Ionogram Scaling for Vertical, Oblique, And Backscatter Sounders: A review of challenges and opportunities

The process of feature identification in ionogram images and subsequent derivation of ionospheric parameters, known as ionogram scaling, enables the characterization of the ionospheric propagation conditions of radio waves in the high-frequency (HF) band. Manual ionogram scaling is a time-intensive and laborious activity; hence, numerous techniques have been developed by many researchers to automate this task. This article presents a review of the various automatic scaling approaches applicable to vertical, oblique, and backscatter ionograms. In addition, we provide a set of recommendations to guide and stimulate further research in this field.