  Using a high-speed and broadband radio frequency (RF) (0.1-1 GHz) spectrum
analyzer developed on the KSTAR tokamak, it is found that several distinct
stages of RF emission appear at the pedestal collapse in high confinement
discharges. Comparison with 2-D electron cyclotron emission (ECE) images has
revealed that each stage is related to the instantaneous condition at the
outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are
intensified with the appearance of a non-modal filamentary perturbation in the
edge within several tens of microseconds before the collapse. Then, the RF
emission becomes broad toward high-frequency range (< 500 MHz) at the burst
onset of the non-modal filament. During the pedestal collapse initiated by the
filament burst, rapid chirping (1-3 {\mu}s) appear with additional filament
bursts. The strong correlation between the RF spectra and the perturbation
structure provides important clues on the stability of edge-localized modes and
on the ion dynamics in the plasma boundary.
