A series of ultra-high-speed Schlieren and shadowgraphy images are captured at 1,000,000 frames per second to
characterise eight instability modes that exist for an underexpanded supersonic jet as it impinges on perpendicular and
inclined surfaces. The use of these high-speed imaging techniques allows the visualisation of these instabilities in real
time, without the need to employ phase-locked methods as has been done previously. It is found that the axial instability
modes show a greater degree of oscillation than the helical modes, and that the degree of oscillation increases as the
impingement distance is reduced. Evidence is also presented that suggests an acoustic feedback loop exists within
inclined impinging jets.