IONOSPHERIC MODEL VALIDATION AT VLF AND LF

A reliable knowledge of radio signal amplitude and phase characteristi cs is required to design and maintain communications and navigational circuits at VLF and LF. The ability to accurately calculate signal lev els as a function of frequency, position, and time is of considerable importance in achieving reliable assessment of communication coverage. Detailed computer models based on multiple mode waveguide theory have been developed. These models have been found to produce good comparis ons between measurements and calculations of signal variations as a fu nction of propagation distance. However, results can be very sensitive to the ionospheric inputs to these computer models. This paper has tw o purposes. The first is to present the results of a systematic compar ison of a set of measurements of signal strength from various transmit ters over a number of propagation paths using a simple model of the io nosphere. The variation of the parameters of this simple model with ba sic propagation parameters is examined. The second purpose is to exami ne the built-in version of this simple model of the ionosphere as impl emented in the Long Wave Propagation Capability. This model is found t o adequately represent a set of in-flight signal strength measurements . It is also clear that there is still room for improvements in this i onospheric model.