transmitted pulses, the average power is increased and radar returns from the height range between about 65 and 90 km have been recorded with a height resolution of 300 m. The special hardware and software used is described. Some observational results from mesospheric measurements are presented.