Free-flight measurement for a small-scale model via synthetic image-based template matching technique

Abstract

This Short Communication proposes a synthetic image-based template-matching technique to track the free-flight motion of small-scale models in hypersonic impulse facilities using a single high-speed camera. The approach iteratively minimizes the zero-mean normalized sum of squared differences (ZNSSD) between measured shadow-graph images and synthetic images. Synthetic images account for partial-area effects (PAE) and a pillbox point-spread function to approximate defocus blur, enabling subpixel sensitivity for models as small as similar to 10 pixels. Applied to a 3 mm diameter, 3 mm long cylinder in a shock tunnel, the approach yields positions consistent with a corner-point tracking approach while substantially improving size and orientation estimates. The measured diameter (13.71 +/- 0.18 pixels) aligns with independent calibration (13.26 +/- 0.68 pixels), whereas the corner-point method underestimates size (11.55 +/- 0.47 pixels) by 2.16 pixels. The proposed method also captures small attitude changes up to-7.5 degrees pitch and 2.8 degrees yaw that the corner-point approach fails to resolve.