Study on System of Low Frequency Signal′s Measurement and Extraction of its Characteristic Quantity

The essay mainly studies a system of low frequency signal′s measurement and the extraction of its characteristic quantity,introduces how matlab analyzes and extracts low frequency signal.MATLAB,a visual programming software,designs testing program for mechanic vibration signal,analyzes and identifies vibration signal.The collected data shows the effectiveness of the system.Based on XM121 modules,the testing system collects and analyzes single channel signal.

Extraction of mode attenuation coefficient from low frequency reverberation signal

A method of extracting normal mode attenuation coefficient from low frequency reverberation signal has been proposed.Pseudo-inverse normal mode filtering method is implemented to get single mode reverberation field firstly.Based on the assumption of separability of modal back-scattering matrix,effective back-scattering matrix element can be calculated using single mode average reverberation intensity.Finally,mode attenuation coefficient is extracted by comparing effective back-scattering matrix elements at different ranges.The extracted mode attenuation coefficients are used to predict sound transmission loss at the same experiment area. Results show that the predicted transmission loss agrees well with the measured data.This method avoids the difficult of treating the coupling between bottom scattering attenuation and normal mode propagation attenuation.Research on extraction of mode attenuation coefficient from low frequency reverberation signal is useful for both geoacoustic inversion and rapid underwater environment assessment.

Extraction of Alpha transmitter signals from single-station observations using the direction-finding method

For the applications of Alpha very-low-frequency(VLF) systems consisting of multiple transmitters, determining the origin transmitter station of received signals is a crucial problem. Based on single-station observations, this study develops a directionfinding method to extract Russian Alpha transmitter signals. First, the amplitudes of Alpha signals received in Suizhou City,Hubei Province(31.57°N, 113.32°E) in the east-west(EW) and north-south(NS) directions are obtained by the power spectrum method. The amplitude ratios of signals in the two directions are subsequently adopted to estimate the propagation angles of the signals with respect to the NS direction of the VLF receiver station. Phase ambiguity in our system is eliminated by comparing Alpha signal waveform with that from the VTX transmitter in India(8.39°N, 77.75°E) as a reference station. Finally, we can determine the quadrant where the incoming wave signals are located relative to the receiver and eventually distinguish the exact Alpha transmitters. Based on the direction-finding results, the amplitudes of each Alpha signal are extracted, and their diurnal variation features are analyzed to verify the performance of our method. These results are of great significance for the further study of Alpha signals and VLF long-distance communication.