Matched filtering for the measurement of conjugately ducted VLF transmissions

The application of matched filtering to the problem of very low frequency ( VLF) radio signals ducted between magnetically conjugate points on the Eart h by ionospheric plasma structures has been examined theoretically and impl emented in recent field experiments. With accurate knowledge of the source VLF transmission the subionospheric component of the received waveform can be effectively removed, and the characteristics of the remaining ducted sig nal can be accurately estimated. Although the technique has been shown not to produce strictly consistent estimates of the subionospheric and ducted s ignal amplitudes, it is able to efficiently recover the transit delays and amplitudes of ducted man-made whistler mode emissions for reasonable values of the relevant signal-to-noise ratios. This is substantiated by the use o f matched filtering to analyze data collected by radio receivers placed at the conjugate ends of a magnetic field line, thereby recovering the amplitu des and transit delays of ducted signals with time resolution as small as 3 0 s. An arbitrary degree of accuracy can be obtained in these measurements through the use of bit stream extraction and comparison. Further correlatio n analysis can be performed to extract Doppler shifts due to drifting ducts , and the small time resolution makes possible studies of ducting under rap idly changing conditions.