Development Of VLF Receiver System And Applications

The reception and analysis of Extremely Low Frequency(ELF:300 Hz-3 kHz)and Very Low Frequency(VLF:3-30 kHz)radio waves provide a powerful tool for ionosphere D region remote sounding,lighting locations,and solar activity monitoring.VLF signals considered in this thesis originate from two major sources,i.e.,lighting strikes and artificial emissions.The radiated ELF/VLF radio emissions mostly propagate in the Earth-Ionosphere Waveguide(EIWG)surrounded by the ground or ocean and the ionospheric lower boundary(e.g.D-region),called sferics.Tweek atmospherics is a kind of dispersive sferics,related to the ionospheric D region.In addition,some energy of the ELF/VLF emissions in the whistler mode leaks upward into the magnetosphere.These waves propagate along the geomagnetic field line and can be received in the geomagnetically conjugate region.Artificial VLF transmitters operate at frequencies between 10 and 60 kHz for naval communications.These transmitters typically communicate with the MSK modulation.Demodulation and detection of the amplitude and phase of MSK transmitter signals has been used to explore the spatio-temporal feature of the ionospheric D region.A new digital low-frequency receiver system has been developed at Wuhan University(WHU)since 2014 for detecting natural and artificial ELF/VLF emissions.The system has been set up in the mid and low latitude region over China(e.g.,Wuhan,Suizhou,Leshan,Mohe)and has obtained massive observational data of VLF signals.This thesis firstly introduces the design and realization of WHU VLF receiver system,including double orthogonal 1?-1mH loop antennas,low-noise analog front-end,and digital receiver.The magnetic antenna is constructed by a multi-turn loop.High performance differential circuits are adopted at the analog front-end.The main three parts of the analog front-end are a matching transformer,a common-base differential amplifier,and a band-pass filter circuit of 1 k-50 kHz with the total in-band gain of 80 dB.The digital receiver system is based on FPGA and consists of a signal condition circuit,a A/D convertor with 250 kSPS and 16-bit resolution,a USB2.0 data transmission module,and the time keeping and synchronization module.In addition,the automatic observation software platform based on VC++ and the data processing platform based on MATLAB GUI have been developed.Based on the WHU VLF receiver system,this thesis presents three VLF applications and initial results:low latitude tweek observations,low latitude whistler observations,and the demodulation of VLF station emissions.Based on the theory of waveguide mode propagation,the exact relations between cut-frequency fcn of tweek atmospheres and reflection height h or electron density ne have been presented in this thesis.In addition,a method has been given to obtain the dispersion curve of tweeks and cut-frequency,the reliability of which is verified by observation data.On the other hand,the study of low latitude whistlers detected at Suizhou focuses on the statistical analysis of morphological characteristics and diurnal variation of whistler occurrences with local time.The dispersions of whistlers is also computed based on the magneto-ionic theory to investigate the diurnal variation of the dispersion of whistlers recorded at Suizhou.Finally,the thesis proposes a digital quadrature demodulation method to extract the amplitude and phase of MSK station emissions.The initial results of the research above verify the reliability and practicability of the WHU VLF receiver system.