Vlf navigation system

Abstract

A VLF navigation system suitable for highly accurate worldwide aircraft navigation utilizing existing VLF transmitting stations on a 'dead reckoning' basis with initial and intermediate fixes. The system is enabled to utilize all existing VLF transmitting stations regardless of station frequency by novel receiver phase-locked loop means which normalizes all station frequencies to phase coherent independent data signals of a common working frequency. In one form of the invention, utilizing hyperbolic coordinates, both RF stations of each station pair utilized for computation are such VLF transmitting stations; while in another form of the invention, utilizing Rho-Rho coordinates, one RF station of each station pair is an on-board frequency standard normalized to the common working frequency, and the other station of each pair is one of the VLF transmitting stations. Computer means, which may be part of the 'on-board' equipment, selects station pairs for optimum station geometry and range, and by comparing the phase differences between the phase coherent data signals for respective station pairs, determines a .DELTA. time and hence a .DELTA. position in suitable coordinates from the initial or intermediate calibration point. Novel multiplexing and counting means, and logic circuitry associated therewith, enable a single counting means to be rapidly sequentially employed by the computer to make the time difference comparison for a series of station pairs. The system is extremely accurate for a number of reasons, including but not limited to the inherent frequency stability of VLF transmitting stations (which are generally atomic clock controlled); the inherent minimal distortion of VLF electromagnetic signals over very long ranges; the fact that errors are not cumulative in the system; and a station pair averaging capability which decreases random errors and permits rejection of the defective signals.