High-resolution algorithms to estimate the depth of near-surface interface using ground-penetrating radar (GPR) can provide a measurement of thin to thick top layer using synthetic data for surface-coupled antennas. In practice, the air-coupled antenna configuration in the target depth estimation needs to determine the air-gap distance between the antenna module and the surface. In this study, the authors present an improved approach of estimating the antenna height for measuring thin to thick top coal layer using the high-resolution algorithms. In GPR, backscattered echoes are generally correlated in nature. In order to apply high-resolution algorithms, the authors use two improved spatial smoothing techniques for fighting against the correlation between echoes. Here, in the existence of air-coal-shale-clay and air-coal-shale-coal interface, step frequency continuous wave GPR is used to measure the thickness of the coal layer. Simulation results on the antenna height estimation using synthetic data models, full-wave model and plane wave model are shown to assess the estimation algorithm’s effectiveness. © The Institution of Engineering and Technology 2021.