On the modelling of tropospheric effects in Ultra-High Frequency radio positioning

Description

Recent technological developments in Ultra-High Frequency (UHF) radio positioning offer the potential to extend metric range measurement accuracy beyond the standard radio horizon. At present, the principal obstacle to achieving this goal is our inability to reliably model, or otherwise account for, signal distortions produced by time and space variations in tropospheric refractive index. Trans-horizon propagation at UHF wavelengths is strongly influenced by small scale irregularities and layering in the refractivity structure of the troposphere which give rise to troposcatter and mode propagation mechanisms. Theoretical and empirical investigations in micrometeorology and turbulence theory over the past twenty years provide a basis for the qualitative understanding of wave propagation in a non-homogenous and time varying troposphere. In this thesis we draw upon these fields to describe the sources and characteristics of tropospheric variability and its effect on the propagation of UHF radio waves and the accuracy of UHF radio positioning. IN particular, we apply this foundation to an assessment of tropospheric effects in Atlantic Canadian waters, by correlating available surface and upper air meteorological data obtained at several stations in Nova Scotia with UHF ranging data collected in cooperation with the Canadian Hydrographic Service. The meteorological data analysed indicate that surface heating effects and weather disturbances may combine to produce day-to-day variations in surface refractive index of the order of several part in 10[subscript 5], and contribute to a strongly layered refractivity structure in the lower atmosphere/ Up to 30% of the twice daily radiosonde ascents recorded during the summer months of 1982 were found to contain extreme refractivity layers within the first tropospheric kilometer. As a result, the average refractivity lapse over the first kilometer of the troposphere is in general poorly correlated with surface refractive index. In addition, significant differences in surface refractivity and the occurrence and characteristics of atmospheric layering were encountered when comparing meteorological data recorded at coastal and offshore locations over distances of a few hundred kilometers. Seasonal variations in monthly mean surface refractivity of the order of 50 ppm were encountered. The range measurement data analysed indicate that the stability of the troposcatter propagation mechanism used in tended rang UHF radio positioning is strongly influenced by the degree of turbulent activity and the extent of non-standard layering encountered in the lower atmosphere. RMS range measurement stability over an 80 km troposcatter link was found to vary from 50-100 ppm during stable atmospheric conditions, to 200-400 ppm or more during periods of maximum turbulence and layering. The greatest periods of instability accompanied change sin prevailing weather conditions and appear related to enhanced scattering from elevated refractivity layers. In the absence of corrective measures, these influences would appear to limit the accuracy of UHF trans-horizon ranging to approximately 100-500 ppm depending upon prevailing atmospheric conditions. Often careful considerations of tropospheric influences this limit could perhaps be reduced to 50-100 ppm. Possible alternatives include detailed observations of surface and upper air meteorology within the survey area, or the use of differential range monitoring coupled with space diversity and recursive filtering techniques. In either case, the effects of a non-homogeneous and time varying troposphere will likely continue to be a limiting factor in UHF system accuracy. In this regard, further study is required to establish appropriate corrective measures and to quantify the degree of improvement possible.