Detection of VLF and ELF Long-Path Signals Radiated from Heated and Modulated Ionosphere Current Systems

This thesis describes the theoretical and experimental evidence of verified detection of very low frequency (VLF) and extremely low frequency (ELF) long-path signals radiated from a heated and modulated dynamo current system at mid -latitudes. A general background of the ionosphere current systems and prior research in VLF/ELF communications is followed by a complete description of the receiving subsystem. The on/off method of ionospheric heating with synchronous detection is the primary experimental technique. Phase reversal of the heating modulation envelope to create four-bit messages with synchronous averaging of the detected message is explained. A laboratory receiver subsystem calibration enables receiver system voltages to be related to the magnitude of the earth's horizontal magnetic field component at the receiving loop antenna. The laboratory calibration found a -20 dB threshold, below background noise level for detection of on/off type signals. This detection threshold was extended to -46.91 dB when 256 sample synchronous averaging was applied to four bit message data. Extensive background noise recording from 1.0 kHz to 5.0 kHz was accomplished using an 80 mHz bandwidth to assist in signal verification. A by-product of these recordings was the finding of a noise amplitude minimum near 3.0 kHz in agreement with the results of Dinger, Meyers, and Davis during a noise study published in 1980. The amplitude spectrum of background noise was compared to the amplitude spectrum of signal plus noise. All amplitudes were related to the magnitude of the horizontal component of the earth's magnetic field, and the system detection threshold. Experimental results show synchronous detection and verification of on/off signals in the range of 1.0 kHz to 5.0 kHz over paths as long as 2708 km from Arecibo, Puerto Rico, to a ship in the Caribbean Sea, and to The Pennsylvania State University at State College, Pennsylvania. Results at 1000 Hz are also strongly supported by a contour propagation plot derived from the theoretical model, centered at the heating transmitter at Arecibo, Puerto Rico. Characterization of synchronously averaged four -bit messages at 1838.235 Hz detected locally at Los Canos, Puerto Rico is described. Study and analysis of similar messages in relation to the receiving system detection threshold is presented. Future research is suggested which is designed to further understand the promising communication potential displayed by experimental results.