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REVIEW OF STEEP-INCIDENCE, VLF, MULTIPLE-FREQUENCY SOUNDING TECHNIQUES AND CAPABILITIES,
NAVAL WEAPONS CENTER CORONA LABS CA
A brief discussion is presented of problems and limitations encountered with classical techniques of vertical ionospheric sounding at very low frequencies. A sounder system is described that solves many of these problems and removes many of the limitations. Developed at the Naval Weapons Center Corona Laboratories, this system is capable of operating in a step-frequency mode with up to 10 frequencies. Pulses as short as 320 micro sec can be transmitted with a 10 duty cycle at each frequency. Continuous measurement of phase and amplitude are obtained for both the normal and abnormal components of the ionospherically reflected waves at all 10 frequencies. A special-purpose transmitter is utilized to give the experimenter control over the nature of the transmissions. Within limitations imposed by the transmitting antenna, the complete system is mobile to allow maximum freedom in the choice of geographic location for sounding experiments. Samples of data which have been collected with this system are presented and discussed. Author