High Frequency Surface Backscatter Coefficients

Abstract

Over-the-horizon radar is a class of high frequency (HF) radar used for long range (1000-3000 km) wide area surveillance. These radars utilise the refractive properties of the ionosphere to illuminate targets beyond the Earth's horizon, and consequently their performance is highly dependent on the ionosphere. Accurate models of the radar ground backscatter are required to accurately assess the ionospheric propagation conditions and thus the expected performance of over-the-horizon radars for operational purposes. The ground backscatter coefficient characterises the amount of radiation scattered back from a surface towards a receiver per unit area. While the backscatter coefficient of the sea is well understood and may be calculated from theory if the sea state is known, the backscatter coefficient of land at high frequencies is not well understood. To calculate the land backscatter coefficients over Northern Australia, a methodology which compares observed backscatter ionograms to those synthesised using HF radio wave ray tracing techniques through model ionospheres was developed. The results from this ionogram comparison method were compared to sea backscatter coefficients calculated from theory using sea state data. Data from the Jindalee Operational Radar Network (JORN) frequency management systems backscatter sounders from September 2015 and March 2016 were analysed and maps of the backscatter coefficients across Northern Australia were developed. The effects of the ray propagation and surface properties, including radar frequency, topography, soil moisture and vegetation cover on the backscatter coefficients were investigated. It was found that desert-like regions had a much lower backscatter coefficient than mountainous/tropical regions. A weak positive correlation between the backscatter coefficient and the soil moisture and surface roughness was observed; however, it was found that the vegetation structure had the largest effect on the backscatter coefficient.