Improvement of the definition evaluation function for TDI CCD remote sensing images by directional wavelet power spectrum

Autofocusing is one of the key techniques for space cameras to ensure a high quality image. The optical system of the camera defocuses in all probability due to many influential factors, such as the impulsion or jitter during launching, changes of temperature and air pressure. So, in order to retrieve the defocus, focus measure algorithm based on power spectrum was studied, where the algorithm was used in TDI CCD space cameras which imaging scenes changed anytime. Firstly, an improvement over Fourier transform power spectrum was proposed, namely wavelet power spectrum(WPS) estimation, which took advantage of the multi-resolution and band-passing characters of wavelet transform. Then, directional WPS was proposed in order to reduce the influence of image-moving-mismatching. Lastly, according to the theory that power spectrum is independent of scenes and defocus made the power spectrum's high frequency losing, the evaluation function for image definition was designed, using the weighted sum of directional WPS. The experimental results of the real pushblooming images show that the new definition evaluation method can reflect the real remote images' defocusing state validly. Moreover, the directional WPS is less sensitive to the scenes' variety compared to FFT PSS, which reduces the error ratio from 0.36 to 0, and the curve gets more saturated. In addition, the error ratio of all the 100 emulational samples is only 0.06. So the proposed algorithm meets the principles of monotony, excellent sensitivity and precision, and suits for the autofocus in TDI CCD remote sensing cameras better.